| Summary: | A series of studies were conducted to investigate the effects of dietary energy and protein supply on behaviour and fertility in the modem high yielding dairy cow. The results demonstrate few direct dietary treatment effects on reproductive perform'ance. However, dietary strategies which alleviate severe negative energy balance in the early post partum period are beneficial to reproductive performance. A more positive energy balance in early lactation was associated with improved ovarian function, namely; earlier commencement to luteal activity, and improved luteal function (essential for embryonic development). In addition, there was a suggestion that a restricted energy allowance in the pre-calving period. could prove beneficial' to fertility in the post-calving period. Significant associations between average daily energy balance and a number of behavioural activities suggests that monitoring of animal behaviour may be a useful indicator of energy imbalance. In particular, standing ruminating in the two hour period post evening milking was strongly negatively correlated with daily energy balance. The expression of this particular behaviour is likely to be a response to stress (displacement behaviour). Oestrous behaviour appears to be unaffected by dietary protein level. The most influential factor on the expression of oestrus was the size of the sexually active group. Multiple cows cycling together appear to produce more intense oestrous cycles with an increased expression of standing immobile on being mounted. Despite being t~e most accurate oestrous behaviour, standing immobile on being mounted was only expressed in 52% of oestrous cycles, making it less useful in oestrous detection. The importance of secondary be~aviours, such as mounting or attempting to mount another cow has been highlighted. Mounting or attempting to mount another cow had a high rate of expression and, when expressed, a high proportion of animals were in oestrus. This behaviour was identified as being the most reliable sign of oestrus.
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