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The Influence of Dietary Fiber Digestibility on the Nitrogen Requirements of Mature Horses^1,2,

University of Maryland, College Park 20742

Abstract

Mature geldings at maintenance were fed different diets in a 4 x 4 Latin square design balanced to account for residual effects in an attempt to determine whether differences in the digestibility of the fibrous portions of feedstuffs would influence dietary nitrogen (N) requirements. Diet 1 contained corn and soybean meal (SBM); diet 2, corn, corn oil and urea; diet 3, corn, SBM, straw and urea; diet 4, corn, alfalfa and urea. Urea supplied 50% of the total N in diets 2 and 3 and 39% of the total N in diet 4. The diets were fed in amounts that met National Research Council (NRC) recommendations for daily digestible energy intakes by mature horses at maintenance and met or exceeded total daily N requirements. True absorbed N was calculated by subtracting the fecal N associated with neutral detergent fiber (NDF-N) from total N intakes; true digestibilities of N ranged from 92.4 to 95.9%. Endogenous and metabolic fecal N excretions ranged from .37 to .56 g N/100 g dry matter intake. Although none of the diets as fed were deficient in N, apparent N digestibility was only 64% of N intake when the horses were fed the diet containing straw (diet 3), compared with 72.5 to 79.6% of total N intake among diets 1, 2 and 4 (P<.01). Fecal excretions of water soluble, bacterial cell-associated and intestinal cell-associated N fractions were greatest when diet 3 was fed. Diet 3 was the only treatment resulting in a significant negative N balance (–25.6% of apparent absorbed N). However, urinary N excretions, as percentages of true absorbed N, were not different. Therefore, the decreased efficiency in the utilization of dietary N caused by the feeding of diet 3 resulted from increased fecal excretions of water soluble, metabolic and endogenous N. These increases were associated with decreases in the digestibilities of NDF and hemicellulose, suggesting that decreased fermentability of the hemicellulose fraction of the straw-containing diet was responsible for the decreased microbial utilization of recycled urea and its metabolite, ammonia. These observations suggest that N intakes should be supplemented when mature horses are fed poorly fermentable feedstuffs.

¹ Scientific Article No. A-3235, Contribution No. 6306 of the Maryland Agr. Exp. Sta.

² Thanks are extended to Ms. Margaret Kempf for her efforts in the preparation of this manuscript. This work was supported in part by the New Jersey Agr. Exp. Sta.

³ Dept. of Anim. Sci.