SELECTION FOR FERTILITY AND MILK PRODUCTION – HOW TO SELECT

Cow fertility has recently received much more attention world-wide in breeding programmes than previously, because of its economic implications. Age at first calving (AFC) and calving interval (CI) are typically the parameters of choice in South Africa, but the success thereof is affected by the farmer’s management choices. As alternative, service records could be used to derive fertility traits such as services per conception (SPC), calving to first service (CFS) and days open (DO). In such selection objectives, milk production of course should not be compromised as it remains the most important economic parameter, but as it is known that fertility and milk yield is often negatively associated, it is important to understand the relationship for effective selection decisions. Thus, the objective of the study cited was to estimate genetic parameters for fertility traits derived from service records and determine their correlation with milk yield.

Service and pedigree records were obtained from on-farm management systems, consisting of 64 464 records from 18 South African Holstein herds. These were analysed with appropriate statistical packages.

The means for the traits were: SPC 2.2 ± 1.6; CFS 89.9 ±34.7 days; DO 138 ±70.2, and milk yield 316 ±108. The heritability of the fertility traits was low (0.04 – 0.06), whereas that of milk yield was moderate (0.29). The phenotypic and genetic correlations between fertility and milk yield were antagonistic (negative) and low, ranging from 0.18 – 0.20 and 0.09 – 0.27 respectively.

Since the heritabilities of the fertility traits studied were low, but nevertheless non-zero, the implications are that there is some genetic aspect which could be utilised in selection, and therefore be used in addition to the standard AFC and CI. Since the genetic correlation between the service-based fertility traits and milk yield was low, the observed relationship was unfavourable, suggesting that when selecting for improved fertility e.g. by fewer services per conception, milk yield will be compromised.

Conclusions: The results showed that cows with the poorest genetic potential for reproductive performance are those having the highest potential for milk production. Thus, although expecting slow progress, fertility and milk production should be selected for simultaneously in order for fertility to be maintained along with further genetic improvement in production.