Model fitting, selection and evaluation of inoculum from slaughtered cattle for ruminant feed evaluation
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Whereas rumen liquor from fistulated cattle is considered the reference source of inoculum for feed evaluation using in vitro gas production (GP) techniques, its use is currently limited by high costs associated with surgical preparation and maintenance of fistulated cattle. Thus, two studies were conducted to evaluate rumen liquor from slaughtered cattle (RLS) as an alternative source of inoculum. Study I examined ability of 4 mathematical models; an exponential with discrete lag phase (EXPL), logistic (LOG), Groot’s (GRTS) and Gompertz (GOMP) models to fit and describe GP data of 4 diets varying in proportions of Chloris gayana hay and a protein concentrate incubated with RLS. Study II evaluated effect of rumen liquor source (i.e. fistulated steers vis-a-viz slaughtered cattle) on GP kinetics, partitioning factor (PF), efficiency of microbial biomass production, metabolizable energy and organic matter degradability as influenced by diet. In study I, model comparison was based on derived GP kinetics, graphical analysis and goodness-of-fit test statistics (i.e. root mean square error-RMSE, adjusted coefficient of determination-Adj-R2, and Akaike’s information criterion-AIC). All models exhibited a sigmoidal shape of the GP profiles. Asymptotic GP, half-life and fractional rate of GP differed (P<0.001) among the 4 models. The RMSE, Adj-R2 and AIC ranged between 1.555 to 4.429, 0.906 to 0.984 and 2.452 to 15.874, respectively, for all diets compared across the 4 models. Based on goodness-of-fit test criterion, GP profiles of diets were more appropriately fitted by GRTS and GOMP than EXPL and LOG models. Based on residual analysis, GRTS model had the lowest and most homogenous residual dispersion as a function of fermentation time for all diets and was thus selected as the most appropriate model that fitted and described GP data in study II. In study II, rumen liquor from fistulated steers and slaughtered cattle as sources of inoculum were evaluated using physico-chemical characteristics, GP kinetics and derived nutritional parameters of diets. The pH and ammonia nitrogen (NH3-N) values for both inoculum sources were within recommended mean buffer pH (6.8 to 7.2) and NH3-N range (50 to 235 mg/l) needed to provide suitable rumen environment for optimal microbial activity. Inoculum from slaughtered cattle produced the highest (P<0.001) asymptotic GP (49.6 ml) and maximum rate of GP (0.837 ml/l) irrespective of dietary substrate. Similarity (P>0.05) in PF values observed for both inoculum sources underscored the effectiveness of rumen liquor from slaughtered cattle as a source of inoculum. Therefore, rumen liquor from slaughtered cattle is as effective as the inoculum from fistulated steers for ruminant feed evaluation using the in vitro gas production technique.