Gordon King, Animal Science, University of Guelph
Currently, on a worldwide basis, animal products provide about one-third of human protein intake and one-sixth of energy intake, but the proportions are much higher in more developed regions. Everything is so good in the industrialized countries of the affluent western world that some individuals or groups now condemn livestock as inefficient, inhumane and unhealthy for both humans and their environment. The nature, extent and perhaps the very survival of modern production systems depends on how the industry responds to these criticisms. Fortunately, progressive producer organizations now adopt a proactive position, providing education for consumers, direct and rapid rebuttal of unsubstantiated criticisms and support for both welfare and efficiency oriented research.
Perhaps one aspect that is not fully appreciated even by many within the animal production industry is the fact that milk, meat, eggs and all other consumable foods of animal origin are obtained entirely through reproductive processes. If mature females did not follow their inerrant urge to reproduce, we would be without eggs, dairy products, meat, many associated by-products, and even less obvious commodities such as honey or caviar. Thus, enhancements of fertility and prolificacy are reasonable approaches to providing more marketable output at lower cost from less breeding stock. Practical and humanely acceptable methods to improve reproductive efficiency should not only enhance profits, but yield current or even increased production from fewer mature animals, thereby reducing potential for environmental degradation.
Although most domesticated animals never reproduce, anyone associated with livestock should realize that, obtaining and maintaining the breeding heard or flock, with all of the associated expenses for housing and feeding, represents a substantial proportion of total production costs. Adequate reproductive efficiency is essential to justify this investment.
The Reproductive Process
The production of livestock products that are of value to humans is achieved through the exploitation of the reproductive processes in a relatively small number of domesticated species. Continuous ovarian activity is necessary to produce eggs and prolonged mammary function is essential to provide surplus milk for human consumption. Similarly, the meat industry depends on the regular birth of young animals, which are then grown and finished to varying degrees prior to slaughter. Other commodities such as draft power or wool also require regular reproduction for a constant supply of replacement animals. The reproductive process which satisfies these demands for work, food or fibre involves a complex series of integrated events. These activities must be precisely synchronized within individuals but some can now be manipulated or regulated to modify individual and group response. To properly evaluate and possibly apply the many currently available procedures and options, livestock unit managers, their advisors, and scientists conducting research and development activities for this industry require a basic knowledge of reproductive function plus a thorough understanding of the various technologies to determine which might be used profitably in their operation.
Reproduction is more than two individuals of opposite sexes simply mating or breeding. A biologist might describe the complete process as the replacement of parents by their progeny. According to his description, the first stage includes the production of haploid gametes by germ cells within the gonads of the first generation, union of gametes to create a diploid conceptus, development of the conceptus through the embryonic and fetal stages, parturition, lactation and various degrees of post-natal care. The offspring must subsequently grow, reach sexual maturity, mate and produce their own progeny for completion of a full reproductive cycle and passage of germ plasm to a succeeding generation.
To those associated with the livestock industry, reproduction is used in a more restricted sense, simply the breeding or mating of domesticated animals or poultry to produce offspring. In food-producing species many offspring are marketed before puberty while many others are castrated so only a few are retained to complete the reproductive process and produce the next generation. The most successful producers of animal commodities are able to market large numbers of immature animals or other consumable products from relatively small numbers of parents.
Evolution of the Reproductive
Reproduction is a complex process which may show striking contrasts even between closely related species. Many lower organisms propagate mainly by vegetative or asexual means, producing endless clones, each identical to the single parent. This asexual proliferation may be through budding and eventual separation of the offspring as in hydra, through simple division of a parent into offspring with shared organelles as in paramecium, or through development of a highly specialized stolon for production of new clones as in salpa. These require no specialized reproductive organs for gamete production and no involvement of different sexes. Certain more complex organisms, such as aphids, bees and some fish, exploit a highly specialized form of asexual reproduction called parthenogenesis which involves regular development of unfertilized eggs to produce large numbers of haploids. Regardless of method, all forms of asexual reproduction can be highly prolific, allowing quick exploitation of suitable environments and nutrient supplies, but do not allow for genetic variation or adaptation in the same manner that sexual reproduction can.
Sexual reproduction involves the combination of genetic material from two individuals. This is accomplished periodically by bacteria or protozoa through the act of conjugation which involves a temporary fusion and exchange of DNA material. Once conjugation is completed, the partners separate and continue asexual propagation. Animals of greater complexity have developed specific organs called gonads to produce haploid gametes which may encounter those of another individual and unite to form diploid zygotes. The interacting gametes are highly specialized cells, spermatozoa produced by the male and oocytes by the female, with the fusion product receiving some of its hereditary material from each parent. Such a mode of reproduction involving the exchange of material between two individuals is adopted occasionally by most living forms and, with the possible but rare exception of parthenogenesis in some birds, is the only type found in homeotherms.
Individual organisms have evolved many diverse mechanisms for sexual reproduction. A few species are truly bisexual with individual components of the same organism producing male and female gametes. These may be self-fertilizing but this is not the usual practice. Some are asynchronous hermaphrodites producing gametes of one sex when young but switching to the other sex as they age. Environmental signals bring many of the synchronous hermaphroditic individuals together and their gametes are indiscriminately released into the environment, usually aquatic, resulting in a high incidence of cross-fertilization. Similar environmental cues induce group spawning in many non-hermaphroditic species. There is undoubtedly considerable gamete wastage by both sexes in this procedure but production is high and some do unite to produce offspring.
