title: The effect of starch-, fiber-, or sugar-based supplements on nitrogen utilization in grazing dairy cows.
abstract: Nitrogen utilization in grazing cows is often low due to high concentrations of rapidly soluble and degradable protein in the pasture-based diet. Broadly, opportunities to improve N utilization lie in either reducing the amount of N consumed by the animal, or incorporating more N into milk protein. The goal of this study was to compare the relative importance of dietary N intake and productive N output for improving N utilization in grazing cows fed either starch-, fiber-, or sugar-based supplements. Also, the Cornell Net Carbohydrate and Protein System (CNCPS; Cornell University, Ithaca, NY) was evaluated as a tool to assess cow performance and improve N utilization in pasture-based systems. Eighty-five cows were randomly assigned to 1 of 5 treatments at parturition (17 cows per treatment). Treatments consisted of a pasture-only control and pasture with a starch- (St and StN), fiber- (FbN), or a sugar-based supplement. The StN and FbN treatments contained additional dietary N. Diets were formulated using the CNCPS to supply similar levels of dietary metabolizable energy, but differing levels of dietary N and metabolizable protein. Nitrogen utilization ranged from 22 to 26% across the 5 groups. Cows fed the St diet had the lowest levels of milk urea N, blood urea N, and urinary N excretion and had the highest productive N output (149 g/d). Cows fed the FbN treatment had similar productive N output (137 g/d) and consumed approximately 100g/d more dietary N than the St treatment, resulting in greater urinary N excretion. Although milk protein yield was moderately greater in the St treatment, quantitatively the difference in N intake (100g/d) had the greatest effect on N utilization and suggests that controlling dietary N intake should be the first priority when attempting to improve N utilization in grazing cows. No effect was observed of supplementing pasture-fed cattle with sugar on production or N utilization under the conditions of this experiment. Predictions of metabolizable energy and protein availability for milk yield from the CNCPS were similar to actual milk yield for all treatments. Model-predicted N utilization and excretion reflected the trends observed in the measured data and suggests that the CNCPS can be a useful tool for formulating and evaluating diets to improve N utilization in pasture-based systems.
authors.affiliation: Department of Animal Science, Cornell University, Ithaca, NY 14853, USA. rjh257@cornell.edu
authors.forename: R J
authors.initials: RJ
authors.lastname: Higgs
authors.forename: A J
authors.initials: AJ
authors.lastname: Sheahan
authors.forename: K
authors.initials: K
authors.lastname: Mandok
authors.forename: M E
authors.initials: ME
authors.lastname: Van Amburgh
authors.forename: J R
authors.initials: JR
authors.lastname: Roche
mesh_terms: D000821:Animal Feed; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D002417:Cattle / Q000378:metabolism*; D003612:Dairying; D004043:Dietary Fiber / Q000008:administration & dosage*; D019422:Dietary Sucrose / Q000008:administration & dosage*; D019587:Dietary Supplements; D005260:Female; D060434:Herbivory / Q000502:physiology*; D009584:Nitrogen / Q000378:metabolism*; D013213:Starch / Q000008:administration & dosage*
chemical_list: D004043:Dietary Fiber; D019422:Dietary Sucrose; D013213:Starch; D009584:Nitrogen