Abstract
Cow live weight is of economic importance in dairy production; however, it is not included in the selection objective for South African dairy cattle. Variance components estimates are a prerequisite to incorporating a trait in the breeding objective. Variance components were estimated for live weight of lactating Holstein cows on two South African dairy herds. Live weight records on 9843 lactating cows, collected over a period of three years, were used. An analysis of variance was carried out to determine fixed effects to include in the model by the least squares method, using the Generalised Linear Models procedure of the Statistical Analysis System. Variance components were estimated by the Restricted Maximum Likelihood procedure using the ASREML programme. A high heritability estimate of 0.74 ± 0.19 is found, which suggests that there is scope for significant response to selection on live weight in the South African Holstein cattle population. A repeatability estimate of 0.86 was obtained. These results form the basis for incorporating live weight in the breeding objective for South African Holstein cattle. Further work, however, needs to be done to develop estimates based on a representative sample of the whole population, as the current study is based on data from only two herds.
Highlights
The Holstein is the most popular dairy cattle breed in South Africa
There is, a pressing need to improve the profitability of milk production in South African dairy herds
In South African dairy cattle, a kilogram increase in live weight was recently reported to result in a decrease in profit of up to ZAR 7.49 [6]
Summary
The Holstein is the most popular dairy cattle breed in South Africa. Presently, about 53,151 Holstein cows are participating in the South African National Milk Recording Scheme [1]. Continued decline in dairy herd profitability poses a serious challenge to the South African dairy industry Due to this problem, the number of milk producers in South Africa has declined from 3899 in January 2007 to 2083 in September 2013 [2]. Live weight is among the traits of economic importance in dairy production, as it affects dairy herd profitability through its effect on cow maintenance costs and income from beef (culled cows). In South Africa, genetic trends show that dairy cattle have been selected mainly for increased yield and improved type [8] [9]. Such narrow breeding objectives hamper improvement in total economic merit. There has been a shift towards more balanced, profit-focused breeding objectives, globally, by including all economically relevant traits [7]
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