Weight is modelled by the Von Bertalanfy function which has three parameters, A, B, and K.

Weight = A*(1 - B*exp(-Kt))**3

where the mean values are A = 633.3 kg, B = 0.601, and K = 0.0036, and t is the age of the cow.

Weights are for non-pregnant, non-lactating cows under healthy conditions. The fetus weight is calculated as

Log10(fetus wt) = 2.932 - 3.347*exp(-0.00406*g)

where g is the number of days pregnant.

Adjustments for diseases are specific to each disease and are described in that section of the website.

Adjustments for cold and heat stress are described in the section on Weather Simulation.

Weights change within lactation due to milk production early in the lactation. Weight at the start of lactation is given by the function in the previous section. Relative changes are given by the factors in the following table, which differ depending on first or later parities.

Week of Lactation | Parity 1 | Later parities |

1 | 1.00 | 1.00 |

4 | 0.981 | 0.968 |

7 | 0.981 | 0.952 |

10 | 0.983 | 0.950 |

13 | 0.998 | 0.952 |

16 | 1.002 | 0.960 |

19 | 1.009 | 0.968 |

22 | 1.022 | 0.976 |

25 | 1.031 | 0.984 |

28 | 1.037 | 0.992 |

31 | 1.046 | 1.000 |

34 | 1.056 | 1.006 |

37 | 1.065 | 1.013 |

40 | 1.074 | 1.024 |

43 | 1.087 | 1.032 |

46 | 1.102 | 1.048 |

Weight varies from day to day, due to unknown effects. The standard deviation of these fluctuations is assumed to be 1.5 kg per day. This noise is added to the expected weight.

Height is modelled by the Von Bertalanfy function which has three parameters, A, B, and K.

Height = A*(1 - B*exp(-Kt))**3

where the mean values are A = 150 cm, B = 0.211, and K = 0.0041, and t is the age of the cow.

Heights do not vary day to day due to environment. Height never decreases until older ages. Before that time the cow will most likely have been culled. Cows will differ in their growth rate. Height is highly correlated to stature.

Heights do not affect diseases, feed intake, milk production, or any financial aspects of the simulation, at this time.

For animals from birth to first calving, dry matter intake can be determined by the following exponential function with two parameters, A and K.

Intake = A*(1 - B*exp(-Kw))

where the mean values are A = 15.36 kg, and K = 0.0022, and w is the body weight of the animal. The standard deviation of intakes from day to day is 1.45 kg.

Within a lactation, dry matter intake is determined from body weight and protein yield with the following formulas.

Intake = (4.6 + 0.011*BW + 12.4*PY)*LAG, for first lactations, and

Intake = (8.4 + 0.006*BW + 12.2*PY)*LAG, for later lactations.

where LAG accounts for a delay due to energy balancing. LAG depends on week of lactation and month of peak milk yield. A table of LAG values is given below.

Week of Lactation | Peak 1 | Peak 2 | Peak 3 |

1 | .77 | .65 | .59 |

2 | .85 | .75 | .66 |

3 | .90 | .81 | .72 |

4 | .94 | .87 | .77 |

5 | .96 | .90 | .81 |

6 | .97 | .92 | .84 |

7 | .99 | .95 | .87 |

8 | 1.00 | .96 | .89 |

9 | .97 | .91 | |

10 | .98 | .93 | |

11 | .98 | .94 | |

12 | .99 | .95 | |

13 | 1.00 | .96 | |

14 | .97 | ||

15 | .98 | ||

16 | .99 |

Adjustments for pregnancy, diseases, and cold or heat stress are given in other sections.

An initial BCS will be genetically determined at the beginning of each lactation. BCS will then change relative to calving date similar to the changes in body weight. BCS is measured on a 1 to 9 scale with a genetic variance of 0.864 and a residual variance of 1.16.

Week of Lactation | Parity 1 | Later parities |

1 | 1.00 | 1.00 |

4 | 0.943 | 0.943 |

7 | 0.876 | 0.876 |

10 | 0.810 | 0.810 |

13 | 0.820 | 0.820 |

16 | 0.830 | 0.830 |

19 | 0.841 | 0.841 |

22 | 0.849 | 0.849 |

25 | 0.857 | 0.857 |

28 | 0.865 | 0.865 |

31 | 0.873 | 0.873 |

34 | 0.881 | 0.881 |

37 | 0.889 | 0.889 |

40 | 0.897 | 0.897 |

43 | 0.905 | 0.905 |

46 | 0.913 | 0.913 |