RESPONSE TO DIVERGENT SELECTION FOR BLOAT SUSCEPTIBILITY IN DAIRY-COWS

An experiment was established with Friesian, Jersey, and Friesian X Jersey cross dairy cattle at Ruakura Agricultural Centre, Hamilton, New Zealand to select for high (HS) or low susceptibility (LS) to pasture bloat. Foundation cows were graded for bloat, assigned to herd according to grade, and mated to selected outside bulls to generate calves in 1973-76. Thereafter the herds were closed to the introduction of outside genes. This paper reports on genetic parameters and provides an update to the first nine calf crops previously summarised. From the 16 calf crops born in 1973-88, 718 animals were graded on a 0-4 scale when challenged with bloat. The paternal half-sib heritability of individual bloat grades at 6 months of age and the repeatability of grades between 6 and 24 months of age were obtained by restricted maximum likelihood techniques. The estimates were 0.12 +/- 0.06 and 0.13 +/- 0.02, respectively. A mean 6-month grade from an average of 4.7 records per animal had a heritability of 0.52 +/- 0.22. In the absence of a control herd, genetic responses from 1977 were evaluated by comparing the divergence between herds. For HS and LS calves born in 1973-76, the average difference in BLUP breeding value (BV) was 0.40 bloat grade units. Thereafter, the mean rate of divergence in adjusted mean bloat grade was 0.035 +/- 0.019 units/year (P < 0.10). The mean generation interval was 3.0 years. By 1988, the herds differed by 0.91 units, or 1.86 genetic standard deviations. Reciprocal crosses between the two herds, born in 1986-88, showed heterosis for bloat susceptibility in the direction of the LS herd mean. First-cross calves from LS dams were less susceptible than those from HS dams. Over all years combined, the two herds also differed by 11.3% in postcalving weight, with LS cows averaging 383 kg and HS cows 344 kg.