REDUCING POSTPARTUM ANESTROUS INTERVAL IN FIRST-CALF BOS-INDICUS CROSSBRED BEEF HEIFERS .3. EFFECT OF NUTRITION ON RESPONSES TO WEANING ANDASSOCIATED VARIATION IN METABOLIC HORMONE LEVELS

Interactions between effects of undernutrition and weaning on the dura tion of post-partum anoestrus and associated variation in milk yield, suckling behaviour, and metabolic hormone levels in Droughtmaster [Bos indicus x Bos taurus (5/8, 3/8)] first-calf cows were studied in 2 su ccessive years. Low body condition score at calving (3.5+/-0.1, scale 1-9) was associated with prolonged post-partum anoestrous intervals in suckled cows, but provided cows were maintaining liveweight, ovarian cyclicity resumed within 50 days if calves were weaned 70 days post-pa rtum This response to weaning was totally inhibited in cows that calve d in low body condition that lost liveweight post-partum (Year 1). In contrast, marked levels of post-partum liveweight loss (23%) did not a ffect resumption of ovarian cyclicity in response to weaning (at 50 da ys post-partum), or time to conception, among cows that calved with hi gh body condition scores (5.8+/-0.2, Year 2). Reduced milk yields and calf weight gains among underfed cows were associated with higher suck ling frequencies and durations (P < 0.05). Acyclic interval after wean ing was negatively related to body condition score at weaning (P < 0.0 1), and positively related to preweaning suckling intensity (P < 0.01) . The time taken to resume ovarian cyclicity after weaning increased b y a factor of 1 2 for every half unit decrease in body condition score at weaning, and by a factor of 1.3 for every 30-min increase in time suckled per day prior to weaning. Within level of nutrition, higher fr equencies and durations of suckling were associated with higher plasma prolactin levels. Plasma IGF-1 levels were consistently depressed in underfed cows: whereas plasma insulin and GH levels were less affected by nutritional treatments. Effects of undernutrition in suckled cows appear to operate at least partly through interactions with milk yield and suckling intensity. Management of post-partum anoestrus in Bos in dicus cows should focus on the conservation of cow body condition and on the strategic use of early weaning.