As organism complexity increases, specialized components have evolved to perform specific functions. In the reproductive system these range from the simple gamete producing and packaging organs of many worms and insects to the very complex systems found in the higher vertebrates. In many of these latter species, in addition to producing and storing spermatozoa, the male has developed a specialized system for conveyance of gametes directly into the female. The evolution of a male copulatory organ, and associated modifications to the female genital system to accommodate this, allowed the progression from external to internal fertilization. The female reproductive system must receive the copulatory organ and subsequently transport the spermatozoa to the site of fertilization for immediate use, or provide suitable storage for subsequent release as needed. In addition to producing the genetic component of the egg or oocyte, females must prepackage sufficient nutrients for embryonic development prior to oviposition or retain and nourish the conceptus internally during its early development.
Birds are entirely oviparous, producing very large eggs which contain a substantial quantity of nutrients, sufficient for complete embryonic development of the germinal component. Since birds are homeotherms, a stable temperature for metabolic processes is essential and eggs must be incubated within a nest or in some other manner which prevents cooling below a critical temperature. In many wild avian species both the male and female participate in the incubation process as well as in the provision of post-hatching care and education of the young.
Eutherian mammals and the few other truly viviparous species retain their conceptuses within a uterus or some other specialized incubation organ. These zygotes are initially quite small but develop and retain a direct connection to the maternal system. The conceptus receives the nutrients required for growth via this route and enlarges continuously throughout gestation. Embryos developing within their dam are also protected from many of the hazards present in the external environment. One additional advantage of viviparity is that the dam-conceptus unit is mobile and can migrate if conditions for survival are better in other locations. In many viviparous species, one, or sometimes both parents provide post-natal care to nourish, protect and educate their offspring.
Reproductive processes have evolved from the simplest asexual types through many stages to the complex mechanisms found in birds and mammals. Much of the change has undoubtedly been mediated by natural selection but the mating seasons, prolificacy and reproductive behaviour of domesticated species have been influenced substantially by human intervention.
Any organism's main reproductive options are concerned with the proportion of available energy that is directed towards reproduction and whether this will be channelled into a large number of small or a small number of large progeny. Natural selection should, in theory, favour individuals producing large numbers. However, in extremely harsh environments a smaller number of offspring, each provided with a greater energy reserve, may have better chances of survival. The general evolutionary trend is towards producing smaller numbers of larger progeny. This is associated with the development of a more complex reproductive system culminating in viviparity, which protects the conceptus by allowing growth to a considerable size and stage of maturity before exposure to the outside environment.
The simplest reproductive strategy is demonstrated by primitive species that simply release gametes into the environment with the expectation that some fertilizations will result and sufficient progeny will develop to maintain or increase the population. In this instance parents have no concern for, or involvement with, their offspring. Where individuals pair off for external fertilization, the female may lay her eggs and depart while the male remains to fertilize these. In some instances the male stimulates oviposition through physical manipulation of the female and ejaculates spermatozoa directly onto the oocytes as they emerge. If any degree of post-natal care is required in these instances, it often becomes the male's responsibility.
Once social groups form, individuals have access to more than one potential mate. Females do not generally benefit from mating with more than a single male but males can produce sufficient spermatozoa to successfully impregnate many females. Male and female birds are equally capable of incubating and looking after their young, thus monogamy with both sexes contributing to pre- and post-emergence care is far more common in avian than in mammalian species. Males of herding, banding or flocking species compete for mating access and defend groups of females from the other males. In this latter instance the males are often substantially larger than females and have well developed secondary sex characteristics such as large horns, antlers, tusks or fangs to enhance their aggressive abilities.
The particular reproductive strategy or pattern evolved by each domesticated species has a substantial bearing on productivity. In feral grazing ruminants, the risk of predation is high so the trend is to produce small numbers of well developed, strong progeny which can be protected by or flee with their dams. Wild pigs leave their young in secluded hideouts or dens while the parents feed so they tend to produce larger numbers of smaller offspring. The gross efficiency of energy utilization by one adult female ruminant which produces a single offspring or one sow which produces a litter is similar. Prolific species show greater efficiency since they produce substantially more offspring in relation to maintenance requirements of the adult population.
However, domestication confounds this simple relationship since, through genetic selection or manipulation, the prolificacy of many species normally producing single offspring could be increased substantially.
In spite of the redundancy of many males, most populations approximate a 1 to 1 sex ratio. Predetermination of sex, and thus alteration of the ratio, would have substantial benefits in livestock production. Effective sex ratio control would permit production of the optimum proportion of males and females to take advantage of any phenotypic differences in sex-limited traits such as milk production or in sex-influenced traits such as growth and feed efficiency.
Embryos can be recovered from donor cows and subjected to karyotyping to determine their sex with only the desired ones being placed in recipients. Although successful, these processes are slow and expensive, limiting the application to only the very outstanding members of the population. If X and Y chromosome-bearing spermatozoa were separated prior to breeding, a large number of progeny with predetermined sex could be obtained. Many techniques have been developed which attempt to do this using physical differences between X and Y sperm but, with the exception of slow and expensive flow-cytometry separation, these have not been particularly successful. Further refinement of the flow-separation or immunological procedures using H-Y antigen would, when these become practical for wide application, have a substantial impact on livestock production practices.
