Influence of main dietary chemical constituents on the in vitro gas and methane production in diets for dairy cows

Background： Modification of chemical composition of diets fed to dairy cows might be a good strategy to reduce methane（CH4） production in the rumen. Notable reductions of CH4 production compared to conventional highroughages rations were more frequently observed for very concentrated diets or when fat supplements were used. In these cases, the reduction in the gas emission was mainly a consequence of an overall impairment of rumen function with a reduction of fiber digestibility. These strategies do not always comply with feeding standards used in intensive dairy farms and they are usually not applied owing to the risks of negative health and economic consequences.Thus, the present study evaluated the effects of seven commercial diets with contents of neutral detergent fiber（NDF）,protein and lipids ranging 325 to 435 g/kg DM, 115 to 194 g/kg DM, and 26 to 61 g/kg DM, respectively, on in vitro degradability, gas（GP）, and CH4 production.Results： In this experiment, changes in the dietary content of NDF, crude protein（CP） and lipids were always obtained at the expense or in favor of starch. A decreased of the dietary NDF content increased NDF（NDFd） and true DM（TDMd） degradability, and increased CH4 production per g of incubated DM（P 〈 0.001）, but not that per g of TDMd. An increase of the dietary CP level did not change in vitro NDFd and TDMd, decreased GP per g of incubated DM（P 〈 0.001）, but CH4 production per g of TDMd was not affected. An increased dietary lipid content reduced NDFd, TDMd,and GP per g of incubated DM, but it had no consequence on CH4 production per g of TDMd.Conclusions： It was concluded that, under commercial conditions, changes in dietary composition would produce small or negligible alterations of CH4 production per unit of TDMd, but greater differences in GP and CH4 production would be expected when these amounts are expressed per unit of DM intake. The use of TDMd as a standardizing parameter is proposed to account for possible difference in DM intake and productivity.

Rumen liquor from slaughtered cattle as inoculum for feed evaluation

Use of nonlinear mathematical models has been majorly based on in vitro gas production(GP) data generated when substrates are incubated with rumen liquor from fistulated steers. However,existing evidence suggests that rumen liquor from slaughtered cattle of unknown dietary history also generates quantifiable in vitro GP data. Fitting and description of GP data obtained from 4 diets incubated with rumen liquor from slaughtered cattle was evaluated using single-pool exponential model with discrete lag time(EXPL), logistic(LOG), Groot's(GRTS) and Gompertz(GOMP) models. Diets were formulated by varying proportions of Rhodes grass(Chloris gayana) hay and a concentrate mixed on dry matter basis to be: 1,000 g/kg Rhodes grass hay(RGH) and 0 of the concentrate(D1), 900 g/kg RGH and 100 g/kg concentrate(D2), 800 g/kg RGH and 200 g/kg concentrate(D3), 700 g/kg RGH and 300 g/kg concentrate(D4). Dietary kinetics for the models were determined by measuring GP at 2,4.8,10.18,24.36,48.72,96 and 120 h. Model comparison was based on derived GP kinetics, graphical analysis of observed versus predicted GP profiles plus residual distribution and goodness-of-fit from analysis of root mean square error(RMSE), adjusted coefficient of determination(Adj-R^2) and Akaike's information criterion(AIC). Asymptotic GP, half-life and fractional rate of GP differed(P < 0.001) among the 4 models. The RMSE, Adj-R^2 and AIC ranged from 1.555 to 4.429,0.906 to0.984 and 2.452 to 15.874, respectively, forall diets compared across the 4 models. Based on the goodness-of-fit statistical criterion, GP profiles of D1 were more appropriately fitted and described by GRTS and GOMP than the EXPLand LOG models. The GRTS model had the lowest AIC value for D2(2.452). Although GRTS model had the most homogenous residual dispersion for the 4 diets, all the 4 models exhibited a sigmoidal behavior.Therefore, rumen liquor from slaughtered cattle of unknown dietary history can be used to derive nutritionally important feed parameters, but choice of the most appropriate model should be made based on fitting criteria and dietary substrates incubated.