DEVELOPING RICE STRAW FOR ANIMAL FEED

Hundreds of millions of tons of rice straw and husks are burned in the field every year simply to get rid of them. This combustion causes very bad air quality through the release of black carbon particles, methane (and larger volatile compounds), carbon monoxide and the generation of ozone. Between them, these emissions cause respiratory illness and premature death in exposed populations, significant losses in crop yield due to the toxic effects of ozone and shading by black carbon, and increased global warming, especially due to methane and black carbon. All of these negative impacts are avoidable if alternative uses for rice straw can be found. Rice straw currently finds little application as animal feed because of its poor quality. Rice straw is composed of roughly 75% polysaccharides and this can provide considerable value as animal feed, unfortunately, this straw is currently hard to digest, partly due to its content of lignin, and more importantly because rice straw has a high content of silica. Silica can make up to 10% of the dry weight of rice straw and this not only make it hard to digest but also highly unpalatable. Our overall aim for this project is to find ways to improve the quality of rice straw as animal feed in order to move it from being burned to becoming a valuable resource for the production of meat and dairy products.

We have been studying rice straw as a potential feedstock for producing biofuels by fermentation of its component polysaccharides. As in uses of rice straw for animal feed, this process is also challenged by straw digestibility and we have been using genome-wide association studies (GWAS) to identify genes that are involved in making straw hard to digest and molecular markers that can be used in breeding studies to improve this characteristic. For this work we have made detailed genetic maps for a collection of 170 diverse rice accessions collected and grown in Vietnam. We have measured how well the straw from these varieties is digested by commercial digestive enzymes (cellulases) and also measured their content of silica and lignin. This revealed that some varieties are more than four times more digestible than others and that silica content in some more digestible varieties are lower than 1%. In the work proposed here, we wish to expand these studies to examine the characteristics of straw from the rice diversity panel in terms of animal feed quality. This will be done in a high-throughput manner in the laboratory in the UK in order to allow us to carry out GWAS to identify markers to help in breeding for improved straw in commercial varieties and to help us identify new genes that affect these characteristics. On a science level, we will focus on increasing our understanding of how silica affects straw digestibility, by looking for cell wall-related genes that affect silica content.

From a point of view of seeing our studies have real impacts on straw use, we will work with scientists in Vietnam and the Philippines to demonstrate the potential value of straw improvement. We will do this by identifying a subset of rice varieties with contrasting animal feed value (high and low) and using these in feeding trials using carabao. Carabao are water buffalo traditionally used as draft animals in South East Asia that are being developed for meat and dairy production in tropical countries. The results of these trials will demonstrate the benefits of using higher quality straw for animal feed and we will use this information to educate farmers of the benefits of growing dual purpose rice varieties for grain and animal feed production, and rice breeders of the benefits of improving straw value in commercial varieties. In the long term our work has the potential to improve the environment and well-being of people in rice growing countries as well as increasing farmer income and the availability of high protein products for local consumption.

The burning of unused rice straw leads to widespread air pollution in rice growing countries. Emissions from rice straw burning are responsible for premature deaths, significant reductions in crop yield and avoidable global warming impacts. Finding cost-effective uses for rice straw could help avoid these damaging emissions, and animal feed applications would have the added value of producing meat and dairy products in regions with protein-poor diets. Rice straw is rich in polysaccharides of considerable nutritive value, but is typically unpalatable and hard to digest. Among angiosperm plants, grasses are distinctive in typically accumulating silica to relatively high levels. Rice is a hyperaccumulator of silica, which can comprise up to 10% of the dry weight of straw. The molecular context and form of silica in plants is poorly understood, and in rice large quantities are found in the cell walls and it is likely that cell wall silica limits digestibility.

Our aim is to develop the basis from which to improve the quality of rice straw as animal feed. We will use genome wide association studies (GWAS) to identify quantitative trait loci (QTL) that influence animal feed quality in rice straw, using a diversity panel of 170 rice accessions from Vietnam. QTL-associated markers can subsequently be used in marker -associated breeding to improve straw quality in commercial varieties. Our previous studies have identified QTL for straw cellulase digestibility, silica content and lignin content. Digestibility studies revealed a greater than 5-fold range in this characteristic and silica levels correlate negatively with this trait. We now propose to use high-throughput assays for animal feed quality to identify QTL for this trait. These studies will also identify rice varieties at either end of the animal feed quality spectrum in our panel, and we will select a small number of these for in-rumen digestibility and long term feeding studies using water buffalo.

Rice straw burning and the climate change both have negative consequences for agricultural production and human health in the Philippines and in Vietnam. In particular, increased tropospheric ozone from biomass burning has negative consequences for plant productivity through inhibiting photosynthesis, whilst black carbon emissions cause significant losses in productivity through shading, and both lead to respiratory ailments and premature deaths among the human population. Diverting straw away from burning into animal feed will not only benefit the environment, but also has the potential to add to the income of rice farmers, and increase food security. The proposed work will identify genetic markers that can be used to improve the animal feed quality of straw from commercial rice varieties. In addition, we will identify existing rice accessions that differ from one another in terms of the animal feed quality of their straw. By carrying out feeding trials with straw with contrasting animal feed quality, we will be able to assess and demonstrate the benefits of feeding better quality straw to animals. We will carry out dissemination activities to demonstrate the potential value of growing rice with high quality straw in providing additional income to farmers and to advise them of which varieties currently available have desirable straw quality. We will also disseminate our results to rice breeders to encourage them to improve straw quality in commercial varieties. We will organise a meeting of appropriate stakeholders from other rice growing nations to disseminate our results and establish relationships to see the benefits of our work are spread widely. The potential for beneficial social and environmental impacts in developing countries and more widely is potential large. Moving rice straw away from being burned in the field will lead to improved air quality, leading to improved health and well-being for those living in rice growing areas. Decreased burning will improve crop yields by avoiding the consequences of tropospheric ozone generation and shading impacts from black carbon, and will also reduce atmospheric warming due to black carbon and methane generation. In addition, bringing more rice straw into use as animal feed will help improve farmer income, allowing the production of additional meat and dairy goods from existing underused resources. Increased meat and dairy production will increase nutrition levels in areas with protein-poor diets and improve food security in rice growing countries.