가뭄이란 조건에서 덮개작물이 옥수수의 수확량을 대폭 증가시킨다

Using cover crops correctly can reduce the stress of drought on cash crops, such as this Kentucky cornfield shown during the 2012 drought.

Farmers planting crops that can’t be sold? That doesn’t sound like a sensible proposition, does it? After all, seed cost money and so does the equipment to get them in the ground. Why grow ‘em if you can’t sell ‘em?

But it turns that an increasing number of farmers are doing just that—buying, planting and tending so-called cover crops. No, they can’t sell them, but they do reap benefits from them, including increased yields of their cash crops like corn and soybeans. Use of cover crops can also help farms survive the droughts expected to be more common in the era of climate change.

Cover crops, which can be many species of  grains, grasses and legumes, are usually planted in the interval between the harvest and planting of cash crops. Sending their roots down into bare soil, cover crops can increase soil carbon, provide slow-release nitrogen, and prevent erosion. But a cover crop/cash crop system is complex. If improperly managed, cover crops can deprive cash crops of water or even reduce yields. Although they make sense in theory, many have wondered how cover crops would work in the real world.

A new survey
Now a new survey of commercial farmers has confirmed that that cover crops increase yields in corn and soybeans, our most common crops. Moreover, cover crops were especially effective under drought conditions.

The survey of more than 759 commercial farmers was conducted last year (2012-13) by the North Central Sustainable Agriculture Research and Education (SARE) program and the Conservation Technology Information Center. The farmers who responded to the survey reported average increases of 11.1 bushels of corn per acre and 4.9 bushels of soybeans per acre. In percentage terms, the extra bushels represent an average 9.6 percent greater yield in corn planted after cover crops compared to crops not preceded by cover crops. The increase in soybeans was 11.6 percent. That’s pretty impressive.

The growers reported yield information from comparable fields that were similar in conditions and rotation except for the cover crops.

Star performers under drought conditions
The yield increases in cash crops planted after cover crops were even greater in states hit hard by drought.

The states most affected by the severe 2012 drought were Illinois, Indiana, Iowa, Kansas, Missouri, Nebraska and South Dakota. The 141 respondents from those states reported the average corn yield was 11.3 bushels per acre, which represented an 11 percent increase in crops grown after cover crops compared to those grown without them. Respondents from the drought-affected states reported even greater benefits in soybeans: an average increase of 5.7 bushels per acre, or 14.3 percent higher yields after cover crops.

The farmers responding to the survey grew cover crops on an estimated 218,000 acres of cover crops in 36 states, mostly in the Mississippi River basin. Not surprisingly, drought-related impacts varied across the country. But the results were solid: farmers enjoyed better corn yields after cover crops in all but one of the states hardest hit by the drought.

Benefits worth paying for
Farmers expected to pay for the ecosystem services provided by cover crops. They were willing to pay median costs of $25 an acre to purchase seeds and $15 an acre for establishment (aerial distribution of seed and termination (killing)) of the cover crop.

The challenges of growing cover crops
Farmers interested in cover crops need to decide which species to use, how and when to plant them, and whether to plant single or multispecies mixes. If the wrong decisions are made, cover crops might not deliver on their potential benefits or may even be detrimental. The survey respondents reported a long list of challenges including cover crop seed availability, increased insect potential, and that cover crops might use too much soil moisture.

Despite the challenges, these farmers had steadily increased their use of cover crops over the last decade. Last winter they reported planting cover crops on an average of 42 percent of their acreage and planned to increase their cover crop acreage this coming winter.

The complexity of the system may explain the correlation of yield increases with experience using cover crops. Growers with more than three years working with cover crops saw a 9.6 percent increase in corn yields, while growers with one to three years reported a still respectible, but lower, 6.1 percent boost in corn.

Drought-resistant systems and drought-tolerant crops 
A complete drought-tolerant package would include appropriate crop choices and specially bred varieties of crops as well as a drought-tolerant system. The crop-centered approach to drought was discussed by my colleague, Doug Gurian-Sherman, in his recent report High and Dry. In addition to highlighting the availability of crops like sorghum and alfalfa that are inherently more drought-tolerant and might be used more often in U.S. agriculture, Doug also discussed the success of conventional corn breeders who have increased drought tolerance at a steady pace of 1 percent a year over decades.

Genetic engineering has yet to play an important role in drought tolerance, only this year introducing its first drought-tolerant variety, Monsanto’s DroughtGard. According to the Monsantowebsite, the variety has produced a 5 bushel (or about 4 percent)  yield advantage  in field tests against competitor hybrids.

But, however successful crop genetics might be, the right choice of crops and varieties cannot compensate for the deficiencies in systems. The fundamental requirement  for combating drought is to keep moisture in soil.  Cover crops can do that–and so much more.

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