Don't believe the hype about cover crops

Tuesday, March 4, 2014

Cover crops have been over-promoted as adding nitrogen and organic matter, or reducing weed growth. Their real function is reducing soil erosion, a real threat in Ontario

by PAT LYNCH

The problem with cover crops in Ontario is that most of them are NOT treated as cover crops.

Cover crops have been promoted to Ontario farmers by various means. You have been told they will add nitrogen. They do not, the exception being red clover. But all the other talk about peas or radish adding nitrogen is just a lot of hype. They do not make nitrogen as red clover does. They just recycle it.

Cover crops have been promoted as adding organic matter. And they do, though not a lot. The organic matter they produce is quickly broken down.

Cover crops are promoted as reducing the weed seed bank by smothering out weeds as they germinate or reducing weed growth to prevent weed seed set. Again, this is an interesting aspect of cover crops, but not their main function.

The main function of cover crops is to reduce erosion. Soil erosion is a real threat to Ontario agriculture. It's a bit like growing older. You can't really see it happening over a short time, but it is occurring. The biggest benefit of growing a cover crop is to "cover" the land when it is most vulnerable to erosion. In Ontario, this is March, April and May.

The erosion in question is water erosion. The reality is that most acres of cover crops are worked in the fall, leaving the soil open to a lot of erosion. Maybe if we had promoted cover crops as cover crops right from the start instead of pussy-footing around about all the minor good things that cover crops do, such as adding nitrogen, cover crops would have started a better life.

To reduce erosion in March, April and May you must have at least 30 per cent residue cover in the spring. Fields that are in wheat or fields that were under corn the previous year and had vertical tillage or are planned for no-till are all right. The fields most at risk are fields that had wheat the previous year and are tilled clean. Or fields that were in soybeans and were worked after harvest.

I can see a major soil erosion event occurring in spring 2015. In 2014, we are headed for the biggest soybean acreage ever in Ontario. We could have three million acres of soybeans planted this year. If history repeats, then maybe 1.1 to 1.2 million acres will be planted to wheat. That leaves close to two million acres of soybean stubble which, if worked, will be at risk of eroding in spring 2015.

What can you do? Why not plant oats after soybeans? In fact, why not broadcast oats into standing soybeans as the leaves drop? You do not want to plant winter wheat since wheat used as a cover crop will increase the soil-borne diseases that hurt wheat yields. I believe using oats that will die when the soil freezes will not significantly increase the soil-borne diseases. The minimum you can do is to not work soybean stubble.

The other question is how to handle a cover crop planted after winter wheat. You can harvest the crop or use some type of reduced tillage. But it is better if you just leave the crop to cover the ground and work it in the spring. That's what I think. BF

Consulting agronomist Pat Lynch, CCA (ON), formerly worked with the Ontario agriculture ministry and with Cargill.