A large number of crop straws (about 800 million tons) are generated in China every year, especially rice and wheat straws. Most straws are burned in farmland after autumn harvest, which leads to heavy fog and haze.

One viable way to reduce straw burning is to use it as roughage resource for ruminants, but most straws are difficult to degrade and have low nutrition value. Another way is to pretreat straws by urea, which can improve the degradation of straw by ammoniation process, and provide non-protein nitrogen at the same time.

However, ruminants will produce huge quantity of methane (CH₄), especially when fed with large amount of roughages. The CH₄ from livestock plays an important role in greenhouse gas emission. Thus, it's essential to reduce CH₄ production from ruminants when feeding them with straws.

Now, nitrate supplementation can make a difference. A research team led by ZHANG Xiumin from the Institute of Subtropical Agriculture (ISA) of the Chinese Academy of Sciences pretreated rice and wheat straws by urea plus nitrate (urea+nitrate) together to improve the degradation of straws and reduce the CH₄ production from ruminants. Their findings were published in Journal of the Science of Food and Agriculture.

The research group pretreated rice and wheat straws by urea+nitrate, analyzed chemical composition change, and tested the degradation of straws and CH₄ production by in vitro ruminal culture.

They found that urea+nitrate pretreatment could destroy the physical structure of straws, increase the neutral-detergent soluble content that degrades easily and fast in the rumen, and increase the in vitro degradation of both straws. Furthermore, the urea+nitrate pretreatment reduced the CH₄ production of both straws and inhibited the population of methanogens.

"This method combines positive effects of urea pretreatment and nitrate supplementation. Urea pretreatment can make the straws easier to degrade by ammoniation process, and the nitrate supplementation can reduce CH₄ production as its toxic on methanogens and hydrogen sink role that compete hydrogen with methanogens. So urea+nitrate pretreatment can increase degradation of straws and reduce CH₄ production at the same time," said ZHANG.

This work was supported by National Natural Science Foundation of China, National Key Research and Development Program of China, China Agriculture Research System and Youth Innovation Promotion Association CAS.