Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/4/2004
Publication Date: 5/10/2004
Citation: Stevens, D.R., Burns, J.C., Fisher, D.S., Eisemann, J.H. 2004. The influence of high nitrogen forages on the voluntary feed intake of sheep. Journal of Animal Science 82:1536-1542. Interpretive Summary: This study created a range of dietary N concentrations and blood plasma osmolalities spanning that of the normal foraging situation and animal physiological states. The data showed that the intake of forage is restricted by N concentration once it exceeds about 40 g kg with over 50% of the N available in the non-protein form. These levels of non-protein N are not common in forage so other factors such as rate of digestion, water-soluble carbohydrates and hedonistic properties should be the focus of future research concerning variations in forage intake. Forage managers, however, still need to consider factors such as nitrate poisoning and the metabolic cost of excreting surplus N when determining the effects of forage N concentrations on animals. Changes in blood plasma osmolality did not appear to be a physiological signal that controls voluntary feed intake under normal physiological conditions.
Technical Abstract: The objective of this research was to determine the importance of nitrogen fiber balance on the voluntary food intake of sheep fed high quality forages that varied in nitrogen (N) concentration and fiber degradability. Whether lambs (n=6/treatment) were fed dried switchgrass (Panicum virgatum L.) as a forage source of slowly degradable fiber (Exp. 1) or dried tall fescue (Festuca arundinacea Schreb.) as a forage source of more rapidly degradable fiber (Exp. 2). In both experiments urea was added to the dried forage at 0, 12 or 24 g N/kg DM to increase the rapidly available N concentration. Acid detergent fiber concentrations were 305 g/kg DM in both experiments, though DM digestibility was 663 and 618 g/kg DM in Exp. 1 and Exp. 2 respectively. Voluntary food intake of the control forage was 28.2 and 19.1 g/kg BW in Exp. 1 and Exp. 2 respectively, and declined for the high urea treatments to 25.2 and 16.2 g/kg 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 N/kg DM in Exp. 1 (P < 0.01) and from 28.4 to 55.9 g N/kg DM in Exp. 2 (P < 0.01). Non-protein N concentrations increased from 28.3 to 53.8% of the total N (Exp. 1; P < 0.01), and from 26.4 to 64.0% (Exp. 2; P < 0.01). Plasma urea concentrations 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 a plasma osmolality increase from 298 to 307 mOsmol/kg (Exp. 1; P = 0.04), and from 299 to 307 mOsmol/kg (Exp. 2; P = 0.06). Varying feed N concentration and the subsequent changes in blood urea and plasma osmolality caused a significant decline in the voluntary food intake of sheep. The response was more severe when forage digestibility was 618 g/kg DM (Exp. 2) compared to 667 g/kg DM (Exp 1.), and may be more important when forage N was greater than 40 g/kg DM and non-protein N was greater than 50% of total N. Key Words: Sheep, Forage, Fiber, Nitrogen, Urea, Intake.