Skin folds and Merino breeding. 4. The susceptibility of rams selected for a high degree of skin wrinkle to heat induced infertility

Abstract

Three experiments were done to establish whether differences in ram fertility could explain some of the differences in reproductive performance observed between two flocks of Merino sheep, one flock of which had been bred for a high (Folds Plus) and the other for a low (Folds Minus) degree of skin wrinkle at the Trangie Agricultural Research Station. In the first experiment the quality of semen collected from Folds Plus and Folds Minus rams at Glenfield, New South Wales, was examined during the period December 1962 until March 1963. From late January until early March 1963, Folds Plus rams had low quality semen which gave a conception rate of 38.6 per cent. Over the same period, Folds Minus rams had higher quality semen which gave a conception rate of 58.6 per cent. Folds Plus and Folds Minus rams were then subjected to a severe heat treatment in mid winter 1963 (experiment 2) which produced severe seminal degeneration in both Folds Plus and Folds Minus rams. Folds Plus rams were more susceptible to heat than Folds Minus rams since the semen of Folds Plus rams remained degenerate for a longer period than the semen of Folds Minus rams and, in a test mating program, fewer pregnancies were observed in ewes artificially inseminated with the semen of Folds Plus rams than in ewes inseminated with the semen of Folds Minus rams. In a third experiment Folds Plus and Folds Minus rams from two independently established selection experiments at Trangie and at Cunnamulla were subjected to a heat treatment. The heated Folds Plus rams from both selection experiments exhibited marked seminal degeneration, but heated Folds Minus rams from both selection experiments maintained high quality semen throughout the experiment. The heated Folds Plus rams from Cunnamulla were more severely affected than the heated Folds Plus rams from Trangie.