Stages and Cycles in Reproductive
Reproductive activity can be divided into a number of arbitrary stages based on gonadal structure or function. One parent is heterogametic and the genetic sex of progeny is established at fertilization by the particular sex chromosome contained in the gamete contributed by this individual. During early embryonic development the genetic sex can be determined only by karyotyping since no phenotypic sex is expressed until the primordial germ cells migrate to populate and organize the genital ridge into a gonad. Once the region becomes sufficiently altered, the gonadal sex can be determined by microscopic examination. As organogenesis progresses the gonads develop into testes or ovaries and produce factors which affect the tubular segments and genital sinus differentiation into the appropriate male or female reproductive system and genitalia. These developmental changes result in the establishment of the phenotypic sex. The precise nature of steroidogenesis during the late pre-natal or neonatal life, programmes the hypothalamic-pituitary axis and brain centres to adopt the typical male or female behaviour patterns, resulting in behavioural sex.
Although there is considerable functional activity and structural development of the gonad and genitalia during mid to late gestation, the system becomes quiescent by birth. Through much of the early post-natal growing period, the size increase of reproductive organs simply parallels the whole body. Once the animal begins to approach adult size it enters the period of puberty during which endocrine stimulation and regulation of the gonad is initiated. The gonads become active and, along with the genitalia, grow faster than other body components. Sexual maturity occurs during this period of rapid growth, providing the potential ability to procreate. The mature adults of many invertebrates and some vertebrates simply mate and die. However, most birds and mammals can survive and function through several or many reproductive periods. Domesticated animals selected as breeding stock embark on a period of reproductive activity which can be long or short depending on the decisions made by their owners and the avoidance of infertility.
Females are born with a finite store of primordial oocytes already present in their ovaries. With prolonged function, this supply could eventually be exhausted resulting in senility. However, almost all domesticated females kept for their commodity-producing potential are culled before this final stage is reached.
Throughout the stage of life when their reproductive system is functional, females of most mammalian species have periods of sexual activity and inactivity. In the domesticated species, females seek out and are sexually receptive to males during the follicular phase and show no interest in the opposite sex throughout the luteal phase of their estrous cycles. Over a longer term, reproductive activity may be governed by pregnancy, lactation and whether the species' pattern is for continuous or seasonal breeding. In the latter instance reproductive activity is confined to specific times of the year and these are separated by periods of seasonal acyclicity with associated anestrus. The mature males in seasonally breeding species also show maximum gonadal function and libido during the mating season. The decline in reproductive activity occurring during the passage from the breeding to the non-breeding part of the year may vary from some reduction in sperm production, ejaculate volume and libido in seasonally-breeding domesticated mammals, to almost complete atrophy of the seminiferous tubules in many avian and some wild mammalian species.
Male and Female Roles in Domesticated
The reproductive process in males involves production of spermatozoa and male sex hormone by the gonad either continuously throughout the year or during the mating season. A large quantity of mature spermatozoa are stored and the male with normal libido should be always willing and able to copulate with a receptive female. He must be physically capable of mounting the female and have properly functioning genitalia to effect intromission and ejaculation. In some species the male may also contribute to post-hatching or postpartum care and protection of progeny. However, for domesticated animals in confinement production systems, the sire's role in a particular reproductive endeavour is finished immediately after copulation.
The female must produce oocytes and sex hormones but this is done in a cyclic rather than a continuous manner. Mature female gametes are not stored so, as oocytes approach maturity, the mammalian female's behaviour alters and she becomes sexually receptive. The necessary signals to initiate ovulation are provided spontaneously in most species, or stimulated by copulation in a few species, so that release of female gametes coincides with the sexually receptive phase. As oocytes leave the ovary they are collected by the fimbria for transport into the oviduct. If mating or insemination occurs the female must transport spermatozoa to the site of fertilization. Should all these events be coordinated properly and successful fertilization result, mammalian dams must then provide the proper environment for conceptus development and eventually give birth. Similarly, birds must oviposit and, unless the eggs are removed to mechanical incubator-hatcher units, they must be tended almost continuously by one or both parents.
Even at birth or hatching, the female's role is not finished since the dam is usually responsible for provision of whatever post-natal care or training is required and mammals must lactate to provide nourishment for their young. However, the female's role in offspring care may be substantially modified, reduced or even totally eliminated in total confinement systems.
Summary of male and female roles in the reproductive process.
Decisions Affecting Reproductive
Population geneticists talk of random mating but these situations probably never occur in higher animals. Even in wild populations with approximately even sex ratios, a few dominant males impregnate most of the females. In domesticated species, a very small number of pedigree approved or superior looking males are usually chosen and perhaps subjected to progeny tests to select the best individuals for use on large female populations. Successful livestock production involves decisions on when to mate and which sires to use on particular dams.
There are also many other possibilities for management decisions and for exercising control over other stages of the reproductive process and options will undoubtedly increase in the future. The interval from birth to sexual maturity is governed by genotype and nutrition. Herdsmen can elect to use breeds or strains which mature faster and either enhance or retard expression of this through feeding practices. Nutritional deficiencies are not compatible with optimum growth, sexual maturity or productivity in any species. Therefore, provision of adequate amounts of a balanced ration is a prerequisite for proper reproductive function and all other aspects of sound livestock production. It is the manager's responsibility to ensure that the nutritional requirements are satisfied and to decide how this is accomplished.
A selection must also be made between a number of mating practices. Mature females and males can range free or be penned together with the expectation that females will be detected by a male and mated at appropriate times. Greater control can be exercised through the use of hand mating or artificial insemination. If either of these practices is adopted, choice can be made on whether or not to mate at a particular estrus and the specific sire that will be used. The technology for other options such as controlled breeding, prolificacy enhancement, embryo transfer, predetermination of sex and induced parturition might also be considered and possibly adopted. The competent manager must have a sound knowledge of reproductive biology and economics to evaluate all the available options and make intelligent decisions on which will be useful in the particular operation.
