Influence of Various Feeds on in Vivo and in Vitro Production of Volatile Fatty Acids

Rumen fluid was collected from two rumen fistulated animals on six different rations and was used to prepare washed cell suspensions of bacteria.Each suspension was used to ferment five of seven different substrates which were prepared from the same feeds as those fed the donor animals.The greatest in vitro production of volatile fatty acids (VFA) from each substrate occurred when the substrate consisted of the same hay as that in the ration of the donor animal.With one exception, the addition of concentrates to a hay diet reduced the YFA concentration in vivo when determined 4 hr.after feeding.The in vitro YFA production also decreased when the donor animal received concentrate in addition to hay.Under in vitro conditions the total acid production, pH, and acetate/propionate ratio was lower than in tureen fluid.When a concentrate mix served as the substrate, these values were further decreased.The in vitro technique used appears to have limited value in estimating the nutritive value of forages and little value in estimating the value of concentrates.