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Unformatted text preview: J. Dairy Sci. 84:17071716 American Dairy Science Association, 2001. Influence of Carbohydrate Source and Buffer on Rumen Fermentation Characteristics, Milk Yield, and Milk Composition in Late-Lactation Holstein Cows G. R. Khorasani and J. J. Kennelly Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB Canada T6G 2P5 ABSTRACT The effects of concentrate-to-forage ratio and buffer on rumen fermentation and production parameters were examined in four rumen-cannulated cows (240 18 d in milk) fed a total mixed ration ad libitum in a 4 4 Latin square design. The treatments were a 50:50 concentrate to forage ratio with [1.2% of dry matter, (DM)] and without (0% of DM) buffer and a 75:25 con- centrate to forage ratio with (1.2% of DM) and without (0% of DM) buffer. Rumen pH declined in response to increased concentrate but was not influenced by buffer. In the absence of the buffer, rumen acetate declined and propionate was elevated at the higher level of con- centrate inclusion. The milk fat concentration was lower for cows fed the high concentrate diet without buffer; however, the addition of buffer to the diet pre- vented the milk fat depression. Milk fat depression was associated with elevated trans-C 18:1 fatty acids in milk, which provides additional support for an inhibitory ef- fect of these fatty acids on mammary fat synthesis. We concluded that the potential of nutrition as a tool to alter milk composition is greater in later lactation as these animals are better able to cope with the negative effects of high grain diets, and the treatment response is greater than in early lactation. ( Key words: lactating cows performance, forage inclu- sion, buffer, rumen digestion) Abbreviation key: (A + B)/P = acetate + butyrate to propionate, A:P = acetate to propionate, FA = fatty acids, HCD = high-concentrate diet, HCNB = concen- trate to forage 75:25 without buffer, HCWB = concen- trate to forage 75:25 with buffer, MCD = medium-con- centrate diet, MCNB = concentrate to forage 50:50 without buffer, MCWB = concentrate to forage 50:50 with buffer. Received November 6, 2000. Accepted February 14, 2001. Corresponding author: G. R. Korasani; e-mail: reza.khorasani@ ualberta.ca. 1707 INTRODUCTION A high level of concentrate fed in early lactation can lower milk fat and raise milk protein concentration, effects that may be modulated by adding buffer (Ken- nelly et al., 1999; Xu et al., 1994). Kennelly et al. (1999) concluded that high concentrate levels and buffers exert their effects by influencing rumen fermentation pat- terns. Feeding high concentrate diets caused a substan- tial increase in milk trans-C 18:1 fatty acids ( FA ), pre- sumably as a result of elevated rumen levels of these FA associated with changes in the extent of rumen bihydrogenation of dietary polyunsaturated FA. Ken- nelly et al. (1999) also reported that the addition of buffer to the diet prevented the elevation of trans-C 18:1 FA in milk fat and related milk fat depression associ-...
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