Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg, VA, 24061, USA.
Department of Animal Science, Faculty of Agriculture, Shahid Bahonar University of Kerman, 76169-133, Kerman, Iran.
Genet Sel Evol. 2020 Oct 2;52(1):56. doi: 10.1186/s12711-020-00577-z.
Seasonal reproduction limits productivity, flexibility, and profitability in commercial sheep production. Hormonal and (or) photoperiodic manipulation can be used to control estrous cycles in sheep and reduce limitations that are imposed by the seasonal anestrous but are often impractical or incompatible with the extensive management systems preferred for ruminant livestock. Thus, the current study investigated the use of selection to improve realized fertility (i.e., the proportion of ewes that lambed) following an out-of-season spring joining period (May and June) in a crossbred sheep population.
Over 17 years, estimated breeding values (EBV) for fertility in selected (S) ewes increased by 0.175 (0.01 per year). The mean EBV for fertility of S ewes was greater than that of control ewes by year 10 (P = 0.02), and the fertility of adult (≥ 3 years old) ewes reached 0.88 ± 0.05 by year 17. Lambing began approximately 140 days after the introduction of rams, and 64% of the S ewes that lambed did so in the first 17 days of the potential lambing season, which indicated that most of the S ewes were cycling at the time of ram introduction and were not induced to cycle by the introduction of breeding males (i.e., the so-called "ram effect"). Animals in the S line had modest increases in body weight and scrotal circumference. A modest negative trend in the additive maternal effect on birth weight was observed but was reversed by additional selection on EBV for maternal birth weight. The heritability of litter size in autumn lambing was low (0.04) and could potentially limit the response to selection for this trait.
Selection improved realized ewe fertility in out-of-season mating, with absolute increases of approximately 1% per year in the percentage of joined ewes that lambed in the autumn. Genetic antagonisms with other performance traits were generally small. A modest antagonism with maternal breeding values for birth weight was observed but it could be accommodated by selection on EBV for maternal birth weight. Our results support results of previous studies that indicate that these selected ewes had one of the shortest seasonal anestrous periods reported for temperate sheep breeds and that spring-lambing lactating ewes from the selection line were capable of relatively rapid rebreeding in the spring.
季节性繁殖限制了商业绵羊生产的生产力、灵活性和盈利能力。激素和（或）光照周期可以用来控制绵羊的发情周期，减少由季节性乏情引起的限制，但这种方法往往不切实际或与反刍动物家畜首选的广泛管理系统不兼容。因此，本研究调查了在杂交绵羊群体中，在非季节性春季配种期（5 月和 6 月）之后，通过选择来提高实际繁殖力（即产羔母羊的比例）的应用。
在 17 年的时间里，选择（S）母羊的繁殖力估计育种值（EBV）增加了 0.175（每年 0.01）。第 10 年时，S 母羊的 EBV 均值大于对照组母羊（P=0.02），第 17 年时，成年（≥3 岁）母羊的繁殖力达到 0.88±0.05。配种后大约 140 天开始产羔，64%的 S 母羊在潜在产羔季节的前 17 天产羔，这表明大多数 S 母羊在配种公羊引入时就处于发情周期，而不是通过引入繁殖雄性动物来诱导发情周期（即所谓的“公羊效应”）。S 系动物的体重和阴囊周长略有增加。观察到出生体重的母体加性效应呈适度负趋势，但通过对母体出生体重 EBV 进行额外选择，这种趋势得到了逆转。秋季产羔时，产羔数的遗传力较低（0.04），这可能限制了对该性状的选择反应。
选择提高了非季节性配种的母羊实际繁殖力，每年秋季产羔的母羊比例增加约 1%。与其他性能性状的遗传拮抗作用通常较小。观察到与出生体重的母体繁殖值存在适度的拮抗作用，但可以通过对母体出生体重 EBV 进行选择来适应。我们的研究结果支持之前的研究结果，表明这些选择的母羊具有温带绵羊品种中报道的最短季节性乏情期之一，并且选择系的春季产羔泌乳母羊能够在春季快速重新配种。
