Sheep & Goats

Rams Matter: the ram effect on ewe fertility

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Abstract

Ewe fecundity and the associated fertility outcome are one of the main determinants of flock productivity and thus profitability in sheep meat production systems. Although the genetic influence of the ewe on her litter size has been studied extensively, what is less well understood is the role of the service ram in determining litter size. The objectives of this study were to investigate the effect of service ram on ewe litter size. Litter size records were available from 12,704 ewes naturally mated to 1,902 service rams across 17,409 mating events from 307 crossbred flocks. Litter size was defined as the number of lambs born per litter; only litter sizes between one (singles) and four (quadruplets) were considered. Variance components for litter size were estimated using a linear animal model containing the fixed effects of contemporary group (flock-week of mating), the interaction between ewe parity and age at first lambing, heterosis and recombination loss coefficients of the ewe and the resultant litter, ewe breed covariates of the eight most prominent breeds; service ram breed and age of the service ram were also included as fixed effects. An additive ewe genetic, permanent ewe environmental and a genetic service ram effect were all included as random effects. Across the population the average litter size was 1.79 (SD = 0.69), ewes mated to younger service rams (4 years). Smaller litter sizes were associated with ewes mated to Vendeen (1.40 ± 0.19) rams; the largest litter sizes were progeny of Belclare (1.93 ± 0.06) and Suffolk (1.95 ± 0.06) rams (P< 0.05). The coefficient of genetic variation for litter size attributed to the ewe and service ram were 11.42% and 4.30%, respectively. The direct ewe heritability for litter size was 0.10 (0.01), the service ram heritability was 0.02 (0.004). A moderate positive genetic correlation was estimated between the ewe and service ram effect (0.49) for litter size. A total of 260 mass sperm motility scores, assessed manually by artificial insemination operators, were available on 137 service rams, no phenotypic litter size data associated with these service rams was included in the genetic analysis. The mean ram litter size estimated breeding values (EBV) for the service rams ranked in the top 20 percentile on sperm motility scores was 2 genetic standard deviations greater compared to rams ranked in the bottom 20 percentile (P< 0.001). Results from this study demonstrate that genetic variation exists among service rams for litter size, this coupled with clear differences in sperm motility based on ram litter size EBV indicate that the potential exists to improve litter size through direct service ram selection.

Keywords: 2026

How to Cite:

McHugh, N., Pabiou, T., McGovern, F., Donovan, A. & Berry, D., (2026) “Rams Matter: the ram effect on ewe fertility”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2248869. doi: https://doi.org/10.31274/wcgalp.23383

Rights: 1