This study was to determine the effects of nonfiber carbohydrate (NFC) sources and rumen degradable protein (RDP) levels in diet on apparent total-tract nutrient digestibility, manure production and nitrogen (N) excretion in dairy cows.
Eighteen mid-lactation multiparous Holstein cows were used in this split-plot study. Cows were randomly assigned to 9 or 11% RDP (16.5% crude protein (CP) for all diets, dry matter (DM) basis) as main plots and cows within each RDP level were further assigned to 3 NFC treatments, including (DM basis): 10% dextrose (D), 5% dextrose, 5% purified starch (DS), and 10% purified starch (S). Each period lasted 4 weeks and samples were collected during the fourth week.
There was no NFC x RDP interaction for response variables. Intake (kg/d) responded linearly for DM (24.3, 24.2, and 22.6), organic matter (22.7, 22.6, and 21.1), CP (4.0, 3.9, and 3.7), and neutral detergent fiber (7.0, 6.9, and 6.5) for D, DS and S, respectively (all P < 0.01). Same pattern was observed for the amount of nutrient digested, but apparent total-tract digestibility (%) was unaffected by dietary treatments, averaging 70.4, 71.8, 72.2, and 41.9 for DM, organic matter, CP and neutral detergent fiber, respectively. Fecal DM excretion (kg/d) was 7.2, 7.2, and 6.7 for D, DS, and S, respectively (P = 0.02), but urine volume (29.3 L/d) and urine N (254 g/d) were not affected. However, fecal N (180, 179, and 173 g/d) decreased, and N use efficiency (100x Milk N/N intake; 26.9, 27.4, and 29.3 %) increased linearly as starch in diet increased. There was no dietary effect on milk protein production (173 g/d). In this study, the substitution of starch for dextrose in the diet reduced DM and nutrient intake, did not alter digestibility, but reduced fecal DM and N excretions.
Future studies will further look into the effect of carbohydrate and nitrogen in diet on nutrient use efficiency, manure excretion, as well as methane emission in dairy cows.
Michel Wattiaux, Professor, Department of Dairy Science, University of Wisconsin-Madison
Fei Sun, PhD candidate, Department of Dairy Science, University of Wisconsin-Madison -Matias Aguerre, Assistant Professor, Department of Animal and Veterinary Sciences, Clemson University
Fei Sun is the presenting author of this presentation and can be contacted for questions regarding this study via firstname.lastname@example.org
This study was funded by the Dairy Coordinated Agricultural Project