THE DIGESTION AND ENERGY VALUE OF WHOLE CROP WHEAT TREATED WITH UREA

Two experiments are reported which have examined the digestibility, energy value and rumen digestion of whole crop wheat (WCW). In Experiment 1 two samples of WCW (WCW1 and WCW2), harvested at the dough stage (growth stages 83-91 ) and treated with urea at 40 g kg-1 dry matter (DM), were studied in terms of chemical composition, apparent digestibility and energy value. The DM contents of WCW1 and WCW2 were 520 g kg-1 and 589 g kg-1, respectively, and WCW2 had a substantially higher cell wall content (587 g kg-1 DM) than WCW1 (438 g kg-1 DM). For both forages, products of fermentation were generally low, indicating preservation by the urea, although in WCW1 lactic acid concentration was 14.1 g kg-1 compared with only 0.8 g kg-1 in WCW2. Digestible organic matter concentrations of the DM (DOMD) in vivo were 657 g kg-1 and 542 g kg-1 for WCW1 and WCW2, respectively. These values were related to low measured metabolisable energy (ME) values (9.8 MJ kg-1 DM for WCW1; 7.4 MJ kg-1 DM for WCW2), with a mean DOMD/ME ratio of 0.0 1 43. Experiment 2 examined the rumen digestion characteristics of the separated grain and straw fractions from WCW harvested at the soft dough stage of growth (growth stage 85) and treated with urea at 0, 20 or 40 g kg-1 DM. On average, grain and straw made up 0.37 and 0.63, respectively, of the WCW DM. Urea use was associated with a significant increase in the DM content of the grain (P < 0.001) and straw fractions (P < 0.01), and the highest rate of urea led to grain of significantly (P < 0.05) higher starch content. Both rates of urea significantly (P < 0.001) increased the proportion of immediately water-soluble DM in the straw fractions (16.7%, 18.2% and 20.2% DM incubated for 0 g urea kg-1 DM, 20 g urea kg-1 DM and 40 g urea kg-1 DM, respectively), whereas the highest rate of urea reduced (P < 0.001) the immediately water-soluble DM fraction in the grain from 24.4 to 19.1% DM incubated. In the grain fraction only, the highest rate of urea also significantly (P < 0.05) reduced the rate of degradation of the insoluble fraction to 0.093 h-1 relative to 0.126 h-1 for the low urea rate. The results also highlighted the fact that in WCW the rates of rumen degradation of the straw and grain fractions are likely to be substantially different.