All products obtained from domesticated animals result through exploitation of reproductive processes so regular parturitions or ovipositions are necessary for the continued operation of any livestock farming enterprise. Therefore, regardless of the specific commodity, success in any commercially oriented unit depends on reproduction, and improving performance should increase profits. Unfortunately, some livestock farmers, and even their so-called "expert" advisers, pay insufficient attention to this direct relationship between production and reproduction. One way to highlight the relative importance of the mature breeding herd is through recalling the proportion of total time, from conception to carcass, that occurs within the body of another animal, or in mechanical devices for poultry.
Beef calves suckle their dams until they reach about six months of age. Then, under reasonable management conditions, they must be fed for an additional 12 to 18 months to reach the optimum size for marketing. Consequently, most producers would say that it takes about 18 to 24 months to completely grow and finish a beef animal. However, that newborn beef calf, weighing 25 to 30 kg, has actually been growing for nine months inside the uterus of its' dam. Thus, the true time required to produce a beef carcass is the nine months of gestation plus the 18 to 24 month growing- finishing period that occurs after birth. The fastest growing-finishing beef animal will actually spend about one-third of the total time from conception to carcass inside the body of another animal. Similar proportions of prenatal to postnatal intervals for some other meat commodities under optimum conditions are illustrated in the accompanying figure. Creating greater awareness of the prenatal stage in any production sequence should illustrate that expenses for maintaining mature breeding stock constitute a substantial proportion of total production costs. Hopefully, this will emphasize the importance of reproductive performance in determining both the biological and the economic efficiency of any livestock enterprise. Once the consequences of poor reproduction are appreciated, the key challenges are recognition of what is actually happening and, whenever poor performance is encountered, what should be done to improve it.
Understanding the Complete Production
All livestock production involves exploitation of reproductive processes. However, although reproduction is a necessary prerequisite for production, most domesticated animals are marketed at relatively young ages and never engage in any breeding activity. This sometimes causes people to overlook the importance of fertility and prolificacy in livestock units.
Since reproduction is essential, all reproductive activities should be critically evaluated and summarized at frequent intervals to monitor performance in relation to previously established goals and to assess prospects for improvement. Reproductive efficiency, although critical in determining profitability, is frequently below optimum. This results in direct economic loss through lowered commodity output and indirect loss through fewer available replacements. The intervals from birth to first production of offspring ('Preparturition Interval') and between subsequent offspring ('Interparturition Interval') are the principal components affecting lifetime reproductive performance. The challenge of optimizing reproductive efficiency involves keeping both of these intervals as short as is practical and economical. Genotype, plane of nutrition and disease incidence all influence the duration of the "Preparturition Interval" for domesticated animals. Similarly, factors like estrous detection efficiency, health status, fertilization rate and embryo-fetal survival affect the "Interparturition Interval" directly; while parturition ease, litter size, maternal instinct and offspring viability also modify performance. Producers who can minimize the interval from birth to first parturition and between all subsequent deliveries should achieve high reproductive efficiency and profitable production.
Requirements for Successful
Cursory assessment suggests that requirements for successful reproduction are not excessive, involving only that a fertile female be mated by a fertile male at the proper time during her estrous cycle, with the resulting conceptus developing successfully through gestation and surviving parturition. A female domesticated animal may, however, consider reproduction a somewhat more formidable challenge. Before any reproduction is possible, animals must grow and develop to reach physical and sexual maturity. From then on, normal ovarian activities with regular estrous periods continue, unless interrupted by pregnancy, severe malnutrition, debilitating disease or transition to the nonbreeding season in certain species. Before producing any offspring that might subsequently be grown and finished for meat, a sexually mature female must develop and ovulate viable oocytes, mate, conceive, and then provide suitable environments for prenatal development throughout gestation. Once offspring are born, they must survive and some must subsequently become parents before a full reproductive sequence is completed. This is indeed a complex process, demanding considerable management expertise to provide appropriate housing, nutrition, disease prevention and supervision if it is to function effectively. Total confinement housing conditions might create even more potentially distressing conditions for domesticated animals so that competent management is of vital importance.
Estrus Cycles and Associated Sexual Behavior in Farm
By the time of birth, female domesticated mammals possess a full complement of oocytes (eggs) contained with primordial or primary follicles located at the surface of their ovaries. No externally detectable evidence of ovarian activity appears through the initial growing period until the animal reaches sufficient physical size to undergo successful conception and gestation. At this time, synchronization of the appropriate gonadotropin controlling hormones from the anterior pituitary gland plus the local regulatory mechanisms within the ovary allow complete follicular maturation and ovulation. Thereafter, successive small groups of selected follicles commence final growth and development toward maturity so the associated estrous cycles commence. Female domesticated animals are anything but promiscuous, displaying only isolated and usually short periods of sexual activity (estrus or heat) coinciding with final follicular maturation. Estrogen, secreted by specialized endocrine cells located in tissues forming the walls of growing follicles, stimulates both the physical and behavioral signs of estrus. Thus, under natural conditions, females only become sexually motivated and receptive near the time when ovulation is imminent and conception is possible. Healthy, nonpregnant dairy cows and sexually mature heifers are polyesters, showing regular periods of estrus throughout the year, unless pregnancy or pathology suppresses the cycle. Estrogen injections can produce estrous-like signs in both intact cycling, pregnant and even ovariectomized females. Unfortunately, veterinarians sometimes overlook this side effect accompanying the therapeutic use of estrogen.
