Impact of simulated field drying on in vitro gas production and voluntary dry matter intake of rice straw

Due to the recent expansion of the motor fuel ethanol industry, which is largely based upon traditional food and feed crops, supplies of ruminant feeds have become limited in many areas of the world thereby creating a need for alternatives. Rice straw, a substantial biomass source worldwide, could fill this role if its nutritional value was higher. Our aims were to determine changes in the nutritive value of rice straw during long term storage, and short term field drying, as well as the voluntary DM intake of rice straw by heifers before, during and immediately after field drying. In Experiment I, rice straw was macerated and stored for 82 days after baling, but there was no effect on chemical components or 30 h in vitro fermentability of neutral detergent fiber (NDF) due to maceration or storage time. In Experiment II, samples of a late maturity tall (M401) and an early maturity short (M202) rice straw variety were collected prior to harvest, during simulated field drying and when fully dried. The M401 had higher acid detergent fiber (473 versus 445 g/kg dry matter (DM); P<0.01), lignin(sa) (38 versus 34 g/kg DM; P<0.01), total Si (55.0 versus 43.0 g/kg DM; P<0.01) and Si in NDF (372 versus 270 g/kg DM of total Si; P=0.01). The M202 produced 14.4, 10.6 and 9.1% more gas than the taller M401 at 4, 24 and 72 h of in vitro fermentation, respectively. Fresh plants produced 6.8, 15.6 and 8.9% more gas than plants collected during simulated field drying and as dry plants at 4, 24 and 72 h of in vitro fermentation, respectively. In Experiment III, voluntary DM intake of rice plants was measured at the same three stages (i.e., fresh, during simulated field drying, dry), and results were consistent with Experiment II, in that DM intake of fresh plants was higher than plants during both simulated field drying and when dry (5.14 versus 4.13 versus 3.69 kg DM/day; P<0.01). That long term storage of straw after baling did not impact levels of its chemical components or 30 h in vitro fermentability of NDF, but gas production of fresh plants was higher than that of plants during simulated field drying and when fully dried, supports the hypothesis that the much higher voluntary DM intake potential of fresh rice plants occurred due to changes during field drying that reduced its fermentability. It seems certain that this depressed DM intake was related to the decrease in fermentability of dry versus fresh plants, although the cause of the reduced fermentability is not clear. However analyses of Si levels in the detergent fiber fractions provided indications that the location of the Si in the structural carbohydrates may be important in this regard.