Abstract
Ruminant feed evaluation systems use constant minimum requirements of rumen-degradable protein (RDP) and often relate this to apparently degradable organic matter (OM). However, studies with tropical forages indicate that RDP: apparently degraded OM might not be constant across high-fiber diets. This was tested with semi-continuous ruminal cultures (Rusitec) using dried contrasting low-protein fiber sources: brachiaria hay (high in fiber, medium lignified), apple pomace (medium in fiber, highly lignified), and sugar beet pulp (medium in fiber and lignification). Each feed was incubated at 14 g dry matter day−1 with 0, 0.85, 1.7, 3.4, 6.8, 13.6, or 27.2 mg g−1 urea. The amount of urea needed to reach a similar basal concentration of ammonia in the incubation fluid was tested for each feed in advance. Apparent fiber and OM degradability were determined after 48 h of incubation. Data was evaluated by regressions and analysis of variance. The response curve of incubation fluid ammonia to urea supplementation was similar in slope in all feeds. Plateaus in apparent OM degradability in relation to ammonia concentration were determined. The ammonia concentration where apparent OM and fiber degradability reached 95% of maximum was approached in the order of pomace < pulp < hay. With regard to fiber degradability, a plateau was reached at ≥ 80 g kg−1 crude protein only with hay and pomace, whilst a linear relationship existed between RDP and OM degradation for pulp. In hay the ratio RDP: OM degraded was equal to 1.6 but was only 1.0 in the other feeds. There was no obvious lack of branched short-chain fatty acids at low RDP. Thus, the hypothesis was confirmed but the demand for RDP seems even higher in tropical forage compared to food industrial byproducts. The efficiency of urea to promote apparent OM and fiber degradation was also variable. Thus, it seems that minimum thresholds of either RDP or ruminal ammonia concentration may not be reflected appropriately by constants.
Highlights
Various tropical grass species and harvest residues are very low in crude protein (CP) and RUMEN-DEGRADABLE PROTEIN (RDP) (e.g., Mathis et al, 2000; Schwab et al, 2005)
The three test feeds differed in fiber properties as intended, with a high fiber concentration found in the brachiaria hay, a high lignification in the apple pomace and medium values for fiber and lignification in the beet pulp (Table 1)
The RDP amount required per unit of Organic matter (OM) apparently degraded was highest (P < 0.05) with the brachiaria hay with values being similar with the two other feeds
Summary
Various tropical grass species and harvest residues are very low in crude protein (CP) and RDP (e.g., Mathis et al, 2000; Schwab et al, 2005). During the dry season, tropical forages have CP values lower than 70 g kg−1 dry matter (DM), which is considered limiting for the maintenance of an adequate ruminal fibrolytic activity (Lazzarini et al, 2009; Sampaio et al, 2009) Such sub-optimal ruminal conditions would be associated with low microbial growth and reduced fiber degradation (Sampaio et al, 2010). The high importance of RDP as nutrient for fiber degrading microbes in the rumen is known, and most feed evaluation systems for ruminants specify a minimum threshold for dietary RDP (60–80 g kg−1 DM) or CP (ranging from >120 to 130 to >160 to 180 g kg−1 DM) (reviewed by Schwab et al, 2005) Another approach for determining requirements for RDP is oriented toward minimum rumen fluid ammonia concentrations. An acceptable NDF digestibility of 59% was found for sheep consuming only brachiaria hay, a wide-spread cultivated tropical grass, with 61 g CP kg−1 DM; this, at a ruminal ammonia concentration of 3.9 mmol l−1 (Tiemann et al, 2008b)
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