Journal of Dairy Science 84 (2001)suppl. 1. - ISSN 0022-0302 - p. 161 - 162.
ID Lelystad, Institute for Animal Science and Health
Refereed Article in a scientific journal
Although CP in the soluble fraction lost during washing of nylon bags is often assumed to be fully and rapidly degraded in the rumen, its actual fate in the rumen is questioned. We developed a filtration procedure that mimics machine washing of nylon bags and enables a further characterization of this soluble fraction. The filtrate of 1-3 g of DM of ten feeds was collected in 100 ml of rinsing water. Feeds were two grass silages (GS1, 25% of DM, and GS2, 53% of DM), wet brewers grain silage (WBGS) and corn gluten feed silage (CGFS), 3 types of corn gluten feed (CGF1 to 3), soybean meal (SBM), lupine meal (LPM) and rape seed meal (RSM). DM and N losses of the filtration procedure and machine washing were compared in quadruplet samples. DM losses during filtration and from nylon bags ranged from 16% in WBGS to 46%in GS1 and were not different between procedures. N recovered in the filtrate ranged from 11.5% for SBM to 56.1% for GS1. The filtrate was centrifuged at 1500 g for 20 min. The pellet was analyzed for N in non soluble particles (NS). Soluble true proteins (TP) in the supernatant were precipitated with TCA (10% in the final volume), and the N in the remaining supernatant was defined as non-protein N (NPN). The NS fraction comprised 0% (GS1 and 2) to 87% (WBGS) of N in the filtrate. Significant amounts of TP were found in SBM (53%), LPM (30%) and RSM (27%). NPN ranged from 13% (WBGS) to 100% (GS1 and 2) of N in the filtrate. The degradation of the protein N (NS +TP) in the filtrate of 6 feeds was measured in rumen fluid using a modified Broderick method. After 4h of incubation the percentages of degraded protein were: 0% (WBGS and CGFS), 24% (CGF 2), 55% (RSM), 79% (LPM)and 80% (SBM). These results show that CP in the soluble fraction, as determined in the nylon bag technique, consists of different protein and non-protein fractions, that are not always rapidly and fully degraded in the rumen. The potential underestimation of the protein value of these feeds, calculated as ((NS + TP)/ N feed) x %undegraded4h,is2- 7 % for the concentrates, and 13 % and 26 % for CGFS and WBGS, respectively
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