Effect of Protein Concentrat ion and Protein Source on the Degradabil i ty of Dry Mat ter and Protein In Situ

Degradability of DM and CP was determined in situ for barley, canola meal, corn gluten meal, barley silage, and four concentrate mixtures. Crude protein percent of concentrate mixtures 1 to 4, respectively, were barley plus canola 1, 15.1; barley plus canola 2, 21.0; barley plus corn gluten 1, 17.1; and barley plus corn gluten 2, 19.6. Degradability of each feedstuff was measured on each of four basal diets consisting of barley silage and one of the above concentrate mixtures fed in the proport ion 40:60 (DM). The resulting CP percent of diets containing concentrates 1 to 4 were 12.3, 16.1, 13.8, and 16.4. Effective degradabili ty for DM and CP, assuming a rumen outflow rate of .08 h -1 , were: barley 77.7 and 79.8; canola meal 60.9 and 66.6; corn gluten meal 18.9 and 11.0; barley silage 47.0 and 81.1; barley plus canola 1, 76.4 and 77.4; barley plus canola 2, 72.5 and 71.8; barley plus corn gluten 1, 72.8 and 55.8; barley plus corn gluten 2, 70.2 and 47.4. With two exceptions (DM degradabil i ty of concentrate 4 and CP degradabili ty of concentrate 2), basal diet had no significant effect on effective CP or effective DM degradability. Crude protein degradability of concentrate mixtures 1 to 4, respectively, as measured (77.4, 71.8, 55.8, and 47.4) was similar to degradabili ty estimated on the basis of their ingredient content (75.7, 72.1,49.6, and 43.0). Received December 19, 1986. Accepted April 27, 1987. 1 Present address: The Agricultural Institute, Moorepark Research Centre, Fermoy, Co. Cork, Ireland. INTRODUCTION Tradit ionally the protein requirements of ruminants have been described in terms of CP or digestible CP. It is now generally accepted that these systems do not describe accurately protein requirements. Their application, therefore, has undoubtedly resulted in ruminant animals receiving both inadequate and excessive protein. A new system has been proposed based on 1) the N requirements of the rumen microbes and 2) the N requirements of the host animal tissue. There are many variations of the system (1, 2, 4, 11, 29) but all have the same conceptual basis. Whereas some of these new systems were published up to 10 yr ago, their use in practice is still limited. One reason for this is the difficulty in accurately determining tureen degraded protein (RDP) and rumen undegraded dietary protein (UDP). Protein degradability in feedstuffs has been measured by a number of procedures such as laboratory solubility tests (5, 13, 25, 31), in situ nylon bag procedures (8, 16, 21, 22, 28, 30), and in vivo measurements using sheep, cattle, and cows cannulated in the rumen and duodenum (9, 18, 20, 27). The in situ technique had been used for many years to determine DM digestibility of forages (19, 26), but it was not until the 1970's that its applicat ion to measuring protein degradability was studied. Since then it has been widely used, and many of the factors that influence protein degradability, as measured by this method, have been investigated. Among the factors studied have been the ratio of sample weight to bag size (16, 21, 28), feedstuff particle size (7, 30), bag pore size (16, 21, 30), and roughage to concentrate ratio in the basal diet (7, 30). Less information is available on the effect of protein source or protein concentration in the basal diet (12, 15). The present experiment was undertaken to determine if the protein source, protein concentration, or protein degradability 1987 J Dairy Sci 70:1841--1849 1841 1842 MURPHY AND KENNELLY in the basal diet inf luenced in situ DM or CP degradabil i ty. A secondary object ive was to de termine whe the r degradabil i ty values determined, in situ, on individual ingredients of a concent ra te mix tu re could be used to predict accurately the degradabil i ty of the comple te concent ra te mixture . M A T E R I A L S A N D METHODS