Submitted to: Animal Reproduction Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2009
Publication Date: 5/1/2010
Citation: Vallet, J.L., Miles, J.R., Brown Brandl, T.M., Nienaber, J.A. 2010. Proportion of the Litter Farrowed, Litter Size, and Progesterone and Estradiol Effects on Piglet Birth Intervals and Stillbirths. Animal Reproduction Science. 119(1-2):68-75.
Interpretive Summary: Mortality of piglets during the birth process ranges from 2 to 9% and represents a significant loss to the swine industry. Previous studies indicate a clear relationship between the time it takes to deliver each piglet and its risk of stillbirth (mortality). However, factors that influence the time it takes to deliver piglets are not fully known. We studied the effects of litter size, the relative position of the piglet in the farrowing process adjusted for litter size, and sow prefarrowing plasma steroid hormone concentrations on piglet birth intervals and the risk of stillbirth. Our results confirmed that prolonged birth intervals for individual piglets influence the risk of stillbirth for that piglet, but not for piglets born later in that same litter. However, paradoxically, the risk of stillbirth accumulates during the farrowing process. Average birth intervals were similar between piglets during most of the farrowing process but were significantly prolonged for the last piglet delivered in the litter. The prolonged birth interval of the last piglet in the litter was associated with a significantly increased risk of stillbirth for the last piglet in the litter compared to previous piglets. Average birth intervals decreased as litter size increased, suggesting that a factor produced by piglets or their placentas may influence the speed of farrowing. Average birth intervals and risk of stillbirth were not associated with prebirth plasma concentrations of progesterone or estradiol. Our results suggest that strategies to reduce the incidence of stillbirth should include hourly monitoring of the birth process as stillbirth increased dramatically when birth intervals exceeded one hour. In addition, attention to delivery of the last piglet in the litter could decrease the incidence of stillbirth in this position. Further studies focused on fetal/placental factors contributing to the speed of delivery of piglets are necessary to discover the factors controlling the relationship between litter size and average birth interval. These could be exploited to decrease farrowing intervals and stillbirth rate in pigs.
Technical Abstract: Stillbirth in swine ranges from 2 to 9%, resulting in a significant loss of piglets. Previous studies clearly indicate a relationship between prolonged birth intervals and stillbirth, but factors influencing birth intervals are not fully known. To characterize birth intervals and stillbirth, farrowing was recorded during 3 farrowing seasons. Blood samples were collected on d 110 and 113 of gestation, and were assayed for progesterone and estrogen. Relationships between estrumate (cloprostenol sodium, an analogue of prostaglandin F2') usage, litter size, proportion of the litter farrowed, progesterone and estrogen concentrations, birth intervals, and stillbirth were analyzed using regression analysis. A clear relationship between birth intervals and stillbirth was observed. Stillbirth rate was unaffected by birth intervals of < 1 h, and increased (P < 0.01) for birth intervals > 1 h. A significant negative association between litter size and birth intervals was observed (P < 0.01). Birth intervals were unaffected by proportion of the litter farrowed until the last piglet in the litter, whose birth interval increased dramatically (1.5 fold; P < 0.01). Stillbirth rates increased as proportion of the litter farrowed increased, and a dramatic increase in stillbirth occurred for the last piglet in the litter. Neither d 110 nor 113 plasma progesterone concentrations were associated with litter size, birth intervals, or stillbirth rates. Curvilinear relationships were present between d 110 or 113 plasma estradiol concentrations and litter size. However, neither d 110 nor 113 estradiol concentrations were associated with birth intervals or stillbirth rates. These results indicate that 1) birth intervals greater than 1 h are associated with increased stillbirth; 2) larger litter size reduces birth intervals; 3) the last piglet in the litter has both a prolonged birth interval and increased risk of stillbirth; 4) plasma progesterone before farrowing does not influence birth intervals or stillbirth; and 5) despite a relationship between litter size and plasma estradiol, plasma estradiol does not influence birth interval or stillbirth. An understanding of the effects of litter size and proportion of the litter farrowed on birth intervals might be exploited to decrease stillbirth in piglets.