# Disease Variables

## True Breeding Values and Ratios

Disease traits were simulated as normally distributed traits per animal, with genetic covariances among the traits and with milk production and growth traits. Each trait had a phenotypic variance of 100.

To convert the normally distributed trait to a probability, 100 was added to the phenotypic trait value (genetic plus residual effects), and then that was divided by 100 to give a value from 0.5 to 1.5 range. Suppose p=0.1 is the probability of a disease occurrence in that parity, season, DIM, then p is multiplied by the cow ratio. Values greater than 1 increase the probability of occurrence, and less than 1 reduce the probability of occurrence. The range of the cow ratios varies with the disease and the heritability of that disease.

## Factors Affecting Disease Incidences

A set of tables were made for the probabilities of each disease occurring, by parity number and by days in milk. The table below contains the odds ratios for the increase in probability of a disease given that the cow has had another disease.

 Cow Has MF LDA Ket Mast. Ret.Pl. Cyst.Ov. Metritis Milk Fever 1 2.61 2.51 1 1 1 1 Ret. Pl. 1 1 1 2.13 1 1 3.53 Mastitis 1 1 1 1 2.39 2.04 1 Metritis 1 1 2.20 1.95 1 2.77 1 Ketosis 1 7.98 1 1 1 1 1 LDA 1 1 6.88 1 1 1 1

Below is a table of the days in milk when a disease most likely occurs during a lactation, and the incidences of the disease by lactation number.

 Trait Earliest DIM Latest DIM Parity 1 Parity 2 3rd + Mastitis 0 305 0.056 0.084 0.087 Lameness 0 305 0.021 0.025 0.025 Cyst. Ov. 31 150 0.032 0.052 0.065 LDA 0 150 0.005 0.005 0.011 Ketosis 0 30 0.003 0.005 0.011 Metritis 0 150 0.096 0.089 0.101 Milk Fever 0 30 0.003 0.005 0.026 Ret. Pl. 0 30 0.013 0.019 0.022

The following table contains odds ratios for the 4 seasons of the year, increases or decreases in probabilities.

 Trait Winter Spring Summer Fall Mastitis 1 1.00 1.11 0.96 Lameness 1 1.04 0.50 0.18 Cyst. Ov. 1 0.64 0.62 0.19 LDA 1 1.18 0.79 1.00 Ketosis 1 1.91 2.04 1.88 Metritis 1 0.85 1.19 0.68 Milk Fever 1 1.01 0.80 0.98 Ret. Pl. 1 1.97 1.70 1.10

Diseases have different effects on production, weights, and reproduction, and for different durations during the lactation. The reductions in performance were in percentages of usual production.

 Trait Milk Days Weight Days Feed Days CR Days Mastitis 0.95 305 1 0 1 0 1 0 Lameness 1 0 1 0 1 0 1 0 Cyst. Ov. 1 0 1 0 1 0 1 0 LDA 1 0 0.77 28 0.70 28 1 0 Ketosis 0.85 21 0.82 21 0.90 21 1 0 Metritis 0.87 14 1 0 1 0 0.62 119 Milk Fever 0.94 21 0.90 21 0.90 21 1 0 Ret. Pl. 0.87 14 1.00 0 1 0 0.62 119

Diseases require labour hours and drug costs, as shown in the following table. Some cases result in death or culling, the percentages are shown below.

 Trait Labour Hours Drug Cost Death % Cull % Dystocia 1 44 0.010 0.022 Mastitis 1 15 0.011 0.070 Lameness .5 26 0.010 0.120 Cyst. Ov. 1 50 0.001 0.050 LDA 1 86 0.020 0.080 Ketosis .67 19 0.005 0.050 Metritis .67 20 0.015 0.060 Milk Fever .5 25 0.040 0.050 Ret. Pl. .67 20 0.015 0.060