DO WE PAY SUFFICIENT ATTENTION TO FERTILITY?

Selection in dairy cattle primarily emphasizes increasing milk yield and solids. This however may be negative to overall fitness, particularly fertility because of the antagonistic association between fertility and milk yield. Thus, although benefiting from yield, profitability may not improve because of deteriorating reproductive parameters such as increased inter-calf period, more AI services per conception, more days open and increased veterinary costs. The question addressed by the authors cited is what is the general picture in SA Holsteins with respect to phenotypic and genetic trends for fertility measures derived from AI service records, acknowledging that the sample analysed may not be sizeable enough to be representative of the SA Holstein breed countrywide.

The data consisted of artificial insemination records of about 64 500 Holstein heifers and cows, which were collected from 18 herds employing on-farm automated milk recording. The data were used to derive the following AI service-based fertility traits: age at first service (AFS), age at first calving (AFC), number of services per conception for heifers (SPCh), calving to first service (CFS), number of days open (DO), number of services per conception for cows (SPC), first service within 80 days post-calving (FS80d), whether cows were confirmed pregnant within 100 days post-calving (PC100d), and whether the cows were confirmed pregnant within 200 days post-calving (PC200d). Trends were determined for the period 1984 to 2011.

There was no clear trend for average breeding values for heifer traits. However, phenotypic trends for heifer traits showed a desirable downward trend, with a decrease of 0.14 and 0.13 months per year for AFS an AFC, respectively. Phenotypic trends for cow fertility traits appeared to increase at the rate of 0.16 and 0.83 days per year for CFS and DO, respectively, with cows requiring 0.02 more services per year for SPC. The average estimated breeding values for the traits FS80d, PC100d and PC200d showed no distinct trend.

It was concluded that, although the phenotypic fertility trends of heifers were favourable, this may not lead to an improvement in heifer fertility due to the overall undesirable genetic trends. The phenotypic improvement might have been from good management and environmental conditions. The 18 herds showed no genetic gains for fertility over the comparison period. Since heritability estimates indicate some genetic aspect that could facilitate improvement through genetic selection, this should be considered imperative for long-term benefits. The included fertility traits could be used in addition to AFC and inter-calving period to serve as basis for selection for reproduction to minimize selection bias and improve fertility in the SA Holstein population.