Residual herbicides - an insurance policy for your GT soys

Monday, March 5, 2012

It only costs two or three per cent of your returns from the crop and is cheap at the price

by PAT LYNCH

There is one main reason why you should use a residual herbicide in glyphosate-tolerant soybeans. It acts as insurance. When you use a soil-residual herbicide, you are buying insurance that you will be able to spray glyphosate later if needed.

There are problems with spraying glyphosate in glyphosate-tolerant soys. The main one is Ontario's weather. Often, weather conditions do not allow you to spray when you should. If it is too windy, you risk the glyphosate being blown off its target. If this has happened to you, you know the consequences are more calamitous than when we used to have off-target movement of hormone herbicides.

Sometimes, the weather is too windy or too wet to enable you to spray on time. Research by Dr. Christy Sprague of the University of Michigan shows that, if weeds reach two inches in height, you can lose three bushels per acre. If they grow to four inches high, you can lose 14 bushels per acre.

Her research also shows that weeds can grow from two inches to four inches in as little as two days. The average time over four years was four days to grow from two to four inches.

Here is what typically happens: you drive by the field and realize that you can see weeds. Depending on your eyesight and how fast you are driving, they could be from one to two inches when you spot them. By the time you get around to spraying, it may be another one to two days. If you are unlucky, it could even be seven days. Each spring, it is not uncommon to be rained out of a field for seven days.

If spraying on time is not a concern, think about resistance management. We know that resistance to glyphosate has already occurred. If you continue to depend solely on glyphosate as your weed control, you will select glyphosate-resistant weeds.

Using a soil-residual herbicide is insurance that you will slow the shift of weeds that occurs in fields only sprayed with glyphosate. These weeds include perennial sow thistle, bindweed, lady's thumb and chickweed. I'm not sure which of these is the worst. When growers abandoned soil-residual herbicides, chickweed thrived.

This little weed, referred to south of the border as "God's ground cover," causes lots of problems, such as root balls that interfere with planting. Less obvious problems that it causes relate to soybean cyst nematode and cut worm. When chickweed lives, soybean cyst nematodes use it as an alternate host to multiply. When chickweed grows into the spring, it attracts cut worms. The adult moths fly in and see chickweed growing and lay their eggs there. Once they outgrow the chickweed or it is killed, they move to the corn crop.

One objection to using a soil-residual herbicide is the cost of the Technical Use Agreement. You may say that since you paid for the TUA, you want to pay less for weed control. That is partially true. The reality is that, while you pay less for weed control if you only use glyphosate, you get less weed control.

There is a TUA built into the price of corn seed. The TUA is to help pay for the cost of developing better soybean varieties. In corn seed, you pay for better genetics, but the cost is built into the price of the seed.

So how much does this insurance cost? Well, it is "about a bushel per acre" to quote an often-used phrase of my friend Horst Bohner of the Ontario agriculture ministry. The full rate of many soil-residual herbicides is around $12 per acre. Lots of growers will use a reduced rate and then come back with a glyphosate treatment to touch up some hard-to-kill perennial weeds, or weeds that had escaped. If you use the full rate of a soil-residual herbicide and have to re-spray, often there is a manufacturer's rebate.

To put it another way, the cost of a soil-residual herbicide is about two to three per cent of the cost of the returns from a crop. That is cheap insurance. BF

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