The bovine estrous cycle usually covers 17 to 23 days so normal, nonpregnant cows should show signs of estrous behavior (heat) every three weeks.
The most infallible sign of estrus is a female's willingness to stand immobilized and allow copulation to take place (heterosexual interaction) or, in the case of cattle and occasionally in other species, to accept mounting by another female (homosexual interaction). Many older references claim that the bovine estrous period lasts for 18 h or more but more recent evidence indicates that it is considerably shorter for confined dairy cows (Hurnik, King & Robertson, 1975, App. Anim. Ethol. 2:55). The observation that estrous periods are quite short for confined, high producing dairy cows has been confirmed by a number of subsequent research investigations conducted in both temperate and tropical regions. Although the information exists in scientific journals, it has not yet been widely incorporated into textbooks or extension publications.
| Duration of
|Approximate time of ovulation||Normal breeding season|
Range 17 - 23
|Mean < 10
Range 1 - 24
|30 h after beginning of estrus||All year|
Range 18 - 22
|Mean < 15
Range 4 - 24
|32 h after beginning of estrus||All year|
Range 12 - 35
Range 24 - 240
|Near end of estrus||Spring & summer|
Range 12 - 19
Range 20 -48
|26 h after beginning of estrus||Fall (variable with breed)|
Range 16 - 22
Range 24 - 72
|On 2nd day of estrus||Fall|
Range 16 - 22
Range 24 - 90
|36 h after beginning of estrus||All year|
Long day breeders - reproductive/gonadal activity in mature animals is stimulated by increasing day length so animals are sexually active in spring and/or summer; e.g. horses and some other wild animals (mostly other equines or related species with long gestations)
Short day breeders - reproductive/gonadal activity in mature animals is stimulated by decreasing day length so animals are sexually active in the fall; e.g. sheep, goats, deer
Photoperiod can be manipulated to regulate reproductive activity.
Subjecting females to altered photoperiod to alter their normal breeding season is used to a limited extent in horses, sheep and goats. The use of reduced or extended light cycles to regulate reproductive activity is much more common in modern poultry management.
Temperature and Moisture
Temperature and moisture interact to regulate plant growth.
Seasonal temperature and rainfall changes control nutrient availability.
Very high or very low temperatures may alter animal behaviour so that
strong signs of estrus are not expressed.
Prolonged exposure to extremes might possibly disrupt endocrine balance and interfere with gonadal function.
High temperatures shortly after mating may cause early embryonic mortality.
Good animal husbandry will provide some protection against extremes of
temperature and moisture.
Provision of windbreaks, shelters or total confinement buildings are necessary in very cold climates.
Shade, often combined with water spraying and fans to insure air movement should be provided during very hot spells. Evaporative cooling works well in hot-dry climates but the effect is reduced in hot-humid regions. Perhaps livestock should not be raised in the humid tropics.
Appropriate feed conservation during periods of prolific growth should alleviate any feed shortages during dry or winter periods.
2. Set up a comprehensive record keeping system
---- entries must be current
---- data must be summarized and evaluated regularly
3. Establish challenging but achievable targets (goals) for all stages. The minimum required would include:
5. Initiate corrective measures whenever necessary.
To be effective, targets must be challenging but achievable. Unfortunately, some livestock owners, government officials or even research scientists, particularly if they do not assume responsibility for or participate directly in the daily management of reproducing animals, frequently have unrealistic expectations. In some instances the cost of additional inputs in relation to what might be achieved, dictate that less than maximum efficiency must be accepted. One important aspect of reproductive management is deciding when it is no longer reasonable to keep mature females. Culling criteria will vary with the type of operation. Large, commercially oriented ranchers should dispose of all open females shortly after the breeding season concludes. Sows that do not breed back within 30 days of weaning should also be shipped to market. Somewhat less rigid standards must be adopted for dairy herds and smaller scale or subsistence farmers. One example of where less than ideal performance must be anticipated and tolerated is in situations where valuable animals are purchased to establish new herds or flocks. Consideration should be given to the fact that no replacement animals will be available for a considerable period so infertile females must be remated for much longer periods than would be the case in their herds of origin. In such instances culling standards and reproductive performance targets should be adjusted accordingly. A suitable goal for almost all livestock farms might be to develop breeding management systems that maximize reproductive efficiency to the extent this can be justified sociologically, ecologically and economically.
A serious difficulty for all farmers today is that available measures of reproductive performance are historical, telling only what occurred in the past. The development of procedures that more accurately predict future fertility present a formidable task. At present, however, the main reason that farmers fail to meet goals is because most do not have any. One immediate challenge is to overcome this deficiency.
Keeping and regularly reviewing comprehensive production records will usually indicate whenever subclinical problems are present. A number of computer programs are available to assist in compiling, organizing and summarizing records. These can be excellent management aids provided that farm operators understand fully that neither hardware nor software have any inherent intelligence. These simply augment rather than replace the user's knowledge and experience.
The science of animal production has advanced to the state where knowledge exists on how to improve many of the components involved with reproductive performance. Unfortunately, for some of these procedures, the cost associated with providing the required inputs may exceed market value of the additional offspring or commodity produced. An additional challenge for producers, and for their professional advisors, is the initial and continuing evaluation of each new or existing technology to determine whether it should be cost effective when used on particular farms. Academics often propose "high tech" and therefore costly solutions for almost everything without appreciating the financial risks associated with farming. Fortunately, most farmers are usually much more conservative than their supporting R&D or extension officers.
