During the sheep breeding season, food supply declines rapidly and rams lose their appetite and spend less time feeding. This combination of factors reduces testicular mass as mating progresses [1, 2]. In addition to the loss of spermatogenic tissue, the number of sperm produced per gram of tissue is reduced, suggesting acceleration of apoptosis [3]. We therefore tested whether underfed rams ejaculate damaged spermatozoa, exacerbating any reduction in fertility caused by reduced sperm output.

Farm Experiment: Two groups of 20 7-month-old rams were fed either a ‘low diet’ (low-quality dry pasture) or a ‘high diet’ (the same pasture plus ad-lib access to concentrate). After 6 weeks, both groups had gained body weight and testicular size but differed in body mass by 25% and in scrotal circumference by 6%. In late summer, the rams were placed with ewe flocks (3 rams per 100 ewes) for 5 weeks, on intermediate feed allowance. By the end of the 5-week mating, ‘high diet’ rams had lost 3% of their body mass and ‘low diet’ rams had gained 10%. Both groups lost about 9% of their scrotal circumference. Pregnancy rate at ultrasound scanning was similar (>95%) for the two groups.

Laboratory Experiment: Two groups of 5 mature rams were fed for 84 days with either a High-energy diet that increased scrotal circumference by 11% or a Low-energy diet that decreased it by 17%. Ejaculate volume, sperm concentration and numbers of sperm per ejaculate were not affected by treatment but, at the end of the treatment period, percentage live sperm was 72 ± 3% with the High-energy diet and 55 ± 2% with the Low-energy diet (P < 0.001).

We conclude that undernutrition can reduce the quality of ejaculated spermatozoa, but the effect of this outcome on flock fertility remains to be determined.