title: Nutrient demand interacts with forage family to affect nitrogen digestion and utilization responses in dairy cows.
abstract: The effect of preliminary feed intake on responses to diets containing alfalfa silage or orchardgrass silage was evaluated using 8 ruminally and duodenally cannulated Holstein cows in a crossover design experiment with a 14-d preliminary period and two 15-d treatment periods. Responses measured were intake, digestion, and utilization of N. Cows were 139 +/- 83 (mean +/- standard deviation) days in milk at the beginning of the preliminary period. During the 14-d preliminary period, 3.5% fat-corrected milk yield ranged from 23.9 to 47.6 kg/d (mean = 36.9 kg/d) and preliminary voluntary dry matter intake (pVDMI) ranged from 14.2 to 21.3 kg/d (mean = 18.6 kg/d). Treatments were a diet with alfalfa silage as the sole forage (AL) and a diet with orchardgrass silage as the sole forage (OG). Alfalfa silage contained 20.5% crude protein (CP; dry matter basis) and orchardgrass silage contained 20.4% CP; AL contained 18.3% CP and 5.6% estimated rumen-undegraded CP, and OG contained 18.8% CP and 6.3% estimated rumen-undegraded CP. Mean N intake was similar between treatments, ruminal N digestibility was greater for AL (30.4%) than for OG (17.7%), and whole-tract N digestibility did not differ between treatments. Intake and duodenal flow of N depended on a treatment x pVDMI interaction; both N intake and duodenal flow increased more for AL than for OG as pVDMI increased. Duodenal flow of microbial N and the efficiency of microbial N production from OM also depended on a treatment x pVDMI interaction in a manner similar to N intake and duodenal flow. However, treatment x pVDMI interactions also indicate that as pVDMI increased and N intake increased for AL compared with OG, a decreasing proportion of the additional N consumed from AL was digested and used for increased milk protein production or body tissue gain. Therefore, when feeding less-filling diets, such as those containing large proportions of legume forage, to high-producing cows, reducing dietary N concentration could increase the efficiency of N utilization and reduce the extent to which greater DMI leads to greater N excretion.
authors.affiliation: Department of Animal Science, Michigan State University, East Lansing, MI 48824, USA.
authors.forename: J A
authors.initials: JA
authors.lastname: Voelker Linton
authors.forename: M S
authors.initials: MS
authors.lastname: Allen
mesh_terms: D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D002417:Cattle / Q000378:metabolism / Q000502:physiology*; D018592:Cross-Over Studies; D003612:Dairying; D004032:Diet / Q000662:veterinary*; D004435:Eating / Q000502:physiology; D005260:Female; D007774:Lactation / Q000502:physiology; D009584:Nitrogen / Q000378:metabolism*
chemical_list: D009584:Nitrogen