Livestock producers appreciate the financial consequences of acute
reproductive disorders such as orchitis in males or recurring abortions in
females. Whenever such conditions occur and professional assistance is
available at reasonable cost, they quickly seek help in an attempt to overcome
a problem. In contrast, many fail to recognize the somewhat greater losses
associated with subclinical conditions that increase the
preparturition-interparturition intervals or reduce prolificacy for almost all
members of a breeding group. Perhaps better extension programs are needed to
educate more farmers on production economics. These should emphasize that small
but continual losses associated with almost every animal are usually much more
substantial than large losses for a few members of the herd or flock.
1. Failure to Mate.
Heifers. i) Delayed development so estrous cycles do not occur
ii) Cycling but not mated (poor detection of estrus)
Cows. i) Cycling but not mated (poor detection of estrus)
ii) Delayed or absent cycles (malnutrition)
2. Fertilization Failures in Mated Females.
i) Insemination at wrong time (poor detection of estrus)
ii) Defective sperm or eggs (unavoidable but the incidence is low)
iii) Incompetent inseminations (poor training or supervision)
iv) Poor semen quality (incidence low unless mishandled after leaving AI center)
3. Pregnancy Failure in Conceiving Females.
i) Early Embryonic Mortality (20-30 % is usual, natures way of
eliminating abnormal embryos)
ii) Abortions (<5 % is usual, may result from infections, toxins or injuries but exact cause is never determined in many cases)
4. Neonatal Mortality. The death of an animal at or shortly after
birth is perhaps the most serious reproductive failure. By this time the
owner's expenses for maintenance of the mature female spans the complete period
of mating and gestation. For meat animals this constitutes a substantial
investment for no return except the possible salvage value of the female. Even
if the dam is remated successfully soon after the death of her offspring, it
will be many months before she again produces viable young. Although dairy cows
will still lactate, the value of a replacement heifer or a veal calf is lost.
Estrus Control One of the difficulties associated with operating any animal breeding enterprise is that almost everything relating to an actual mating occurs when the female is ready, rather than at the manager's discretion. Thus, procedures to manipulate ovarian activity so that ovulation is regulated to allow mating at predetermined times would be useful. Today, no technique precisely regulates ovulation but reasonably effective pharmacological and physical methods are available to synchronize estrus in mature, cyclic females, or to stimulate follicular phases in near pubertal or acyclic animals. Influencing ovarian activity so that a number of females show estrus around the same time has a number of practical uses in livestock production. Several physical and pharmacological methods for estrus control are available.
To be acceptable for routine use in commercial livestock production units, any estrus-controlling procedure must fulfill a number of essential criteria. One obvious requirement is that it must be effective in regulating ovarian activity in many of treated females so that most will experience estrus around the same time. In general, the higher the efficiency and more precise the control, the better the agent. A second desirable characteristic is that fertility should not be depressed at the estrous period following treatment. A very slight reduction might be tolerated if control is precise, but an ideal agent would not suppress the chance of conception or normal embryonic development. In addition to being effective in regulating activity and not depressing fertility, ease of administration is also important. Physical methods, such as those involving light manipulations, are only practical in groups that are or can be confined under appropriate conditions for the necessary time. Similarly, administration by injections, implants or incorporation into the ration are all possible with dairy cows but may require special provisions for pastured heifers and be totally inappropriate for some beef herds maintained on rangelands. Several other requirements for an ideal cycle-regulating procedure are absence of undesirable side effects, production of no potentially toxic tissue residues and economy. Any compound or procedure under consideration should be evaluated for its ability to satisfy all of these criteria.
Artificial Insemination. Artificial insemination (AI), which has been practiced on a worldwide scale for more than half a century, is still the major technology available for improvement of farm animals. Since only a few outstanding male animals are needed to mate a large population of females, selecting the best males by performance and progeny testing programs is extremely important. The opportunity for genetic improvement through these testing and breeding programs would be extremely limited without AI for widespread dissemination of semen from the superior sires at a reasonable cost. The dairy industry in many countries has certainly made excellent use of this technology. Poultry, pigs, sheep, goats, horses and even fish can also be inseminated but, with the exception of turkeys, applications in these species not yet extensive.
Artificial Insemination technology has been around for so long that some people seem to have forgotten that its first objective is to get cows pregnant. Now, almost all of the major decisions in some bull studs seem to be made by sire analysts rather than by technical supervisors. All to often bulls with desirable proofs are retained even though their fertility is suspect. However, one rarely encounters farmers who stop using AI because they are not satisfied with the genetic quality of the resulting calves. Most of those who quit do so because they do not get enough calves born. Fortunately, governments in some temperate regions are relaxing monopoly regulations so that bull studs must face competition. Farmers now have choices between local cooperatives, private services or "do-it-yourself" options. With competitive alternatives available, AI studs that maintain satisfactory pregnancy rates along with improved genetic quality survive. Those that fail to do this will disappear.
In the tropics a number of AI centers offered frozen semen based programs that worked reasonably well so long as donors supplied all equipment, maintained some supervisory staff in the project and subsidized the routine operation. Once aid stopped, however, many projects floundered. A much more logical and sustainable approach for regions that feel they have a real need for AI might be to begin with, or revert back to fresh semen technology and continue with this until the processing and distribution staff, inseminators and farmers all become experienced. After high fertility is achieved routinely with the fresh semen technology, a sire proving program is in place and the local government can assume all of the continuing operation cost, a change to frozen semen might be contemplated.
Check the following links for additional information on the methods and applications of AI.
