Impacts of silica levels, and location in the detergent fiber matrix, on in vitro gas production of rice straw

Abstract

Rice straw can be used as a forage for cattle, generally those whose intake potential exceeds their energy need, to provide economic savings in cattle production systems. This study evaluated how simulated field drying of rice straw, its silica level and location in the detergent fiber matrix, impacts in vitro gas production. We also determined how in vitro gas production is impacted by removing Si from rice straw entirely by growing it in a Si-free hydroponic solution. In Experiment 1, rice plants grown in controlled conditions were analyzed fresh (i.e., within 60min of harvest), and when fully dried (i.e., 25°C for 7d), for dry matter, ash, acid detergent fiber (ADF), neutral detergent (ND) extracted ADF (ND/ADF), Si in ADF (ADF-Si) and ND/ADF (ND/ADF-Si), as well as in vitro gas production. Fresh straw had a higher proportion of ADF-Si (539 versus 485mg/g total Si; P<0.01) and less Si in ND extracted ADF (196 versus 340mg/g total Si; P<0.01), than the same plants after drying, which may have caused the higher in vitro gas production of fresh straw. However, in Experiment 2, the ADF-Si, ND/ADF-Si and total Si were not predictive of in vitro gas production in a set of 39 commercially grown field samples of rice straw, possibly due to the narrow range of Si values in the samples or because the location of the Si in the fiber matrix, or Si itself, is not predictive of the digestibility of its organic matter (OM). In Experiment 3, rice grown under controlled conditions in Si free or Si containing hydroponic media, in two sub-experiments, had similar gas production per unit OM. The general lack of impact of rice straw Si levels, either total or relative to location in the detergent fiber matrix, on in vitro gas production seems to demonstrate conclusively that Si is not causative to the low fermentability of rice straw OM.