Feeding level and dietary energy source have no effect on embryo survival in gilts, despite changes in systemic progesterone levels R. Z. Athorn A,E , P. Stott A , E. G. Bouwman B , A. C. Edwards C , M. A. Blackberry D , G. B. Martin D and P. Langendijk B A School of Animal and Veterinary Sciences, The University of Adelaide, Roseworthy Campus, Roseworthy, SA 5371, Australia. B South Australian Research and Development Institute, Roseworthy Campus, Roseworthy, SA 5371, Australia. C ACE Livestock Consulting Pty Ltd, PO Box 108, Cockatoo Valley, SA 5351, Australia. D UWA Institute of Agriculture, University of Western Australia, Crawley, WA 6009, Australia. E Corresponding author. Email: rathorn@rivalea.com.au Abstract. This study was designed to assess the effect of feeding level and dietary energy source on luteal function, systemic progesterone concentration and embryo survival in gilts during early gestation. At Day 0 of pregnancy, 104 gilts were allocated to one of four experimental diets (LStarch: 1.2 · maintenance requirement (M) Starch diet (43.3% starch), n = 31; HStarch: 2.4 · M Starch diet (43.3% starch), n = 21; HFat: 2.4 · M Fat diet (13.5% fat), n = 23; and HFibre: 2.4 · M Fibre diet (7.2% fibre), n = 23). On Day 5 of gestation, no significant difference in circulating concentration of systemic progesterone was seen among the treatments. However, on Day 15 of pregnancy, gilts on the HStarch diet had a significantly lower concentration of systemic progesterone than did gilts on both the LStarch and HFat diets (P < 0.05; 24.8 Æ 2.4 v. 32.7 Æ 2.4 and 36.1 Æ 2.1 ng/mL, respectively). At Day 35 of gestation, there was also a tendency for gilts on the HStarch and HFat diets to have a higher total luteal weight than for gilts on the LStarch diets (7.2 Æ 0.2 and 7.1 Æ 0.2 v. 6.7 Æ 0.2 g (P < 0.05)). No difference in embryo survival was seen among the treatments. From the present study, we can conclude that altering feeding level and dietary energy source did not affect embryo survival, despite the fact that systemic progesterone concentrations were affected on Day 15 of gestation. Also, luteal weight was greater for those gilts on the high feeding level than for those on the low feeding level when fed the same energy source. Received 5 January 2012, accepted 14 June 2012, published online 23 October 2012 Introduction In pigs, embryonic mortality can reach up to 40%, severely reducing the potential litter size and, ultimately, the number of pigs weaned per sow per year (Pope 1994; Kemp et al. 2006). Progesterone is known to be an important driver of endometrial function and, as such, is important for early embryo development, implantation and survival of embryos (Foxcroft 1997; van den Brand et al. 2000; Spencer et al. 2004). High feeding levels have been shown to reduce systemic progesterone concentrations during early pregnancy (Prime and Symonds 1993), and therefore have been linked to a decrease in embryo survival in gilts (Dyck and Strain 1983; Jindal et al. 1996). However, the effects of feeding level on embryo survival have been equivocal, with other studies finding no (Pharazyn et al. 1991a; Quesnel et al. 2010) or even positive (Virolainen et al. 2004) effects of an increased feeding level on embryo survival and pregnancy rate (Dyck and Strain 1983; Virolainen et al. 2004). This paradox may be due to the fact that these studies focussed on the relationship between embryo survival and progesterone concentrations in the systemic blood circulation, rather than concentrations in the blood supply to the uterus. The supply to the uterus is not synonymous with systemic supply, because a ‘local’ supply of progesterone directly from the ovary to the uterus, via counter current exchange and venous–arterial anastomoses, occurs (Krzymowski et al. 1990). Therefore, while a high feed intake decreases systemic concentrations of progesterone, it may actually increase the local supply of progesterone to the uterus via an increase in the secretion of progesterone by the ovaries. Therefore, the amount of progesterone that is supplied to the uterus, and the effect of feeding level on this supply, will ultimately depend on the balance between the systemic clearance of progesterone and the ovarian production of progesterone. Even though a high feeding level increases the rate of metabolism of progesterone by the liver (Prime and Symonds 1993), there are indications that a high nutritional status may increase progesterone secretion by luteal cells. Insulin-like growth factor 1 (IGF-1) increases with a high feeding level, especially so in starch-rich diets, and it has been suggested that it may influence early luteal function and thus CSIRO PUBLISHING Animal Production Science, 2013, 53, 30–37 http://dx.doi.org/10.1071/AN12004 Journal compilation Ó CSIRO 2013 www.publish.csiro.au/journals/an