Availability to ruminants of nitrogen in senesced C4 tropical grasses

Abstract

Context: Nutritional standards usually assume that ∼10% of the total nitrogen (TN) in forages is indigestible and hence not available to the ruminant. Senesced tropical C4 grasses often contain TN concentrations that are marginal or deficient to meet the nutrient requirements of ruminants, and low TN availability will exacerbate N deficiencies.Aim: The aim of the study was to estimate the availability (i.e. digestibility) to ruminants of TN in mature and senesced C4 grasses, using data derived from previous experiments and published data.Methods: In Dataset 1, forages grown in subtropical or tropical environments, including C4 (n = 143) and C3 (n = 15) grasses, were analysed for parameters including TN, acid detergent fibre (ADF), and N insoluble in ADF solution (ADIN). ADIN was used as a measure of unavailable TN. The Dataset 2 analysis included published measurements of TN and ADIN in C4 (n = 187) and C3 (n = 45) grasses.Key results: In Dataset 1, TN averaged 9.7 and ADIN 1.45 g N/kg diet DM in C4 grasses. ADIN concentration could be predicted from TN and ADF concentrations by multiple regression (R2 = 0.50; P < 0.001). The ratio ADIN/TN averaged 170 g/kg and increased exponentially with increasing ADF concentration (R2 = 0.43; P < 0.001). Also, ADIN/TN was inversely related to TN concentration and DM digestibility. In C4 grasses containing >400 and >500 g ADF/kg DM, ADIN/TN averaged 190 and 230 g/kg, respectively, and in those containing <10 g TN/kg DM, ADIN/TN averaged 194 g/kg. In these low-quality C4 grasses only ∼80%, and as little as ∼50%, of TN was available to ruminants. Dataset 2 produced similar results, although C4 grass forages were generally of higher quality, most having been harvested during vegetative growth. ADIN/TN was much lower in C3 grasses (89 g/kg).Conclusions: In senesced, low-quality C4 grasses containing >400 g ADF, <10 g TN or <530 g digestible DM/kg, the proportion of TN available to ruminants is substantially lower than that assumed for forages in general.Implications: Low availability of TN in many senesced C4 grasses needs to be considered when evaluating the adequacy of dietary TN for ruminants grazing senesced tropical grass pastures.