The effects of restricted suckling and prepartum nutritional level on reproductive performance of primiparous crossbred beef cows : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Animal Science at Massey University

The objective of this experiment was to study the effect of suckling restriction by fitting nose plates to calves on the reproductive performance of first calving crossbred beef cows managed at two nutritional levels in mid-gestation. Reproductive activity was recorded in 21 two-year-old Hereford x Friesian and Simmental x Friesian primiparous cows before and after restriction of calf suckling. Cows were allocated two pastures allowances from day 112 to 212 of gestation so that the high nutrition (HN) group gained 0.75 kg/day, whereas the low nutrition (LN) group gained 0.17 kg/day during mid-gestation. Suckling was suppressed for 7 days by fitting nose plates to 46-day-old calves in the restricted suckling (RS) group, whereas the normal suckling (NS) group remained as a control. Ultrasound examinations and blood sampling were carried out at weekly intervals from day 18 and day 25 post partum, retrospectively, to evaluate ovarian activity and serum progesterone concentrations. Calf grazing and suckling activity was observed for 3 h per day on 2, 5 and 3 days before, during and after treatment, respectively. Dominant follicles (DF) in the ovaries occurred in all cows by day post partum and underwent a pattern of growth and atresia before first ovulation. Restricted suckling cows had a mean 18 days shorter interval from calving to conception than NS cows (76.0 [plus or minus] 5 vs 94.1 [plus or minus] 6 days, P [is less than] 0.05), whereas genotype and nutritional treatment had no effect on interval to conception. However, the beneficial effect of suckling restriction on reproductive performance was apparent only in those cows fed a high pasture allowance in mid-gestation and consequently calved in better condition than NS cows. Restricted suckling calves spent more time grazing during the period of attachment of nose plates and differences in behaviour remained over a further 5 days after nose plates were removed (62.4 [plus or minus] 7 vs 38.6 [plus or minus] 7 minutes spent grazing; P [is less than] 0.001), but were lighter at weaning (146.7 [plus or minus] 3 vs 162.4 [plus or minus] 3 kg; P [is less than] 0.01). Despite the lower calf liveweight at weaning it is concluded that manipulation of suckling through fitting nose plates to calves can be successfully used to shorten the interval from calving to conception in primiparous cows fed higher pastures allowances during mid-gestation and consequently calved in better body condition