This study assessed the effect of feeding level on progesterone concentration in the caudal vena cava during early pregnancy in gilts. Twenty-four Landrace gilts were allocated to either a high (2.8±0.02) or a low (1.5±0.01 kg day⁻¹) feeding level at Day 0 of pregnancy. Serial blood samples were collected every 15 min for 3 h before and 3 h after feeding on Days 6 and 9 of pregnancy. Embryo survival and development as well as in vitro luteal progesterone production were assessed at Day 10 of pregnancy. Progesterone concentration in the vena cava was pulsatile with gilts on the high feeding level having more pulses compared with Low gilts on Day 9 of pregnancy (P<0.05). On Day 6 the number of pulses did not differ significantly between treatments; however, the average progesterone concentration in the vena cava tended to be higher in the gilts on the high feeding level (P<0.10). Embryo survival at Day 10 was 92±3% for High gilts compared with 77±3% for Low gilts (P<0.05). No difference in embryo development between the treatments was seen. There was no difference between treatments in in vitro secretion of progesterone by luteal tissue. In conclusion, a high plane of nutrition positively affects progesterone secretion by the ovaries in early pregnancy.

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This study employed a unilateral ovariectomy model to investigate the relevance of the local supply of progesterone (ovary) compared with the systemic supply of progesterone, in terms of embryo survival in the ipsilateral uterine horn as opposed to the contralateral uterine horn. Thirty gilts were unilaterally ovariectomised (ULO) during the luteal stage of their first oestrous cycle. Half of the ULO gilts were fed at 1.2 maintenance requirement (M), while the other half were fed at 2.4M. Across ULO gilts 0.8 more embryos survived in the ipsilateral horn compared with the contralateral horn at Day 35 of gestation (P<0.05). In ULO gilts on the 2.4M feed level the difference (+1.3; P<0.05) between the ipsi- and contralateral horn was more pronounced than on the 1.2M feed level (+0.4; NS). The higher feed level reduced circulating levels of systemic progesterone on Day 5 of pregnancy but not embryo survival at Day 35. However, post-implantation embryo survival was lower on the low feed level. In conclusion, these data indicate that local progesterone supply from the ovaries to the uterus contributes to the probability of embryo survival.

This study assessed carry-over effects of energy level during the early antral phase and subsequent follicular phase on follicle recruitment and ovulation rate. Gilts (n=45) were fed a standard diet to a low (L, ~1.2kg day(-1)) or high (H, ~2.7kg day(-1)) level during the early antral (luteal) phase, and subsequently fed a H or L feed level during the follicular phase, resulting in four treatment groups (HH, HL, LH and LL). Follicle size at the end of the luteal phase was greater for gilts fed a high feed level previously (3.3vs3.0mm; P<0.05). During the follicular phase, high feeding increased follicle size at Day 5 (6.9vs6.2mm; P<0.005) and plasma oestradiol concentration (P<0.05). Nevertheless, a low feed level during the luteal phase reduced ovulation rate (14.4vs13.2; P<0.05) and embryo number (12.6vs10.5; P<0.05), and this was not counteracted by feed level during the follicular phase. Plasma progesterone concentration after ovulation was lower for LL gilts than for other treatments (P<0.05). These results indicate that undernutrition during early antral follicle development may have a residual effect on follicle recruitment and quality.

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 ± 2.4 v. 32.7 ± 2.4 and 36.1 ± 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 ± 0.2 and 7.1 ± 0.2 v. 6.7 ± 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.