Breed genotype effect on ewe traits during the pre-weaning period in hair sheep under the tropical Mexican conditions

• Genetic components on pre-weaning traits were estimated on 4 hair sheep breeds.
• Katahdin litters were heavier.
• Heterosis was only observed for litter weaning for the Pelibuey × Katahdin ewes.
• Non-genetic factors had a greater influence on the traits studied.

The present study was carried out on a commercial farm in southwest of Mexico to determine breed group effects of hair sheep (Pelibuey, Dorper, Katahdin, and Blackbelly and their crosses) and their genetic components (breed additive, breed maternal and heterosis effects on litter weight at birth, adjusted litter weaning weight at 60 days, and lambing interval. The dataset included records from 1821 four-purebred ewes and crossbreed groups (representing 9 different crosses between the four pure breeds). In total, there were 6371 lambing events from 2007 to 2013. The first model included the effects of breed, parity, litter size, season of lambing, year of lambing and season × year interaction. Those significant effect (P < 0.05) were included in the models to estimate the breed additive, breed maternal and heterosis effects. Breed additive and breed maternal effects were expressed as deviations from the Pelibuey breed. Breed, parity and litter size were significant sources of variation for all the traits (P < 0.05), but season and year interaction were significant only for litter birth and weaning weight (P < 0.05). Heterosis effect was only observed for adjusted litter weaning weight at 60 days of age for the Pelibuey × Katahdin crossbred ewes (P < 0.05). Under the conditions of this study, direct heterosis and maternal effects between the four breeds studied has shown no significant effect on these traits. Non-genetic factors had a greater influence on the traits studied than the genetic makeup of the ewe.