Artificial Insemination in Dairy Cattle - University of Florida
Artificial Insemination for Beef Cattle - Oklahoma State U. [PDF viewer required]
Embryo Transfer. A full century has passed since the first successful transfer of rabbit embryos was described. The procedure remained one of scientific rather than practical interest until the research conducted on cattle transfers at Cambridge, England, in the late 1960's and early 1970's. These scientists established the basis for practical applications with commercial service now available in Canada and throughout the developed world.
Bovine embryo transfer (ET) began as an expensive surgical procedure done with the donor under general anesthetic and in an aseptic environment. Subsequent research and development produced nonsurgical programs that still requires considerable technical skill but without the hazards accompanying anesthesia and major surgery. Current technology allows the embryo collection and transfer right on the farm so lactating cows can serve as embryo donors without being moved from their home environment. Thus, embryos can be collected from the best females and placed in the uteri of inferior females (recipients) to develop. The recipient mother serves only as an incubator for the calf and does not contribute anything to its genotype.
A good site for ET information, including a discussion of costs involved and who should use it, is provided by the Cooperative Extension Service at Oklahoma State University.
Procedures such as, in vitro fertilization (IVF), embryo cloning and even predetermination of sex are now being carried out on research stations. IVF is now available through some cattle breeding centers and other manipulative reproductive technologies might soon reach the practical application stage even for commercial producers. However, the more conventional mating practices will continue as the only reproductive procedures for most of the livestock in the world for the foreseeable future. Producers should attempt to maximize reproductive efficiency through the application of conventional or innovative technology only to the extent this can be justified through better return on investment.
In domesticated mammals that normally have multiple offspring, the ovulation rate, percentage fertilized and embryonic survival govern the actual number born at each parturition. Sheep genotype has a marked effect on the number of oocytes ovulated with means below two for most breeds. In contrast, ovulations usually exceed three in the Finish Landrace, Booroola Merino and Romanov breeds. Goats also show substantial interbreed variation in numbers born, ranging from a very low incidence of twinning in Angora or Pashmina does to several prolific Indian breeds that average four kids per parturition. Similarly, ovulation rate and potential litter size in pigs may range from 5.5 in wild strains to more than 20 in the French hyperprolific Large White and the Chinese Meishans.
Profitable pork production requires prolific sows, appropriate care so most piglets survive to weaning and efficient growth to market weight. The total cost of keeping a sow for one year includes her feed, housing, labor, health care, depreciation and interest on capital, with the feed constituting the major expense. Some reduction in maintenance costs might be possible through making better use of existing space but this increases the risk of distress to the animals through overcrowding. A better possibility for higher profits would be through increasing the actual number of piglets weaned by each member of the breeding herd. An assessment of sow productivity in large Ontario herds indicated that Piglets Weaned per Sow per Year averaged 16.75. The mean preweaning mortality was 18.6% but ranging in individual herds from just over seven to almost 37%. Larger herds made better use of facilities than smaller herds as indicated by more piglets weaned per farrowing crate in larger units. The conclusion from this survey was that potential for improvement in Canadian herds is substantial.
A number of countries have Government supported research and development programs investigating incorporation of hyperprolific breeds such as the Chinese Meishan into rotational crossbreeding systems. Results show improvement in litter size but, to date, gains are offset by losses in carcass quality. Future developments might increase litter size somewhat through better incorporation of desirable and avoidance of detrimental traits into synthetic strains, or by further selection for increased uterine capacity within existing purebred populations. Experience in Canadian commercial pig units suggests reproductive efficiency might improve marginally through reducing farrowing interval and increasing litter size. Alternately, a dramatic increase is readily achievable through reducing deaths between birth and weaning. Such losses consistently fall below 10% in the best herds but increase to more than 20% in many units, indicating the means for better neonatal survival exist but are not always applied. The investment in and potential value of individual piglets surviving through pregnancy and birth is substantial so any reduction in subsequent losses would have considerable economic significance. Perhaps pork producers should redirect strategy. Rather than place undue emphasis on increasing litter size born, priority should be to keep more piglets alive through developing sows consistently able to farrow and raise 11 or 12 vigorous piglets to weaning.
Prolificacy has an additional dimension in reproductive efficiency since it can be manipulated in most species. Cattle and other large domesticated mammals give birth usually to only a single offspring. This combined with late sexual maturity and a long gestation period puts them at a potential disadvantage. To survive and compete with more prolific species, commercial beef producers must insure that most of their cows produce calves annually. Practical methods to increase multiple births from commercial cattle might be developed sometime in the future but these already exist for pigs and small ruminants. Size of pig litters can be enhanced through selection within breeds for greater uterine capacity or by crossbreeding using highly prolific breeds. Sheep productivity could increase through wider application of alternate management programs. A large Agriculture Canada research project demonstrated that intensification combined with appropriate hybrid strains and management procedures, substantially increase lambing efficiency and quality. The Canadian sheep industry declined substantially in the later half of this century, yielding much of the domestic market to imports. Perhaps adoption of new production strategy such as that developed by Agriculture Canada might provide sufficient increases in prolificacy and product quality so Canadian lamb might again compete in domestic and world markets.
