EVALUATION OF IN VITRO GAS PRODUCTION METHOD AND
ASSOCIATIVE EFFECTS OF RUMEN DIGESTION BETWEENPASTURE AND SUPPLEMENTARY FEEDSAye Sandar ChoAnimal Production System laboratory
Objective: Evaluation of feedstuffs is necessary for provision of a nutritionallybalanced ration. A ration formulated for ruminants is often a mixture of individual
feeds. The nutritive value of the feeds in mixtures are assumed to be additive.
However, adding supplements particularly NFC to pasture based system candrastically change ruminal conditions and often has deleterious effects on forage
digestion which is not additivity. This phenomenon is commonly referred to as
associative effects. In vitro gas production (GP) method has been used as a measureappearance of fermentation products and can study kinetics of digestion by rumen
microorganisms. Currently, to increase the energy intake of dairy cows in the pasturebased systems, supplements such as concentrate ingredients rich in starch or fiber andsilages are included in the diet. However while associative effects are often discussed
in ruminant nutrition, at least theoretically, they are seldom taken into account in feed
formulation. The objectives of this experiment were to evaluate the usefulness of invitro gas production method to identify associative effects on ruminal digestion
between pasture and supplementary feeds.
Materials and Methods: Pasture was incubated with corn (C), barley (B), cornsilage (CS) and grass silage (GS) at proportion of 0, 25, 50, 75, and 100% respectively
in experiment 1. In experiment 2, pastures was incubated together with corn, barley,beet pulp (BP) and hay (H) at proportion of 75-25% and pasture + grain + fiber (threeway mixtures) at proportion of 70-20-10%. In both experiments, rumen fluids weretaken from ruminal fistulated cows fed pasture. Gas production was measured
as described by Theodorou et al (1994). GP were monitored up to 96h and parameters
were obtained by simple exponential equation. Organic matter (OM) effective
degradability was obtained by France et al., (2000) model. The prediction was donefor each observed value using the fitted parameters obtained from pure ingredients.
Associative effects on parameters were tested by comparisons with the values
predicted from those of single feeds assuming additive effects.
Result: (Experiment 1) Almost all feed mixtures had positive associative
effects on lag time. Grain and CS supplementation showed positive associative effects
on the potential GP, IVDMD and IVOMD. For rate of GP, negative associative effects
were observed for corn and CS supplementation, whereas, such effects were notobserved in barley supplementation. Associative effects were less stimulated by GS
supplementation and in summer.
(Experiment 2) Both of all two-way and three-way mixtures had positive
associative effects on lag time. For two-way mixture, corn supplementation showed
positive effects on potential GP, IVDMD and IVOMD and negative effects on rate ofGP. Such effects were not observed in other two-way supplements and on ED. Almost
all three-way mixtures showed positive associative effects. Although the mixtures
included corn tented to negative effects on rate of GP, the mixtures included BP tented
to positive effects on digestibility. For ED, negative associative effects were found for
P+C+H mixture, whereas, positive associative effects were found for P+B+BP
mixture.
From both experiments, it is concluded that in vitro GP method is a useful toolfor detecting associative effects of feed mixtures and associative effects on ruminal
digestion were observed among pasture, starch and fiber source supplements.
Associative effects were more pronounced on three-way combinations of feeds thantwo-way.