# Cattle body weights

Environmental and genetic effects on calf weights
Cow body weights

## Environmental and genetic effects on calf weights

Weights at birth, 3, 6 and 9 months were available for 403 calves; at 9, 12, 15 and 18 months for 306 calves; and at 18, 21, 24 and 30 months for 189 calves, born in the years 1974 - 1980, 1974 - 1979 and 1974 - 1978 respectively. Analyses of variance laid out in Table 37 show the significance of effects on calf weight at these different stages.

The estimated least squares means for body weight at the different stages are shown in Table 38.

Figure 7 illustrates mean growth of all calves from birth to 30 months of age. The coefficients of variation of weights at birth, 3, 6, 9, 12, 15, 18, 21, 24 and 30 months were 15%, 17%, 16%, 16%, 19%, 17%, 18%, 17%, 19% and 17% respectively.

Effect of year of birth

Table 37 indicates that significant year effects existed for weights at all ages except 15 months. The weight differences between extreme years at birth, 3, 6, 9, 12, 15, 18, 21, 24 and 30 months was 15%, 12%, 26%, 17%, 14%, 13%, 15%, 21%, 28% and 17% respectively of the mean weights.

Causes of variation between years in this environment can be annual rainfall affecting pasture availability, annual rainfall affecting the disease situation, changes in management techniques, genetic progress etc. Correlations and regressions between the 7 year means for weight at each stage from 3 months to 30 months and the 7 year means for rainfall from birth to each stage were all negative and non-significant.

To determine any linear trends in body weights from 1974 to 1978 for 21, 24 and 30 month weights; from 1974 to 1979 for 12, 15 and 18 month weights and from 1974 to 1980 for birth, 3, 6 and 9 month weights; the regression of the least squares constants on year of birth (represented as 1 - 5, 1 - 6 and 1 - 7) were calculated (Table 39).

Table 39. Correlations and regressions of the least squares constants for weight on year of birth

 Trait Correlation Regression Birth 0.65 0.30 3 month weight 0.23 0.30 6 month weight 0.51 1.84 9 month weight 0.38 1.48 12 month weight 0.45 1.28 15 month weight 0.87 2.67* 18 month weight 0.84 4.40** 21 month weight 0.97 8.91** 24 month weight 0.98 13.09** 30 month weight 0.92 8.99**

*P = <0.05 **P =< 0.01

The regressions were significant from the 15 month stage onwards and indicate that weight for age increased at each stage by the amounts listed in Table 39, per year over the relevant period.

Effect of month of birth

Table 37 indicates that significant month of birth effects existed for weights at all ages except 12 and 24 months. Figure 8 shows the weights of animals in each of the month of birth groups, corrected for au other effects evaluated.

The effects of month of birth on later body weights are usually related to the stage in the wet and dry weather cycle at which an animal reaches a given age; and relative rankings of month of birth groups might be expected to change from stage to stage. This situation is clearly illustrated in Figure 8.

Correlations and regressions between the 12 monthly means for weight and the 12 monthly means for rainfall from birth are given in Table 40 for the seven ages, 3, 6, 9, 15, 18, 21 and 30 months when the effect of month was significant.

Table 40. Correlations and regressions of weight (kg) at seven ages and rainfall (mm) between birth and that age.

 Age Correlation Regression 3 months 0.84 0.011 6 months 0.86 0.019 9 months 0.68 0.013 15 months 0.74 0.019 18 months 0.80 0.029 21 months 0.55 0.028 30 months 0.79 0.046

Table 40 indicates that the correlations and regressions were all positive and significant at these stages.

Effect of dam parturition number

Table 37 indicates that the effect associated with dam parturition number was significant at all ages from 3 months to 18 months. Table 38 shows that at 3, 6, 9, 12, 15 and 18 months calves from first parturition cows were 12%, 7%, 6%, 12%, 10% and 9% lighter than the mean weight of calves from the three other parturition groups.

Effect of sex

Table 37 indicates that the effect of sex was significant at all ages from birth to 30 months. At birth, 3, 6, 9, 12, 15, 18, 21, 24 and 30 months males were 5%, 4%, 7%, 11%, 10%, 11%, 14%, 15%, 12% and 8% heavier than females.

Repeatability of early calf weights

The repeatabilities of calf birth, 3 month and 6 month weights considered as a characteristic of the cow are indicated in Table 41. Data are from 403 calves born to 158 cows.

Table 41. Repeatability of early calf weights.

 Trait Repeatability s.e. Birth weight 0.15 0.06 3 month weight 0.36 0.06 6 month weight 0.33 0.06

Phenotypic correlations between calf weights at different ages

The phenotypic correlations available between calf weights are shown in Table 42. These were all positive with correlations between birth and early weights being lower than those between all other later ages.

Table 42. Phenotypic correlations between calf weights

## Cow body weights

Weights, at calving and when calf weaned at 6 months, were available for 103 individual cows over 165 parturitions. Only cows that had complete data on their linear measurements were used in this analysis so numbers were rather small. Analyses of variance laid out in Table 43 show the significance of effects on cow weight at the two stages.

Table 43. Analysis of variance of cow weights at calf birth and weaning (6 months)

 Source d.f. Mean squares Calving Weaning Origin 1 7176* 6425* Cows 102 1443** 1364** Year of calving 5 648* 654 Month of calving 11 702** 1131** Parturition calving number 3 325 67 Sex 1 321 92 Remainder 41 235 349

*=P< 0.05; **= P< 0.01.

The estimated least squares means for the two cow body weights are shown in Table 44.

Table 44. Estimated least squares mean of cow weights at calf birth and calf weaning at 6 months (kg)

 Variable Number Weight at calf birth Weight at calf weaning Overall mean 165 232.26 235.21 Origin Foundation 103 220.36 223.95 Born on centre 62 244.15 246.47 Year of calving 1975 7 209.82 216.32 1976 26 221.16 229.54 1977 6 216.58 253.62 1978 59 232.16 226.43 1979 46 235.72 246.94 1980 21 278.09 238.40 Month of calving January 3 265.70 218.57 February 6 211.70 266.66 March 11 221.01 250.50 April 23 212.18 244.64 May 36 221.74 268.42 June 14 224.10 231.24 July 13 220.88 244.93 August 15 217.56 222.14 September 14 240.59 231.88 October 11 244.47 220.99 November 13 221.88 169.66 December 6 285.27 222.96 Parturition number 1 48 225.63 222.96 2 49 233.64 231.58 3 34 238.11 239.84 4+ 34 230.65 246.45 Sex of calf Male 92 229.44 235.72 Female 73 235.07 233.70

The mean cow weight at parturition was 232 kg and at weaning of calf, 235 kg, thus an average of 3 kg was gained during the 6 months suckling period.

The repeatability of cow weight at parturition was 0.78 ± 0.06 and at calf weaning 0.67 ± 0.08. The phenotypic correlation between cow weight at parturition and calf birth weight was 0.37.