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ABSTRACT
The objective of the study was to evaluate the effect of energy sources on milk production and reproduction of dairy cows on a pasture-based feeding system. Following parturition, 201 Holstein cows from the Elsenburg herd were fed ad libitum cultivated irrigated kikuyu-ryegrass pastures for 22 weeks. Cows were divided into three groups according to concentrate supplementation, of which the energy was provided by starch and/or fat. These were grouped according to parity, body live weight and milk yield of previous lactation. The control consisted of low levels of standard supplement while treatments consisted of supplements of high starch-low fat (HSLF) and high starch-low fat low starch-high fat (HSLF-LSHF) given to cows on pastures. Treatment P-HSLF was a glucogenic regime made of maize. Treatment P-HSLF-LSHF was a high starch/high fat combination treatment, whereby a glucogenic diet was offered for the first 60 days of lactation as per treatment P-HSLF and followed from 61 to 154 days by a lipogenic (low starch-high fat) diet made of wheat bran and calcium salts of palm fatty acids. Cows were milked twice a day and concentrate supplementations were fed after each milking. Milk yields and body live weight of dairy cows were affected (P < 0.05) by the concentrate levels while milk composition was not affected. The number of services per conception (SPC) and the number of days from calving to first service (CFS) did not differ (P > 0.05) between treatments for both primiparous and multiparous cows. However, the pregnancy rate (PR) of cows at 150 days was affected (P < 0.10) by the feeding programme, with P-HSLF and P-HSLF-LSHF being higher compared to the control. This evidenced that manipulating levels could have influenced the hypothalamic-pituitary-ovarian axis through improved energy balance (EB) and optimize the carry over effects into fertility. Since studies of feeding different energy sources on reproductive performance are still scarce and inconsistent, further investigations with large numbers of animals and emphasis on EB and metabolism are therefore needed to assess the effects of types of energy sources and to understand factors affecting reproduction in dairy cows.