Breeding to Reduce Susceptibility to Bloat in Dairy Cattle

An experiment with Friesian x Jersey cross cattle was established in 1972 at Ruakura, in order to select in 2 closed herds for high (HS) or low (LS) susceptibility to pasture bloat. By the 1990 calf crop, there was a difference in adjusted bloat score between herds of 0.94 units (1.93 genetic standard deviations). The repeatability and heritability of a single bloat score were 0.13+0.02, and 0.12+0.06, with a heritability of 0.52+0.22 for mean bloat score. Annual death rates from bloat have averaged 2.6% in the HS herd and 0.8% in the LS herd (0% in the LS herd for the 1979-90 calf crops). Genetic markers were sought to distinguish between HS and. LS animals, using hypervariable probes on cattle born up to 1990. Some possible markers were identified. In an independent group of 35 HS and 17 LS calves born in 1991, one "LS marker" was present in 3 HS and 16 LS sample, while one "HS marker" was present in 14 and 1 samples, respectively. DNA samples from susceptible animals in unrelated industry herds are presently being analysed to test for consistency of these findings.