Rambouillet and Romanov reciprocal breed effects on survival and growth traits of F1 lambs and on reproductive traits of F1 ewes.

Objectives were to estimate reciprocal effects of Romanov and Rambouillet breeds on survival, growth, and reproductive traits of F1 progeny and direct breed effects (Suffolk and Composite - ½ Columbia, ¼ Hampshire, and ¼ Suffolk) on survival and growth traits of the subsequent terminally sired lambs. Mature Rambouillet ewes (n = 243) were exposed to 20 Romanov rams over two seasons producing 621 lambs for evaluation of growth and survival traits with 274 F1 ewes being evaluated for reproduction traits through 4 yr of age. Similarly, mature Romanov ewes (n = 116) were exposed to 20 Rambouillet rams producing 601 lambs for evaluation of growth and survival traits with 176 F1 ewes being evaluated for reproduction traits through 4 yr of age. A total of 433 of those F1 ewes produced 3,431 lambs (1,552 litters) from 1,634 exposures to terminal sires over 4 yr. Terminal sires consisted of 38 Suffolk and 44 Composite rams. Reciprocal crossbred ewe lambs were produced from dramatically different uterine and neonatal environments, with litter size at birth from Romanov dams exceeding those from Rambouillet dams by 1.52 lambs (P < 0.001) and birth weight of lambs from Romanov dams averaged 3.41 kg compared with 4.26 kg from Rambouillet dams. Differences in BW were still evident at 140 d (P < 0.001) for dam-reared lambs. However, reciprocal ewe first breeding BW of both types were similar (P = 0.38). Minimal differences were observed in performance of reciprocal cross ewes through 4 yr for productivity, longevity, or progeny growth and survival. One exception was BW at 140 d where an interaction of dam breed with terminal sire breed reached significance for both dam-reared (P = 0.05) and nursery-reared (P = 0.02) lambs. This interaction was due to the lower weight of Composite-sired lambs out of reciprocal cross ewes born from Rambouillet dams. Composite rams increased number born (P < 0.01) and number weaned (P < 0.05) of the reciprocal cross ewes. Suffolk rams increased (P < 0.001) BW and growth rates from birth to 140 d of terminal progeny. Thus, there were little cumulative differences accrued over the 4 yr and no differences were detected for cumulative kilogram of lamb generated at 140 d per ewe exposed. The practical outcome of this evaluation was that performance levels of both types of Romanov crossbred ewes was similar allowing the industry to produce the desired crossbred ewes without needing large purebred ewe flocks of the less numerous Romanov breed.