The relationships between body weight, scrotal characteristics and environmental effects (year, birth type and age) and fertility were studied in a total of 254 Horro rams. Fertility varied between years (P < 0·01), but not with body weight at mating within the range (30–55 kg) observed, nor with type of birth (both P > 0·05).
Testicular diameter and circumference were strongly correlated to body weight (r = 0·95 and 0·89 respectively; both P < 0·001), but weight of testes only weakly so (r = 0–28; NS). Scrotal circumference was more highly correlated to body weight in ram lambs (r = 0–85; P < 0–001) than in adults (r = 0·51; p = 0·06).
It is concluded that adequate levels of fertility (82 ± 1·3%) are expressed by 30 kg Horro rams, and that when feed supplies are limited there are no benefits to be gained from maintaining rams of this breed at higher weights.