Using primiparous autumn-lambing ewes, this study investigated nutritional effects on (i) pulsatile LH secretion, (ii) responsiveness of the pituitary-ovarian axis to exogenous GnRH, and (iii) oestrus and ovarian cyclicity during the early post-partum period. At lambing (25 October ± 0·3 days), 32 primiparous ewes rearing 2·1 ± 0·04 lambs were equally allocated to four dietary treatments in a 2 ✕ 2 factorial designed experiment. Diets comprised ad libitum hay and 1·5 kg per ewe per day of one of four concentrates (12·5 MJ metabolizable energy per kg dry matter) containing either fibrous (F) or starchy (S) ingredients with (P) or without (O) the inclusion of 120 g/kg fish meal. Throughout the study (lambing to 17 December) ewe body condition score was assessed and ewes and their lambs were weighed at weekly intervals. Blood samples were obtained from all ewes to measure circulating concentrations of progesterone and oestrous behaviour was monitored using vasectomized rams. On day 12 post partum, blood samples were obtained from all ewes at 2-h intervals (09:00 to 17:00 h) to measure concentrations of insulin and urea nitrogen, and at 15-min intervals (09:00 to 21:00 h) to measure pulsatile LH secretion. All ewes received 25 i. v. injections of GnRH (250 ng GnRH in 2 ml 0·9% saline) at 2-h intervals commencing 4 h before the end of the 15-min blood sampling period and their ovaries were examined via laparoscopy on day 17 post partum. There was no effect of dietary treatment on ewe live weight or body condition score throughout the study but inclusion of fish meal in the ewe diet increased lamb growth (FP/SP 255 (s.e. 8·9) v. FO/SO 234 (s.e. 8·2) g/day, P < 0·05). Circulating concentrations of insulin on day 12 post partum were lower in ewes given the fibrous compared with the starchy diets (P < 0·05) while dietary inclusion of fish meal increased (P < 0·001) urea nitrogen. Pulsatile LH secretion on day 12 post partum was not affected by dietary treatment. For ewes on diets F P, FO, SP and SO, the numbers that experienced an LH surge during the period of GnRH administration were 1, 2, 1 and 0, and that ovulated by day 17 post partum were 3, 5, 0 and 5 (FP/SP v. FO/SO, P < 0·05) respectively. Dietary treatment did not affect the intervals from parturition to the onset of ovarian cyclicity or oestrus (overall means were 23 (s.e.1·0) days and 38 (s.e.1·6) days respectively) but ewes on the fibrous compared with the starchy diet had a higher (P < 0·05) incidence of short (≤ 10 days) first ovarian cycles. Results of this study, which involved young growing animals, demonstrate that (i) inclusion of fish meal in the ewe diet influenced the sensitivity of the pituitary-ovarian axis to exogenous GnRH, and (ii) a fibrous compared with a starchy diet was associated with an increased incidence of premature luteal regression.

The objective of the present study was to evaluate the efficacy of a prostaglandin F2α (PGF2α) and progesterone (P4) treatment in initiating oestrus and ovulation post partum (PP) in Holstein-Friesian cows. Using four herds, the treatment protocol consisted of a single intra-muscular injection of PGF2α (Estrumate) between days 12 to 14 PP followed 48 h later by progesterone treatment via intra-vaginal CIDR insertion for a period of 7 days. Milk samples for progesterone determination were collected three times weekly from 7 to 65 days PP. The ovarian activity and reproductive performance of treated (T, no. = 153) animals and untreated control cows (C, no. = 315) were compared. Treatment was effective (P < 0·001) in reducing the mean interval to PP commencement of luteal activity from 29·62 (s.e. 0·82) days to 22·09 (s.e. 0·70) days. The mean interval to first PP oestrus in the T animals was significantly reduced (P < 0·001) from 55·62 (s.e. 1·58) days to 44·91 (s.e. 1·44) days and the incidence of silent ovulation in cycles between days 21 to 65 PP was reduced (P < 0·001) from 56·28% to 42·27%. In two herds under one management regime and with a similar block-calving pattern (no. = 280 animals), the treatment protocol was beneficial to the overall reproductive performance as there was a significant shortening in the mean interval to first PP service (75·82 (s.e. 1·93) v. 80·86 (s.e. 1·32) days) and in the interval to PP conception (83·07 (s.e. 2·49) v. 88·90 (s.e. 1·95) days), both P < 0·05.