Genetics of susceptibility to bloat in cattle

Abstract

A herd of identical twin cows was graded for degree of bloat when grazing red clover pasture in the spring and autumn of 1972–73 and 1973–74. Sources of variation in the resulting bloat grades were analysed. Observer differences or interactions were very small. The main sources of variation were between grading times (30%) and between cows (20%). Afternoon gradings were on average higher than morning gradings. About 50% of the cow variation was between twin pairs, suggesting a heritable component. An interaction occurred between cows and grading period in that some cows varied more between grading periods than others. This variability was not related to mean grade. Cows highly susceptible to bloat showed a straight line. relationship with the whole herd mean (as a measure of pasture bloat potency), and cows of extremely low susceptibility had a curve asymptotic to a maximum grade of about 1.5.