The 2010 growing season is in full swing. Hopefully everyone at least has a tentative plan for weed control regardless of crop.

I say tentative because every year unexpected circumstances arise that throw the proverbial “monkey wrench” into our weed control plans. Last year it was cool temperatures and excessive rains from late-April through mid-May.

Every year I hear people say there is no “normal” year, and I think that is a true statement. But maybe, after the year we had last year, 2010 will bring more average environmental conditions. As we move through the early part of the 2010 growing season, this is a good time to discuss the importance of proper timing for postemergence herbicide applications.

Over the years, weed scientists across the United States have studied yield loss in different crops growing in competition with weeds. Some studies determined the amount of yield loss associated with increasing weed densities. Others investigated how long a crop could tolerate weed competition prior to sustaining yield loss.

Each crop possesses a critical period during which weeds should be controlled to prevent yield loss. This period varies among crops and regions and can be influenced by weed species, environment, or crop factors such as variety and row spacing.

For example, seedling corn grows faster than seedling cotton, so the critical period of weed control for cotton is longer than for corn.

Because so many factors can influence this critical period of weed control, a good rule of thumb is that weeds should be controlled for the first four to six weeks following emergence to avoid yield loss. Starting with a clean field by using a good burndown program, tillage, or a pre-emergence herbicide is the best way to allow the crop to get ahead of the weeds.

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A common practice that has evolved since the introduction of herbicide-resistant crops is to delay the first postemergence herbicide application to allow more weeds to emerge so they can all be killed at once. Even before the widespread detection of herbicide resistance in weeds, this type of logic was misguided, and this practice has several negative consequences.

Yield loss due to weed competition is nearly inevitable when the first postemergence herbicide application is delayed. Weed control will most likely be incomplete because weed density or size was too great at the time of application making the weeds more difficult to control. Lastly, multiple herbicide applications may be required for complete control. Realistically, all fields cannot be treated at the ideal herbicide timing due to environmental conditions, logistics, etc., but they should be treated as close to the appropriate times as possible.

These concepts translate to all crop-weed situations. In research conducted last year at Stoneville, rice yields were 25 percent higher when barnyardgrass was controlled one week after rice emergence versus four weeks after emergence. Cotton yields were better at Stoneville in 2010 when glyphosate plus Dual Magnum was applied to cotton in the one-leaf stage compared with the four-leaf stage.

A few years ago when I was with the LSU AgCenter, we documented a 40 percent soybean yield reduction when the first glyphosate application was delayed from one week after soybean emergence to four weeks after emergence. Research from North Carolina State University reported a 14 percent reduction in corn yield when the postemergence herbicide application was delayed from four weeks after planting until six weeks after planting.

The importance of early herbicide application is even more critical in areas with glyphosate-resistant Palmer amaranth because the early application timing may provide the only opportunity for postemergence control. Much has been said and written about the value of residual herbicides for managing glyphosate-resistant Palmer amaranth, and in irrigated cotton, residual herbicides offer the best opportunity for controlling it.

However, when glyphosate-resistant Palmer amaranth is targeted with a postemergence herbicide, you cannot control it too early, particularly in cotton and soybeans where the postemergence herbicide options for it are limited.

A PPOase-inhibiting herbicide (Flexstar, Ultra Blazer) in soybeans or Ignite in a LibertyLink system will not control glyphosate-resistant Palmer amaranth greater than about four inches in height. The Flexstar label suggests a rate range for controlling Palmer amaranth with three to six leaves, while the Ignite label recommends treating 3- to 4-inch Palmer amaranth.

In either case, Palmer amaranth should be treated when it is very small. In fields with glyphosate-resistant Palmer amaranth, if the majority of plants are 4 inches in height, then it is a safe bet that a large number of plants will be 5 to 6 inches tall and will likely not be controlled with the postemergence herbicide.

When people ask me at what size glyphosate-resistant Palmer amaranth should be treated, I usually cut the label suggestion in half so that most plants in the field to be treated will be within the recommended size.

In modern crop production, a large percentage of costs are associated with the bag of seed. Also, we have had to endure some pretty significant yield losses due to weather late in the season over the last two years. These factors are out of our control for the most part, so we cannot afford to lose yield to early-season weed competition. Starting with a clean crop and timing postemergence herbicide applications appropriately are things we can control to avoid yield loss and improve profit margins.

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