Factors Affecting Blood Urea Nitrogen and Its use as an Index of the Nutritional Status of Sheep

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Abstract
Three experiments were conducted to study factors affecting blood urea nitrogen (BUN) and the use of BUN as an index of the nutritional status of ewes. In experiment 1,315 ewes of varying ages grazing on mature annual grass were flushed by supplementing daily with 0, 90, 180, 270, 360 and 450 g pelleted alfalfa hay just prior to and during the breeding season. Lambing percentage (lambs born × 100/ewe present at lambing) increased from 98% (no supplement) to 130% when 360 g pelleted alfalfa were consumed (P< .10). Both BUN (P< .05) and weight gain (P< .05) increased during the flushing period when alfalfa was fed. Simple correlation coefficients (r) were: supplemental N intake vs. BUN, 0.99; BUN vs. weight gain, 0.95; BUN vs. lambing percent, 0.95; supplemental N intake vs. gain, 0.95; and weight gain vs. lambing percent, 0.97. In experiment 1, 315 ewes of varying ages grazing on mature annual grass were flushed by supplementing daily with 0, 90, 180, 270, 360 and 450 g pelleted alfalfa hay just prior to and during the breeding season. Lambing percentage (lambs born x 100/ewe present at lambing) increased from 98% (no supplement) -to 130% when 360 g pelleted alfalfa were consumed (P < .10). Both BUN (P <.05) and weight gain (P < .05) increased during the flushing period when alfalfa was fed. Simple correlation coefficients (r) were: supplemental N intake vs. BUN, 0.99; BUN vs. weight gain, 0.95; BUN vs. lambing percent, 0.95; supplemental N intake vs. gain, 0.95; and weight gain vs. lambing percent, 0.97. In experiment 2, blood samples from 10 each of lambs, yearlings and ewes were taken five times during a day to determine the effect of animal age, time of day, and variation in BUN levels among animals. Neither age nor sampling time had any significant effect on BUN level but variation between animals within age class was significant (P < .01). The minimum number of samples necessary for 95% of the values to be within ± 10% of the sample mean was calculated to be 10. In experiment 3, 19 to 25 ewes from each of 26 commercial flocks throughout northern California were sampled for BUN prior to breeding. Since the standard deviations were rather constant irrespective of BUN level, the minimum sample size necessary for 95% of the values to be within t 10% of the sample mean varied between flocks with regression of Y = 31.79 -.95X (r = -.761) where Y is the sample " size and X is BUN value. Copyright © 1974. American Society of Animal Science . Copyright 1974 by American Society of Animal Science.

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