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Engineering More Digestible Grass Could Reduce Livestock Pastures And Give Us Cheaper Biofuels
Engineering More Digestible Grass Could Reduce Livestock Pastures And Give Us Cheaper Biofuels
When you look at livestock through the lens of environmental sustainability, one of the biggest drawbacks is the sheer volume of grass these animals eat before finding their way onto our dinner plates.
In a world with unlimited grazing land, this might not be such a bad thing. But on a planet with finite land resources, this need for grass creates a stumbling block in our efforts to meet an ever-growing demand for meat.
But what if we could figure out a way to help cows and other animals get more out of the grass they eat? Recent research suggests we may be headed that direction, with scientists discovering a gene which can be turned down to make grass more digestible – and more efficient.
Grasses owe their evolutionary success partly to sturdy cell walls that make them difficult to digest. This deters many herbivores, and poses problems even for those animals which are adapted to eat it. Unlike humans, cows and sheep have stomachs that allow them to graze on grass. Still, they can’t release all of the grass’s energy. Much of a plant’s calorific value is tied up in a robust cell wall, making it inaccessible to livestock.
More digestible grass would make it possible to raise cows on smaller pastures, reducing the pressure put on the land by beef and dairy production. It also has the potential to boost bioenergy efforts. Crops such as maize, sugarcane and rice have those same robust cell walls, and this makes them difficult to process into biofuels.
Trailers filled with sugarcane sit at Biosev SA’s Santa Elisa mill in Sertaozinho, Brazil, in 2015, ready to be converted to ethanol. 
The parts of the sugarcane plant which aren’t made into sugar are a perfect source of fuel, yet it takes large amounts of energy to turn them into bioethanol. They already are used in Brazil, where they’re given a high energy “pre-treatment” followed by treatment with enzymes to release sugars from the cell walls. These sugars are then fermented to make bioethanol. In theory, more digestible biomass should lower the amount of energy and enzymes needed, making bioethanol production both cheaper and more environmentally friendly.
Previous efforts to find more digestible grasses have generally had little success. Sometimes the methods have been quite simple, such as screening plants to identify those with high digestibility. Others have worked on suppressing genes. One crop has been brought to market, brown-midrib maize, in which a natural mutation decreases lignin production. It attracts a premium in the US as an animal feed, but the mutation has the side-effect of reduced yield.
New research findings
The new research, released recently in New Phytologist, identifies a key gene involved in the stiffening of grass cell walls, and demonstrates that suppressing it can increase the release of sugars, making the grass more digestible.
Scientists have known for a long time that ferulic acid contributes to strong grass cell walls. This small molecule creates cross-links in the cell wall, making them stronger. It has been hard to identify the gene responsible for adding ferulic acid to the cell walls, which encodes an enzyme, although a collaboration between researchers in the UK, Brazil and the US has now had success. The project was led by Dr. Rowan Mitchell from Rothamsted Research in the UK and Dr. Hugo Molinari at the Laboratory of Genetics and Biotechnology at Embrapa Agroenergy, part of the Brazilian Agricultural Research Corporation (Embrapa).
Mitchell first identified a possible gene in 2007. He compared the fully sequenced genomes of rice, a grass and Arabidopsis, a non-grass commonly used as a model organism in the lab. By looking for a gene that was highly expressed in grasses but not in non-grasses, and judging that it was the right type of gene based on the sequence of the protein it encodes, he came up with a likely candidate.
Demonstrating its role in the lab, however, has proved difficult. In the 10 years since the discovery, the team has tried various approaches, and were finally successful using RNA interference (RNAi). Using genetic modification techniques, they added a gene which suppresses the target gene to around 20 percent of normal activity.
This increased the release of sugars by up to 60 percent and makes it a promising target for improving grass crops for both bioenergy and animal feed.
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