Pulse crops in rotation  
You’ll Get More Than Nitrogen Benefits

By Lorne McClinton

For Pulse Crop Monitor

If you put in pulse crops for the nitrogen (N) benefit, you’ll get that, but you’ll also get some other yield enhancing agronomic benefits you may not be expecting.

New research by Adrian Johnson, Hugh Beckie and S.A. Brandt at Agriculture and Agrifood Canada at Melfort proves what farmers have long reported: yields increase when crops are grown on pulse stubble. The ‘why’ is now becoming much clearer.

"There is definitely a legume benefits," says Johnson. "A nitrogen benefit from the pulse crops, which results in increased yields and higher protein levels, has been found across all regions of Saskatchewan. In Swift Current, a semi-arid region, we found spring wheat yields increased 16 percent with a 0.8 percent increase in protein. We saw smaller increases in grain yields further north, but still found increases in protein."

How much nitrogen peas and lentils leave in the soil for the following crop depends on your soil and location.

"In dry regions, we found about 20 pounds of N per acre in lentil stubble," says Johnson. "How much N is carried over is a function of soil type and production potential. At the Scott research station in the Dark Brown soil zone, we found ¼ pound of N per acre per bushel. That’s quite a range. The higher the production and greater the moisture, the greater the nitrogen benefit."

Seeding cereals on even the highest yielding pea stubble doesn’t necessarily mean you don’t still have to ad nitrogen. Agronomy Manager John Harapiak with Westco in Calgary says, "Just because you get better crops on pea stubble doesn’t mean you shouldn’t try to capture all of your crops potential yield. Even on high yielding pea fields, fields that yielded 90 bushels per acre, we still got tremendous response to added nitrogen."

"All things being equal, pea stubble gives you the best potential for your highest yielding crop. However, if farmers cut back too much on fertilizer, or eliminate fertilizer altogether, they aren’t capitalizing on the yield potential inherent in the crop."

Don Boles, who farms near Three Hills, Alberta, says, "It’s a bit risky to follow a general rule of thumb that a pound of residual nitrogen is provided for every bushel of pea production. I’ve grown peas for seven years. Experience as shown me that I need to fertilize my pea stubble to maximize my wheat yield and protein content."

Boles says he thinks residual nitrogen is variable, and adds that he believes the rotational benefit of peas isn’t primarily nitrogen carry-over. : I think farmers who don’t fertilize after peas may be shortchanging their crop. In my rotation, peas-wheat-canola-barley, I use the same fertilizer for wheat grown on peas as for canola on wheat," he says.

Johnson also emphasizes that the legume benefit is not only nitrogen. "You break the disease cycles when you change the crop sequence. Difference in wheat yields on pea and canola stubble can be explained by adding nitrogen to the canola stubble. You could never add enough nitrogen to wheat on wheat stubble and get the same yield as wheat on pea stubble," he says.

I drier regions, where disease tends to be a smaller problem, farmers can use the crop’s rooting pattern to plan their rotations. Lentils and peas have shallow roots. The bulk of these roots are in the top 24 inches to 30 inches of the soil. In, contrast, wheat and canola roots can grow down as far as 6 feet.

"Understanding how plants use resources and how rotations can be based on available water will continue to receive attention," Johnson says. "For example, if you have a high yielding wheat crop, followed by a dry fall and very little winter moisture, you may go into your next year with limited surface moisture reserves.

"Planting a crop like field peas that can efficiently utilize surface moisture and growing season precipitation is a good idea. You’ll probably get a reasonably good crop. If you plant a deep-rooted crop like wheat or canola, yields might be limited because the deeper soil profile has never been recharged.