Even on the West Side of the San Joaquin Valley, where average rainfall is a mere 7 inches per year, farmers can reap the benefits of winter cover crops without the expense of irrigation, University of California research has found. Growing a winter cover crop helps retain soil nitrogen – keeping it from leaching into groundwater – improves water infiltration, reduces runoff, increases soil organic matter and boosts long-term soil fertility.

Moreover, a vigorously growing cover crop can smother winter weeds, reducing or eliminating the need for herbicides or tillage between crops.

“Despite the many and varied benefits of cover cropping that are increasingly seen by farmers in other parts of the country, the vast majority of Central Valley farmers currently do not use them,” said Jeff Mitchell, UC Cooperative Extension specialist in the Department of Plant Sciences at UC Davis. Mitchell, a cropping systems expert, is based at the Kearney Agricultural Research and Extension Center in Parlier.

The costs and benefits of winter cover crops are being examined in an ongoing trial at the UC West Side Research and Extension Center in Five Points. Initiated in 2000, the trial is led by Mitchell, William Horwath, a professor in the Department of Land, Air and Water Resources at UC Davis, and Dan Munk, UC Cooperative Extension advisor in Fresno County, a cotton and soils expert.

Mitchell said the West Side trial addresses valley farmers’ primary concern about cover crops – water.

“When water is short, as it has been in many recent years, farmers wonder how inserting an extra crop that doesn’t bring an immediate return on investment makes sense,” Mitchell said. “But our work over the last 12 years has demonstrated that cover cropping ‘on the cheap’ – relying only on rainfall for irrigation – supplies many benefits and doesn’t cost much.”

Rainfall during the November to March winter growth period in Five Points averages 7 inches, slightly less than the 30-year average annual rainfall of 7.6 inches for the site. Winter rainfall has varied considerably during the trial, from a low of 2.9 inches in 2003 to a high of 11 inches in 2006.

From 2000 to 2010, a cover crop mix of triticale, ryegrain and pea was grown at a seed cost of $55 per acre (2012 dollars). In 2011 and 2012, the researchers used a mixture of fava bean and “tillage radish” for the cover crop, at a cost of $50 per acre.

Tillage radish is a large-rooted winter annual being marketed for its ability to improve soil condition. It’s thick, tuberous roots break up the soil surface. When it is killed in the spring and the roots decompose and shrivel, it leaves behind channels that help with aeration and water infiltration.