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.

中文翻译：

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对绵羊秋季羔羊选择的直接和相关反应

季节性繁殖限制了绵羊商业化生产的生产力，灵活性和盈利能力。激素和（或）光周期操作可用于控制绵羊的发情周期，减少季节性发情所施加的限制，但通常不可行或与反刍动物优选的广泛管理系统不兼容。因此，当前的研究调查了杂交绵羊种群在春季反季节之后（5月和6月）使用选择来提高已实现的生育能力（即羊羔的羊羔比例）的情况。在过去的17年中，选定（S）母羊的育性估计育种值（EBV）增加了0.175（每年0.01）。到第10年，母羊生育能力的平均EBV高于对照母羊的EBV（P = 0.02），到17岁时，成年（≥3岁）母羊的受精率达到0.88±0.05。羔羊在引入公羊后约140天开始产羔，而有羔羊的64％的母羊在潜在的头17天开始产羔。羔羊季节，这表明大多数羊在引入公羊时都在循环，而不会因引入育种雄性而诱导循环（即所谓的“公羊效应”）。S系动物的体重和阴囊周长适度增加。观察到增加的孕产妇对出生体重的负面影响适度的趋势，但通过对孕产妇出生体重进行EBV的其他选择而被逆转。秋季羔羊产仔数的遗传力很低（0.04），并可能限制对选择该性状的反应。选择提高了非常规交配中的已实现母羊繁殖能力，秋季接合的母羊所占的百分比每年绝对增加约1％。具有其他表现特征的遗传拮抗作用通常很小。观察到适度的拮抗作用，与母体繁殖体重有关，但可以通过选择EBV来适应母体出生体重。我们的结果支持以前的研究结果，这些结果表明，这些选出的母羊是温带绵羊品种中报告的最短的季节性发情期之一，并且来自选育系的春季lam哺乳母羊能够在春季进行相对较快的繁殖。具有其他表现特征的遗传拮抗作用通常很小。观察到适度的拮抗作用，与母体繁殖体重有关，但可以通过选择EBV来适应母体出生体重。我们的结果支持以前的研究结果，这些结果表明，这些选出的母羊是温带绵羊品种中报告的最短的季节性发情期之一，并且来自选育系的春季lam哺乳母羊能够在春季进行相对较快的繁殖。具有其他表现特征的遗传拮抗作用通常很小。观察到适度的拮抗作用，与母体繁殖体重有关，但可以通过选择EBV来适应母体出生体重。我们的结果支持以前的研究结果，这些结果表明，这些选出的母羊是温带绵羊品种中报告的最短的季节性发情期之一，并且来自选育系的春季lam哺乳母羊能够在春季进行相对较快的繁殖。