EFFECT OF SUPPLEMENTARY UREA, GLUCOSE AND MINERALS ON THE IN VITRO DEGRADATION OF LOW QUALITY FEEDS

Increasing levels of ammonia-N in the rumen fluid used for in vitro incubation were achieved by supplementation of the ration of the donor cows with urea and by addition of urea either with or without glucose to the rumen fluid after collection. The ration of the donor animals consisted of wheat straw (80%) and maize silage (20%). During the second half of the experiment the basal ration was supplemented with a mineral mixture. Wheat straw, Guinea grass and two rice straw varieties were incubated with the various kinds of rumen fluid. Parameters studied were: solubility, apparent organic matter disappearance after 48 hours of incubation ($OMD_{48}$), rate of organic matter degradation from 0 to 24 hours of incubation ($k_1$) and from 24 to 95 hours ($k_2$). The concentration of ammonia-N in the rumen fluid at which 95% of the maximal $OMD_{48}$ and k1 were reached (88.2 and 100.0 mg/l) were independent of the feed. With regard to the $k_2$ the required ammonia-N concentration to reach 95% of the maximal $k_2$ differed per feed. Mineral supplementation increased the OMD48 and $k_1$, but not the solubility and $k_2$. Glucose addition in combination with urea had no beneficial effect compared to urea supplementation alone.