Modern poultry strains probably operate closer to their biological maximum than any of the other domesticated species so prospects for obtaining more eggs per year from breeding stock are limited. Any further progress with laying hens will likely involve slight increases in the efficiency of feed conversion or possibly in obtaining slightly larger rather than more eggs from each bird. Mature body size is substantial in both broiler and turkey breeding hens. Efficiency might be enhanced further through maintaining current egg production from slightly smaller broiler hens, thereby lowering feed requirements. The potential for increased turkey marketing seems to favor larger carcasses so there is little prospect for reducing body size of mature breeders. Also, as birds get heavier, it may be difficult to maintain present egg production. Perhaps the most feasible means of further improving reproductive efficiency will be through selection for better hatchability of turkey eggs and feed utilization by breeding stock.
Fish are clear winners in prolificacy but, like other cold blooded animals, grow rather slowly and only during warmer seasons in temperate climates. Thus, several years are required to reach sexual maturity but appropriate reproductive manipulations could shorten this. The currently raised species in Canadian aquaculture are all carnivorous, usually consuming very expensive diets. Further progress is possible to reduce feed costs but it is this present discrepancy in price for feed ingredients that allows ruminants to compete with their more prolific relatives.
The following table, while somewhat arbitrary, attempts to place a few
of the many aspects associated with reproductive performance in relative
perspective. One consideration is the length of time females must be maintained
before any salable outputs are produced. Chickens have the advantage of rapid
growth and early sexual maturity so can begin providing economic returns by 6
or 7 mo. Sows in intensive confinement units are expected to farrow by one year
and the first litter can be weaned three to six weeks later. Early maturing
ewes might lamb before one year of age but many do not mate until they are
yearlings and are more than two years by the time they wean the first lambs.
Fish and other poikilotherms in cold climates grow rather slowly, requiring
several years to reach breeding condition so there are no returns before this
time. Beef cows may give birth to their first calves between two and three
years of age but must nurse these for approximately 6 mo. before the weanling
calves can be sold. Early maturity is advantageous since it reduces the
interval during which animals must be maintained without providing any returns,
but other aspects must also be considered when assessing reproductive
Comparative Performance for Various Meat Animals
|Mature wt, kg||450||50||180||2.5||2.0|
|First return2, mo of age||30 - 42||16 - 28||12 - 14||6.5||36|
|Gross return3, $/year||450||140||700||60||150|
lactating, hen & trout laying eggs.
2First sale of weanlings, chicks or fry.
3Sale of annual outputs of weanlings, chicks or fry.
The values in this table attempt to equate economic efficiency for the various species by dividing the anticipated annual return by the daily metabolizable energy required for females at the height of production. This simple comparison across species emphasizes the importance of prolificacy since return increased dramatically in direct proportion with higher numbers of offspring produced. On the basis of this isolated result, one might conclude that fish and poultry should be much more profitable than cattle or sheep. Additional factors must be considered, however, since individual animals differ substantially in the types of feed necessary to satisfy nutrient requirements. Balanced rations for mature beef animals can be formulated almost entirely from quality forages costing around $75 per tonne. Alternately, the common fish in temperate aquaculture systems are carnivorous, consuming diets costing more than $1,500 per ton. It is this discrepancy in feed costs that allow ruminants to compete favorably with the more prolific species. Any cost effective methods to improve prolificacy should increase profits generated by all domesticated animals, but might be particularly advantageous for those currently producing the fewest offspring per year.
Currently, on a worldwide basis, animal products provide about one-third of human protein intake and one-sixth of energy intake, but the proportions are much higher in more developed regions. Everything is so good in the industrialized countries of the affluent western world that some individuals or groups now condemn livestock as inefficient, inhumane and unhealthy for both humans and their environment. The nature, extent and perhaps the very survival of intensive confinement production systems beyond this current decade depends on how the industry responds to these criticisms. Fortunately, progressive producer organizations now adopt a proactive position, providing education for consumers, direct and rapid rebuttal of unsubstantiated criticisms and support for both welfare and efficiency oriented research. Perhaps one aspect that has not yet received sufficient emphasis is the fact that milk, meat, eggs and all other consumable foods of animal origin, including even less obvious commodities like honey or caviar, are obtained entirely through reproductive processes. Therefore, enhancements of fertility and prolificacy are reasonable approaches to providing more marketable output from less breeding stock. Practical and humanely acceptable methods to improve reproductive efficiency not only enhance profits, but should yield current or even increased production from fewer mature animals, thereby reducing potential for environmental degradation.
Lactation or milk secretion is programmed to coincide with parturition. This process involves the extraction and modification of constituents from the blood by specialized epithelial cells lining the alveoli or glandular tissue of the mammary system. The galactopoietic hormones (TSH, GH, ACTH, steroids and possibly Prolactin) initially increase and then maintain the numbers of secretary cells, facilitate transfer of nutrients into these cells by increasing blood supply and vascular permeability, and enhance synthetic activity by cellular organelles. The resulting secretion (milk) is stored within the lumen of the alveoli, their emptying duct system and in the gland cistern. When appropriately stimulated, myoepithelial cells associated with the glandular and upper duct portions of the system contract, forcing fluid down into the lower ducts. Also, the stimulus causes the sphincters or contractions at each junction to relax, facilitating milk passage out the ducts toward the teat or nipple openings. Mammary massage, kneading or the initial suckling provides the inducement for the milk let down reflex. Go to the material on Milk Production and Biosynthesis for additional information on mammary structure and function.
Neonates nurse frequently, usually more than once per hour for the first few hours or days after birth. As they grow and their digestive capacity increases, the frequency of nursing declines. Once the offspring develop ability to ingest and digest solid feed, nursing frequency decreases, resulting in a decline and eventually cessation of milk synthesis. As lactation stops, the mammary system undergoes involution, shrinking back almost to its pre-stimulus size.