Reproductive Responses to Daily Injections with Porcine Somatotropin Before Mating in Gilts

Abstract

Abstract. Litter size and progeny birth weights are lower in gilts than in sows. Somatotropin (ST) is an important regulator of ovulation, fetal growth and survival. We therefore investigated effects of pST treatment of gilts for two to four weeks before mating on ovulation rate, behavioural estrus, fetal growth and survival, litter size and birth weights.In Experiment One, gilts were injected with 0, 30, 60 or 90 μg pST/kg/day for 14 days commencing 7 days after first estrus. Reproductive tracts were collected and corpora lutea and follicle numbers counted 5.5 days after second estrus. Ovulation rate (P=0.031) and number of medium-sized follicles (P=0.059) correlated positively with pST dose. In Experiment Two, gilts were injected with 0, 12.5, 25 or 50 μg pST/kg/day for 21 days from first estrus, and mated at second estrus. Numbers of corpora lutea, follicles and fetuses were counted at day 31 of pregnancy. Numbers of medium follicles and ovary weights were positively related to pST dose. In Experiment Three, 31 week old (1 st replicate) or 27 week old (2 nd replicate) gilts were injected daily with 0 or 12.5 μg pST/kg/day until mating 25.9 ± 0.6 days later, and delivered at term. Pre-mating pST increased total litter size in younger gilts in the 2 nd replicate only (P<0.05). In conclusion, injecting gilts with pST before mating does not consistently alter ovulation rate, increases the number of medium follicles available for recruitment at the second mating after treatment and increases subsequent litter size in younger gilts. Key words: Embryo growth, Growth hormone, Litter size, Ovulation rate, Pig (J. Reprod. Dev. 56: [540][541][542][543][544][545] 2010) eproductive performance is poor in primiparous pigs (gilts), compared with multiparous dams (sows). Litter size in the pig is lowest at their first parity, and then increases in either the second or third parity depending on breed [1][2][3]. The low litter size of gilts appears to be primarily due to low ovulation rates, with small effects of parity on embryo survival, and similar rates of still birth across parities [2,[4][5][6][7]. Birth weight is also lower in gilt than sow progeny [1,3], resulting in decreased survival and postnatal growth. Gilts can comprise 30-50% of modern commercial herds (B Luxford, personal communication), and increasing the litter size and/or birth weight of gilt progeny would therefore have a substantial impact on overall profitability. Somatotropin (ST) or growth hormone (GH) is an important regulator of ovulation in a range of mammalian species. In transgenic mice and sheep constitutively expressing ST ovulation rate is increased but fertility decreased, which is due to decreased mating rates in mice, and increased fetal loss in sheep [8][9][10]. Conversely, resistance to ST reduces litter size in the last week of pregnancy by 35-55% in GH receptor/GH binding protein knockout mice, largely due to reduced ovulation rate [11,12]. In transgenic pigs continuously expressing ST at relatively high levels, ovulation is completely...