Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/27/2003
Publication Date: 2/1/2004
Citation: FREETLY, H.C., LEYMASTER, K.A. RELATIONSHIP BETWEEN LITTER BIRTH WEIGHT AND LITTER SIZE IN SIX BREEDS OF SHEEP. JOURNAL OF ANIMAL SCIENCE. 2004. v. 82. p. 612-618. Interpretive Summary: Nutrient requirements during pregnancy are determined partially by the number of fetuses and the total weight of the fetuses. Current nutrient recommendations for pregnant ewes are based on a single breed type that infrequently has more than two lambs during each pregnancy. Current U.S. sheep productions systems have incorporated breeds of sheep that have large litter sizes, subsequently current nutrient recommendations fail to predict pregnant ewe requirements. This study established that in most breeds of sheep that litter weight does not increase proportional to increases litter size. The ratio of lamb weight over number of lambs decreases as litter size increases. The study also established that breed differed in how fast this ratio decreased. This study suggest nutrient recommendations for the pregnant ewe need to account for litter size as well as breed type. Development of robust predictors of nutrient requirements during pregnancy that can be used across breeds may require a mathematical model that describes the fundamental biology of pregnancy rather than the current method of predicting requirements as a function of the ewes body weight.
Technical Abstract: Metabolizable energy requirements of the ewe increase during pregnancy due to increases in fetal metabolism and maternal metabolism. Fetal metabolism is related to total weight of the fetuses. Fetal number is a primary contributor to fetal weight. Litter birth weight represents the culminated fetal growth of the litter and can be used to estimate the affect of fetal metabolism on energy requirement of the ewe. Based on the data in other species, we hypothesized that litter weight in sheep would increase curvilinearly at a decreasing rate with increasing litter size. Birth weights of lambs born to yearling (11 - 15 mo) and mature ewes (> 34 mo) were collected on litters born to Dorset, Rambouillet, Suffolk, Finnsheep, Romanov, and Composite III ewes mated to produce straight bred lambs. Litter birth weight expressed as a function of litter size increased curvilinearly at a decreasing rate for mature Rambouillet, Suffolk, Finnsheep, Romanov, and Composite III litters (P < 0.042). Breeds differed in the rate at which litter birth weight decreased with increased litter size. In yearlings, litter weight increased at a curvilinearly decreasing rate for Suffolk ewes (P = 0.002). The quadratic term for the relationship between litter weight and litter size did not differ from zero for Finnsheep (P = 0.39) or Romanov litters (P = 0.07). The hypothesis that litter weight will increase curvilinearly at a decreasing rate with increased litter size is supported by experimental results.