Determination of Tannin levels in multi-purpose Kenyan Trees and fodder Crops, their variation and effect on Protein digestibility in ruminants
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Energy and protein are the major limiting nutrients in dairy production on the small scale mixed farms in Kenya. Commercial feed supplements are expensive and therefore multipurpose fodder trees (1-1PT) and forage crops are advocated. as the alternative supplements, because they are inexpensive, able to provide green forage even in dry season and have high protein content. These trees have tannins whose levels, seasonal and altitude distribution have not been established. Previous studies have indicated that tannins may have either beneficial effects like bloat control and increased protein bypass, or deleterious effects like the reduction feed intake and digestibility of protein in animals fed on tanniferous feed. The objectives of this study were to determine the tannin levels in the MPT, as influenced by altitude and season, and the effect of these tannins on ruminal degradation and intestinal digestion of the diet. Samples of four multipurpose fodder trees and four forage crops viz. leucaena, sesbania, gliricidia, calliandra, velvet bean, green leaf and silver leaf desmodium and cassava, were collected at Mombasa (low altitude, below 300 m ASL.) in the wet season and at Embu (high altitude, 1500 m ASL.) in both wet and dry season. Tannin and protein content were determined using gravimetric and wet oxidation nitrogen determination methods respectively. The effect of tannin on protein degradability was determined by comparing the polyethylene glycol (pEG) treated with untreated forage samples using the mobile nylon bag technique. The treated and untreated samples were incubated separately in the rumen of four Holstein cows with both rumen and duodenal cannula for 0, 6, 12, 24, 48 and 96 hours. Afterwards two sample of each species from time 12 and 24 hours were inserted into the intestine through duodenal cannula, and recovered from the feces. The dry matter (DM) and protein of samples recovered from the rumen and feces were determined and these values were fitted in a non linear regression equation P = a + b( 1-e-ct). 11 The results indicated that the MPT had ytterbium-precipitatable tannins ranging from 16.08 (±2.39)% of the DM in Gliricidia sepium to 30.31 (±2.42)% in Desmodium. intortum. The tannin content varied significantly (P<0.05) with species. The altitude did not have significant effect on tannin content, but tannins within species behaved differently with season. The protein content differed significantly (P<0.05) with species. Proteins were significantly (P<0.05) higher in wet than in the dry season. The altitude had no effect on protein content. The tannin : protein ratio also varied significantly (P<0.05) with species. Species also had a significant interaction with season. Tannins significantly (P<0.05) reduced the rumen effective degradability of both DM and crude protein in all species, resulting in large quantities of undegraded dietary nitrogen(N). Tannin also significantly (P<0.05) decreased the total tract digestion of the DM (DMD) and depressed the digestible crude protein (DCP) of leucaena, calliandra, cassava and D.intortum, but had no effect on gliricidia, sesbania and velvet bean DCP. A large proportion of the rumen undegraded dietary protein that reached the intestines was degraded in most species except calliandra. Tannin significantly (P<0.05) altered the degradability constants a, b, and c for both DM and CP by reducing fraction a and the rate of degradation c and increasing the b fraction. From the study it was concluded that the MPT and forage crop species, despite their high protein content, are not good protein supplements. Their tannins reduce both the CP digestibility and the DM degradability in the rumen. Lastly, potentially viable treatments that reduce the effect of tannin on digestibility are suggested.