title: The influence of high-nitrogen forages on the voluntary feed intake of sheep.
abstract: The objective of this research was to examine the effect of high concentrations of nonprotein nitrogen (NPN) on the voluntary food intake of sheep fed high-quality grasses. Wether lambs (n = 6 per treatment) were fed dried switchgrass (Panicum virgatum L.; Exp. 1) or dried tall fescue (Festuca arundinacea Schreb.; Exp. 2). In both experiments, urea was added to the dried forage at 0 (control), 12, or 24 g of N/kg of DM to increase the NPN concentration. Acid detergent fiber concentrations were 305 g/kg of DM in both experiments, although DM digestibility was 663 and 618 g/ kg of DM in Exp. 1 and Exp. 2, respectively. Voluntary feed intake of the control forage was 28.2 and 19.1 g/ kg of BW in Exp. 1 and Exp. 2, respectively, and decreased for the high-urea treatments to 25.2 and 16.2 g/kg of BW in Exp. 1 (P = 0.07) and Exp 2 (P = 0.03), respectively. Total feed N concentrations increased from 29.5 g to 45.7 g of N/kg of DM in Exp. 1 (P < 0.01) and from 28.4 to 55.9 g of N/kg of DM in Exp. 2 (P < 0.01). Nonprotein N concentrations increased from 28.3 to 53.8% of the total N in switchgrass diets (Exp. 1; P < 0.01), and from 26.4 to 64.0% in tall fescue diets (Exp. 2; P < 0.01). Plasma urea concentrations of the lambs increased from 3.1 to 6.6 mM (Exp. 1; P < 0.01) and from 2.9 to 5.8 mM (Exp. 2; P < 0.01) as the amount of urea added to the diets increased. These changes resulted in an increase in plasma osmolality from 298 to 307 mOsm/kg (Exp. 1; P = 0.04), and from 299 to 307 mOsm/kg (Exp. 2; P = 0.06). Increasing feed N and NPN concentrations through the addition of urea caused a significant decrease in the voluntary feed intake of sheep fed tall fescue and switchgrass. These responses showed no significant cause-and-effect relationship between voluntary feed intake, plasma urea concentrations, and plasma osmolality.
authors.affiliation: Department of Crop Science, North Carolina State University, Raleigh 27695, USA.
authors.forename: D R
authors.initials: DR
authors.lastname: Stevens
authors.forename: J C
authors.initials: JC
authors.lastname: Burns
authors.forename: D S
authors.initials: DS
authors.lastname: Fisher
authors.forename: J H
authors.initials: JH
authors.lastname: Eisemann
mesh_terms: D000821:Animal Feed; D000818:Animals; D003902:Detergents; D004043:Dietary Fiber / Q000008:administration & dosage / Q000378:metabolism; D004063:Digestion; D002149:Energy Intake / Q000187:drug effects*; D008297:Male; D009584:Nitrogen / Q000008:administration & dosage / Q000378:metabolism*; D009994:Osmolar Concentration; D011897:Random Allocation; D012756:Sheep / Q000378:metabolism*; D014508:Urea / Q000008:administration & dosage* / Q000097:blood
chemical_list: D003902:Detergents; D004043:Dietary Fiber; D014508:Urea; D009584:Nitrogen