Effect of feeding diets with different type of carbohydrates on dry matter intake, rumen fermentation, and productivity of lactating dairy cows
The objective of this study was to investigate the effect of feeding different types of carbohydrates on DMI, rumen fermentation, and milk production of lactating dairy cows. Our hypotheses were that both high sugar and high starch diets will decrease rumen pH compared to a basal diet, but a high sugar diet has higher DMI and milk fat yield than a high starch diet. Six ruminally cannulated peak-lactating dairy cows (DIM = 75 ± 12.2; BW = 630 ± 59.2 kg) were used in a replicated 3 × 3 Latin square design with 21-d periods. Cows were fed diets consisting of 35.5% barley silage and 64.5% concentrate mix on a DM basis. Control diet (CON) contained 27% starch, 4% sugar and 28% NDF, high starch diet (STA) supplemented with additional steam-rolled barley grain contained 32% starch, 4% sugar and 26% NDF, while high sugar diet (SUG) supplemented with sucrose contained 27% starch, 9% sugar and 26% NDF. All diets were formulated to contain 17% crude protein. Although DMI was not different among these three diets, mean rumen pH of STA and SUG diets was lower than CON diet (6.29 and 6.23 vs. 6.38; P < 0.01). However, there was no significant difference between STA and SUG diets. In addition, duration of rumen pH < 5.8 was not different between STA and SUG diets, but it tended to be longer for SUG diet than CON diet (117 vs. 30.1 min/d; P = 0.08). Concentrations of total VFA and NH3-N in rumen fluid were not different among the treatments. However, compared with CON, STA and SUG diets had lower acetate proportion (62.0 vs. 59.6 and 59.4 mol / 100 mol; P = 0.01), but there was no difference between STA and SUG diets. Although DMI, milk yield, milk component yields and milk fat concentration did not differ among the treatments, concentrations of milk CP (3.14 vs. 3.08 and 2.97%; P < 0.01) and MUN (16.2 vs. 13.3 and 14.9 mg/dL; P< 0.01) were higher for SUG diet compared with STA and CON diets. These results suggested that feeding high sugar and high starch diets to lactating dairy cows might decrease rumen pH without affecting DMI or milk fat yield, but that high sugar diet may increase milk CP content.
Keywords: sugar, starch, milk production