Short-term effects of herbage allowance on milk production, milk composition and grazing time of cows grazing nitrogen-fertilized tropical grass pasture

Abstract

The short-term effects of varying the quantities of herbage allocated (15, 25, 35, 55 kg DM cow-1, day-1) to lactating Jersey cows strip grazing 3-week regrowths of nitrogen-fertilized Panicum maximum cv. Gatton panic pastures, upon the diet selected, milk production, milk and milk fat composition and grazing time, were measured. Sixteen cows were used in a latin-square design with experimental periods of 1 4 days. With increasing herbage allowance milk yields rose, averaging 8.9, 9.9, 10.4 and 10.6 kg milk cow-1, day-1 respectively. A lower solids-not-fat percentage in milk from cows receiving 15 kg DM cow-1, day-1 and a lower percentage protein from cows receiving 15 and 25 kg DM cow-1 day-1 indicated a lower intake of digestible energy by cows at these lower herbage allowances. Molar percentages of short-chain fatty acids in milkfat (C6, C8, C10, C14) increased linearly and oleic acid (C18:1) decreased linearly as herbage allowance increased, showing that a higher proportion of the milk fat was coming from energy in the feed and a smaller proportion from body reserves. Cows selectively grazed leaf from the uppermost layers of the swards. As herbage allowance was restricted, a higher proportion of pasture was eaten and this came mainly from pasture growing 10 cm or more above ground level. Total time spent grazing was similar for all herbage allowances but the proportion of night grazing was consistently lower at the higher intensities of grazing.