Postpartum anoestrum in the pasture grazed New Zealand dairy cow : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy, Department of Veterinary Clinical Sciences, Massey University
Postpartum anoestrum (PPA) is that period following parturition before ovulatory ovarian cycles have been re-established. This period lasts 20 to 30 days in normal, well-fed dairy cattle. To achieve an average interval between calvings of 365 days so that calving continues to occur at an appropriate time of the year, cows must resume cyclic activity, display behavioural oestrus, be mated and conceive by 83 days postpartum. An extended period of PPA compromises achievement of this target. Extended periods of PPA may result from either a failure to resume ovulations (anovulatory anoestrus) or a failure of expression or detection of behavioural oestrus (non-detection of oestrus). The study population for this thesis was drawn from the research herds of the Dairying Research Corporation, Hamilton and from commercial herds from the Waikato region around Hamilton. All herds calved seasonally between July and September and were milked twice daily. First calving occurred at approximately 2 years of age (i.e. heifers) and thereafter at 365 d intervals. Nutrition was predominantly from ryegrass/white clover pasture. The 10 year average rainfall of the distribution 1230 mm with higher rainfall in spring than summer. Average daily temperature ranges from a low of 8-9°C in July to a maximum of 18.3°C in January. It was demonstrated that primiparous cattle had a longer PPA (40.2 vs. 27.2 ± 6.2 days for 2 year old and older cows, mean ± SED, respectively) and that Friesians had longer PPA intervals than Jerseys (39.3 ± 3.1 vs. 27.9 ± 2.7 days, respectively). Increasing the stocking rate resulted in an increased PPA interval (30.2 ± 2.8 vs. 27.1 ± 2.9 days, for high and low stocking rates, respectively). Body condition score (1 = thin, 10 = fat) at calving was inversely related to the PPA interval (regression slope = -7.9 days, P<0.05). Cows that had not commenced cycling 1 week before the planned start of mating (PSM) had lower condition scores (-0.3 ± 0.1), smaller total ovarian mass (-1.3 ± 0.2, arbitrary units), higher serum urea concentrations (0.31 ± 0.16 mmol/L) and lower blood glucose concentrations (-0.14 ± 0.06 mmol/L) than cows which had ovulated by this time. Significant differences in the proportion of cows not detected in oestrus and anovulatory anoestrus were demonstrated among the 8 farms studied. These data indicate that age, breed, stocking rate, body condition score and between farm factors influenced the PPA interval. Large ovarian follicles (>9 mm) were present by 10.3 (± 0.7) days postpartum and regular turnover of follicles occurred in the ovaries of all cows examined by daily transrectal ultrasound. An average of 4.2 (± 0.6) large follicles appeared before the first ovulation which occurred at 43.4 (± 5.3) days postpartum. The largest follicle in anovulatory cows had lower intrafollicular concentrations of oestradiol (E2), testosterone (T) and progesterone (P4) than in cycling cows when ovariectomy occurred at approximately 60 days postpartum (47 vs. 372 ± 2.1 ng/ml; 1.4 vs. 10.0 ± 2.3 ng/ml, and 7.8 vs. 16.0 ± 1.8 ng/ml for E2, T and P4, respectively). However, there were no differences in the diameters, the number of granulosa cells or the rates of growth of the largest follicles between the anovulatory and cycling cows. A luteinising hormone (LH) surge and ovulation was induced in 10 of 10 and 9 of 10 anovulatory heifers, respectively, following treatment with 250 μg of gonadotrophin releasing hormone (GnRH) when the largest follicle was >10 mm in diameter and growing, at 3 to 4 weeks postpartum. Sufficient GnRH receptors and releasable LH must have been present in the pituitary and the largest follicle must have been sufficiently mature to ovulate in response to the LH surge. However, only 3 of 9 ovulating heifers continued to ovulate beyond the first, short (<10 day) cycle. Oestradiol treatment (0.5 mg i.m.) of PPA cows when either a small, growing follicle (5 to 9 mm) or a large plateau follicle (>10 and ± 1 mm for 72 h) was present on the ovary resulted in 8 of 15 and 5 of 15 cows having an LH surge and ovulating, respectively. This illustrates that the E2 positive feedback mechanism, a prerequisite for ovulation in a normal oestrous cycle, failed in nearly half of these PPA cows. PPA cows had a lower LH pulse frequency and a higher LH pulse amplitude but similar mean LH concentration before and 3 and 10 days after ovariectomy compared to cyclic cows when ovariectomy occurred approximately 60 days postpartum. The LH parameters increased by a similar amount in the PPA and the cycling cows following ovariectomy. Exogenous E2 treatment at 10 days post-ovariectomy resulted in a significant decrease in LH pulse frequency and an increase in LH pulse amplitude in the PPA but not the cycling cows. The GnRH pulse generator in the PPA cows appears to be suppressed by both ovarian and extra-ovarian factors. Additionally, hypersensitivity to negative E2 feedback on LH pulse frequency was observed. Undernutrition and low body condition score have been hypothesised as contributing to increased negative E2 feedback in cattle. Treatment of anovulatory cows with P4 for 5 days at 2 to 3 weeks postpartum resulted in a significant shortening of the intervals from calving to first ovulation, calving to first oestrus and calving to conception (30.7 ± 0.4 vs. 34.2 ± 1.0, 35.8 ± 2.6 vs. 40.0 ± 1.8 and 85.0 ± 3.0 vs. 93.4 ± 2.3 days, respectively, P<0.05) when compared with herdmates. Progesterone treatment produced a 'priming' effect, as the duration of the first postpartum luteal phase (9.5 ± 0.4 vs. 5.6 ± 0.9 days) and the proportion of cows detected in oestrus at the first postpartum ovulation (83.3 vs. 37.0, P<0.05) were both increased. Treatment of anovulatory cows for 7 days with P4 and 400 i.u. of equine chorionic gonadotrophin (eCG) increased the probability of first service and conception occurring, compared to untreated cows. Low body condition score at the time of treatment reduced the probability of first service and conception, but the increase in probability of first service or conception following P4 and eCG treatment was the same among cows with either low or medium body condition score. Supplementation of a white clover/ryegrass pasture diet with pasture silage did not alter the intervals from calving to first ovulation, calving to first oestrus or calving to conception when compared with control cows fed pasture only. Silage supplementation did reduce first service conception rate (37.5% vs. 53.3%, P = 0.09). The proportion of cows not in oestrus by the date of the planned start of mating varied among herds possibly due to differences in the age structure, breed and nutritional management. Further research is required to identify management and animal factors associated with an unacceptably high proportion of the herd not detected in oestrus by this date. Failure of the E2 positive feedback mechanism, low LH pulse frequency and low intrafollicular steroid concentrations were identified in PPA cows. Increased sensitivity of the E2 negative feedback mechanisms due to depleted body fat reserves and/or poor postpartum nutrition associated with prolonged periods of negative energy balance postpartum may be the major mechanism for extended PPA. An understanding of the control of GnRH and LH release from the hypothalamus and pituitary respectively, will be required before the patho-physiology of PPA can be fully understood. Treatment of anovulatory cows either early (2 to 3 weeks) postpartum or immediately before the planned start of mating shortened PPA intervals. The mechanisms appear to involve a 'priming' effect on expression of behavioural oestrus and on the length of the first luteal phase. This thesis increases the understanding of the factors that influence the PPA interval, the endocrinology of PPA and the treatment of PPA cows.