Evaluation of Dorset, Finnsheep, Romanov, Texel, and Montadale breeds of sheep: V. Reproduction of F1 ewes in spring mating seasons.

Objectives were to estimate effects of sire breed (Dorset, Finnsheep, Romanov, Texel, and Montadale), dam breed [Composite III (CIII) and northwestern whiteface (WF)], mating season (March and May), and their interactions on reproductive traits of mature F1 ewes in spring mating seasons. A total of 1,099 F1 ewes produced 1,754 litters of 2,995 lambs from exposures to Suffolk rams during March and May mating seasons in 1995 through 1999. Fertility rate and ewe longevity were measured. Number born and litter birth weight were recorded, and number and weight at weaning and 20 wk of age were analyzed separately for dam- and nursery-reared litter mates. Total productivity from 4 to 6 yr of age for each ewe entering the breeding flock was calculated as the sum of 20-wk weights for dam-reared lambs and separately for nursery-reared lambs. Interactions of sire breed x mating season, ewe age x mating season, and ewe age x dam breed were often significant. Interactive effects of sire breed and mating season on fertility rate (P < 0.001) were primarily due to differences in magnitude. Fertility rates of sire breeds for March and May matings, respectively, were 92 and 89% for Romanov, 91 and 72% for Finnsheep, 90 and 52% for Texel, 88 and 52% for Montadale, and 83 and 62% for Dorset. Sire breed x mating season also affected number born (P < 0.03); March and May values were 2.12 and 2.05 for Romanov, 2.00 and 1.94 for Finnsheep, 1.39 and 1.41 for Texel, 1.37 and 1.51 for Montadale, and 1.37 and 1.55 for Dorset, respectively. Interaction of sire breed x dam breed on fertility rate (P < 0.01) was due to change in rank as well as magnitude. Romanov- and Dorset-sired ewes out of CIII dams had greater fertility rates than Romanov- and Dorset-sired ewes out of WF dams. The opposite situation existed for ewes by Finnsheep, Texel, and Montadale sires. Differences between dam breeds (CIII and WF) in total productivity of dam-reared lambs were not detected, whereas ewes exposed in March (78 kg) were more productive (P < 0.01) than those exposed in May (68 kg). Means of sire breeds for total productivity of dam-reared lambs were 47, 65, 70, 70, and 111 kg for Texel, Montadale, Dorset, Finnsheep, and Romanov, respectively (P < 0.001). Superior reproduction of Romanov sired ewes was primarily due to greater fertility rate and prolificacy at each mating season and ewe age. Use of Romanov-crossbred ewes would increase fertility during spring mating, an important constraint of the sheep industry.