Question

In beef cattle, calves that are born from heifers (cows that have never had a calf before) are often less matured compared to calves born from multiparous cows (cows which have had >1 calf). The cause of this phenomenon is that the nutrients consumed by the heifer are distributed not only to maintenance, gestation and lactation, but also to the heifer's own growth. Therefore, calves from heifers receive less nutrients, and are often smaller, both at birth and at weaning. This smaller size gives them a disadvantage compared to calves of the same age born from older cows. We can adjust for the bias caused by the age of the mother by scaling records for calves born from younger cows. This scaling will give us an idea of what the calves could have looked like had they been born from an older mother a) Considering the aforementioned, Table 1 shows a sample of data for weaning weight in beef cattle. Calculate the heritability of this trait. Assume that the additive genetic variance is var(G) = 0 = 300 kg. Table 1 Weaning weight (kg) Age of cow at calving (years) 220 235 265 315 260 280 285 230 b) Table 2 shows adjustment factors for the effect of Age of Cow at Calving (AOC) on the weaning weight (WW). Again, calculate the heritability of WW for the data in Table 1, but adjust WW for AOC. Assume the same additive genetic variance as in a). Table 2 Age of cow at calving (years) Adjustment factor (kg) +30 +20 +10 5 or + c) What happened to the heritability when you adjusted the WW? Explain your observations.

Answer 1

Ans:

a)

Formula to calculate heritability of trait as follows;

H2=V9: Vp

Vg = genetic variance

Vp = phenotypic variance

Vg = 300kg

Vp can be calculated as

Call Age of cow at calving Weaning weight in kg 220 235 WN 265 315 5oUAWNN 260 280 00 O 285 230

Variance in weaning weight can be calculated in excel function descriptive statistics function;

Mean Standard Error Mode Median First Quartile Third Quartile Variance Standard Deviation Kurtosis Skewness Range Minimum Maximum Sum Count Age of cow at Weaning weight in kg 5.25 261.25 1.0978875821 11.3290870897 #VALUE! 4.5 262.5 2.75 233.75 7.5 281.25 9.6428571429 1026.7857142857 3.105295017 32.0434972231 -1.332938272 -0.63580431 0.5295614218 0.3272725336 95 220 315 2090 00 ENCO

hence Vp of weaning weight = 1026.78

substitute all values in the above formula to calculate heritability

H2 = Vg/Vp

H2 = 300/1026.78

     = 0.28

b)

Call Age of cow at calving WN 5ovo AWNN Adjusted Weaning weight in kg 250 265 285 325 260 280 00 O

Vp can be calculated in excel as above;

Mean Standard Error Mode Median First Quartile Third Quartile Variance Standard Deviation Kurtosis Skewness Range Minimum Maximum Sum Count Age of cow at calving Adjusted Weaning weight in kg 5.25 272.5 1.0978875821 10.0445436503 285 4.5 272.5 2.75 257.5 7.5 285 9.6428571429 807.1428571429 3.105295017 28.4102597162 -1.3329382716 0.9918082857 0.5295614218 0.4859269106 95 230 325 2180 E NO O

therefore Vp from above table= 807.14

Hence substitute all the values in formula to get H2

H2 = Vg/Vp

     = 300/807.14

    = 0.37

c)

Heritability in weaning weight is 0.28, whereas after adjusting weaning weight as per the table the heritability is 0.37

When adjusted for weaning weight heritability was increased by ~0.1.

This shows that adjusting the weaning weight has decreased the Vp i.e variance in adjusted weaning weight.

Altogether it indicates that heritability of the trait will increases when adjusted for weaning weight.