Crops: The Lynch File - Spray your weeds early to reduce yield loss

Monday, May 4, 2009

You can lose up to one bushel per acre a day once weeds get to two inches, according to one researcher. Acting early can prevent major losses in yield

by PAT LYNCH

Do you believe you should wait for all the weeds to emerge before you spray your Roundup Ready crop? You may think you are saving money by just spraying once.
Or you may believe that, if you wait long enough, all the weeds will be up and subsequent flushes will not affect yield. You hope the crop will canopy before these weeds can affect yield.

This is a good strategy if your plan is to keep spray costs to a minimum.

It's not a good strategy if you want to make the most profit per acre.

Researcher Dr. Clarence Swanton from the University of Guelph has shown that early weeds reduce yield by what he calls shade interception.

He believes that these early weeds force the crop plants to change how they grow, thus reducing yield. Others believe that the roots from these early weeds physically interfere with water and nutrient uptake of crops to reduce yield. Either way, early weeds reduce yield.

Dr. Christy Sprague from the University of Michigan believes you can lose up to one bushel of soybeans per acre per day once weeds get to two inches. She has further research showing that weeds can grow at a rate of one inch per day. Ideally, once weeds get to that height, you can spray and prevent major yield loss.

The reality of growing conditions in Ontario is that you can get rained out of the fields for days in late May and early June. Too often, these rains start just when weeds are reaching the two-inch level. If you plan to spray today but are rained out for a number of days, you will lose yield. While Dr. Sprague's research suggests a loss of one bushel an acre per day, the amount of yield loss is determined by weed species, height and pressure. The yield loss that Dr. Sprague found was with a heavy pressure of broadleaf weeds.

One plan is to spray early and use a residual product along with glyphosate. If you have missed this window, the alternative is two applications of glyphosate. And if concern about yield loss is not enough to encourage you to spray early, think about how much harder bigger weeds are to kill. The options include higher rates of glyphosate or switching to non- glyphosate products.

Be especially concerned about some weeds which glyphosate has difficulty in controlling once they get past a certain stage. These weeds include Spreading Atriplex, Wild Buckwheat and some types of lamb's quarters. There are lambs quarters that have higher than normal levels of calcium. This allows the plant to deactivate the glyphosate and keep growing. The solution is to spray these types early when they are small. Alternatively you will have to use a higher rate.

For some growers, the switch to Roundup Ready crops has made weed control simpler. For too many producers, maximum yield is being held back because of early weed growth. BF

Pat Lynch CCA (ON) is head agronomist for Cargill in Ontario.