THE EFFECT OF HOST DIET ON THE GAS-PRODUCTION PROFILE OF HAY AND HIGH-TEMPERATURE DRIED GRASS

One of the sources of variation in the in vitro gas production (GP) te chnique is the inoculum source that is used and this could be greatly affected by the diet that is given to the donor animal. To study the e ffect of the host diet on the gas production profiles of grass hay and high temperature dried grass, two cows were offered either II silage : barley diet (80:20 dry-matter (DM) basis; GSB) or barley straw (ad l ibitum; STR). An adaptation period of 3 weeks was used and each animal experienced each diet type twice. Rumen fluid and solids were collect ed at the end of each 3-week period and used to inoculate the substrat e cultures. The volume of gas produced (ml/g DM) was 379 and 289 for G SB and STR respectively; gas yield (ml/g organic matter degraded) was 442 and 411. The maximum fractional rates of degradation (per h) were 0.067 and 0.061 and the time (h) taken to reach these rates were 5.3 a nd 12.6. None of these differences was significant. There were also no significant differences in the concentration of total volatile fatty acids (VFA) in the final incubation media, or in the molar proportions of individual VFA. These results suggest that the fermentation stoich iometry was not affected by donor animal diet and, while microbial act ivity from STR was lower, this did not significantly affect the GP pro file. The difference in microbial activity between the two diets was p erhaps minimized by taking the sample of rumen contents before the mor ning meal.