22 Energy for Growth

22.1  History

Only in recent times have sciences such as physics and chemistry become divorced from practial subjects such as raising animals for us to eat. People were far more sensible in the old days.  Many of the scientists who were interested in agriculture were also interested in chemistry or physics, and vice versa.  Thus, in the history of this topic, energy for growth, some of the great names of physics and chemistry were actively thinking about biological and agricultural phenomena.

Energy is the capacity of doing work, while metabolism is the overall balance of constructive and destructive changes  within living organisms.

22.2 The respiratory quotient

The RESPIRATORY QUOTIENT (RQ) is the volume of carbion dioxide exhaled divided by the volume of oxygen used. The RQ was introduced in 1849 by Regnault and Reiset who devised a closed‑circuit apparatus for the measurement of oxygen consumption and carbon dioxide production. Small animals with a high surface to volume ratio were found to have a greater rate of respiration per unit of body weight than large animals. It was concluded this was due to the greater rate of heat loss in small animals.

Although oxygen consumption is increased after any large meal, the increase is most dramatic after a meal with a high protein content.  By the 1860s,  calorimeters had been developed for the combined measurement of oxygen consumption, heat production, and the elimination of carbon dioxide and water. With this apparatus, it was possible to show the RQ's of various energy sources.

                 Carbohydrates = 1.0

                 Protein                 =  0.8

                 Fat                        =  0.7


Where are we going?  What is the point of all this?  We are trying to explain fat deposition in meat animals.  Look at the feed energy required to deposit this fat in a meat animal.  Look at what happens to this energy when humans eat it.  So, fat is a bad thing, eh?  Let's have meat without fat, eh?  Well, don't forget - meat totally without fat is bland and seems dry, so who wants to eat it? So, fat is sort of bad, and sort of good - both at the same time - a lovely problem for the bright and tidy minds of keen students to ponder.

22.3 Feed intake and digestion

 22.4 Energy distribution models

Although the growth equation of von Bertalanffy may be translated into energy terms, the other growth equations in which deceleration is a function of body weight or of age present more of a problem. One approach to the problem is given by Sir John Hammond's  idea of body tissues competing for circulating nutrients.

There are a number of astute points about this model;

In the pregnant female, the uterus attains a priority for food distribution  between A and B. The model also corresponds fairly well with our present understanding of the endocrine control of the flow of energy. In ruminants, for example, the energy flow to skeletal muscle is increased by  somatotropic hormone (STH) while the flow to adipose tissue is increased by insulin . Also, nitric oxide now is recognised as controlling blood flow in a number of organs, as in the effect of FGF (fibroblast growth factor) in modulating blood flow in skeletal muscle . In other words, circulating factors may control nutrient availability patterns between tissues by diverting the flow of blood-borne nutrients, as well as by switching on and off the energy assimilation systems in various tissues.

Hammond's model was also supposed to explain the effect of reduced energy intake on muscle development. But this did not survive experimental testing.  For example, with pigs at live weights of 150 kg on a submaintenance diet, the tissues of the carcass are affected in reverse order to their order of anatomical development. Fat is reduced most, bone is reduced least, and muscle is intermediate in reduction.  For another example, with Angus steers on a resricted diet reducing live weight, bone and connective tissue may be relatively unaffected, muscle mass may be reduced, but there may be only relatively slight reductions in carcass fat content. Chemical analyses shows the loss of muscle weight is from protein, and not to dehydration or to mobilization of intramuscular fat.

To cope with all these defects in the original Hammond model,  Berg and Butterfield  in 1976 proposed an alternative model of nutrient distribution during growth. Most researchers still agree with this modification.

Energy distribution between tissues is now envisaged as a combination of nutrient availability and tissue capacity. Hammond's model resembles growth equations in which specific growth rate is a function of age, essentially of physiological age.  As animals become older they become capable of reproduction and extensive fat deposition, and their priorities change accordingly. Conversely, the model of Berg and Butterfield resembles growth equations in which specific growth rate is a function of body size, because of the implied limits to tissue volume in all except adipose tissue. Thus, according to these two models, the animal "knows" either its physiological age or what size it should be.
<>This is difficult to explain!  Here are some ideas which have been proposed.


   22.5 Conclusion

  This lecture is more about what we don't know than what we do know! It might be possible to combine all these proposed growth‑controlling mechanisms into a single system with each tissue compartment being given

This could be incorporated into an algorithm with
 It would, however, be extremely difficult to place meaningful limits on parameters 1 to 3, and to program metabolic interactions resulting from inputs 4 to 10. The reason is, although the vascular system may resemble a branching pipeline system, it carries a number of different fuels at once. Metabolically, it is compartmentalized (glucose, glycerol, amino acids, fatty acids, ketone bodies, lactate, pyruvate, high density lipoproteins, low density lipoproteins, very low density lipoproteins, etc.) with a comparable number of interacting hormonal and biochemical regulators. However, in cattle, blood glucose concentration may in fact be correlated with the rate of growth in live weight). Required muscular work (7) is a particularly difficult parameter to evaluate since, as well as diverting energy to a mechanical output, work also stimulates muscle growth so that muscle mass and physical activity might be correlated.

The real conclusion?  We really do not understand how animals grow, nor how they distribute nutrients between different types of tissues. We may think we grow the animals but, really, the animals grow themselves. If you become an animal scientist, you might help solve some of these great mysteries!

Further information

Structure and Development of Meat Animals and Poultry. Pages 463-476.