REPRODUCTIVE-PERFORMANCE OF F1 CROSSBRED DAIRY-COWS USED FOR DRAFT - EFFECT OF WORK AND DIET SUPPLEMENTATION

Forty pregnant F1 crossbred cows (20 Holstein-Friesian X Boran and 20 Simmental X Boran) were assigned to a 2 X 2 (work X diet) factorial ex periment as follows: supplemented-non-working (SNW), supplemented-work ing (SW), non-supplemented-non-working (NSNW) and non-supplemented-wor king (NSW). Working cows pulled sledges 100 days/year (pull = 350 to 4 50 N, 4 h/day, 4 days/week). Conception and oestrus at fixed times (20 0 and 365 days post partum) were analysed using linear logistic models . Proportional hazard models were used for analysing 'failure' time da ta such as time to first oestrus or time to conception. Diet supplemen tation significantly decreased days to first oestrus and days to conce ption in non-working and working cows. SW cows had similar reproductiv e performance to NSNW cows. In supplemented cows, work significantly d elayed days to conception. However, by 365 days post partum, conceptio n rate was similar for SNW and SW cows. Body condition at calving sign ificantly affected post-partum reproductive ability of non-working and working cows. Natural grass hay alone could not support potential rep roductive ability of crossbred cows. Work output of supplemented cows may be associated with longer calving intervals. The economic trade-of fs between longer calving intervals and work output should be examined in detail.