Ram mating performance in Border Leicesters and related breed types. 3. Relationships of ram serving capacity, testis diameter, liveweight, breed and age with flock fertility

Abstract

Relationships of ram serving capacity (a measure of serving frequency in pen tests), testis diameter, liveweight, breed and age with flock fertility were studied in 40 rams joined individually for 7 days at high ewe:ram ratios. Rams were either Border Leicester, BLI (a high prolificacy line derived from the Border Leicester and Merino and since released as the Glen Vale breed) or BLI x Border Leicester, and aged either 1.5 or 2.5 years. Joinings were over 2 periods, 2-9 April or 16-23 April. Breed, age, period and interaction effects were also examined. On average (± s.e.), the number of ewes with mating marks in 7 days increased by 2.03 ± 0.52 ( P < 0.001) per serve per hour of serving capacity and by 0.40 ± 0.18 ( P < 0.05) per kg liveweight. Relationships with pregnancy rate in marked ewes differed between classes of rams. Larger testes were of most benefit when rams were of high serving capacity ( P < 0.0 1 ) or low liveweight (P<0.01). Only serving capacity was related to number of ewes pregnant, the number increasing by an average (± s.e.) of 1.73 ± 0.57 ewes per serve per hour (P<0.01). This suggests that a ram's capacity for more frequent service was more important to fertility than was extra testicular tissue. BLI x Border Leicester, BLI, and 2.5-year-old Border Leicester rams impregnated 30-40% more ewes than did 1.5-year-old Border Leicester rams. We estimate that the 1.5-year-old Border Leicester rams could be satisfactorily joined to about 46 ewes and the other rams to about 60 ewes.