Top Crop Manager

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Herbicide application timing more important

Value increases as crop prices climb.

March 10, 2008  By Ralph Pearce

At this time of high commodity prices when growers should be doing everything to maximize their earning potential, a somewhat disturbing trend has arisen. In the spring of 2007, Robert Moloney, an agronomist with FS Partners in Mitchell, Ontario, was fielding an uncommon number of calls from growers seeking advice on spray timing. With some probing questions, he was finding that many growers had been letting weeds climb to troublesome heights; in conventional soybeans, he was hearing about lamb’s quarters with more than four leaves.

It may be a worst case scenario but the effect is obvious; waiting too long to spray for a weed like lamb’s quarters can lead to significant losses in yield.

It was a point of surprise that he expressed during a breakfast meeting in May 2007. In spite of relatively high prices for corn, soybeans and wheat at the time, most growers he was talking to seemed more concerned with crop stage than weed stage. “They were forgetting that in conventional soybeans, especially if we get lamb’s quarters that are beyond the four leaf stage, they become tough to kill,” reminds Moloney. “If they’re beyond eight leaf, they’re really tough to kill.”

The same scenario applies with grasses, he says. Although the weed might be just an inch in height, it may have tillers on it. “At that point, a CleanSweep program isn’t going to do it, so you’re going to end up with a two pass program and probably throwing in a grass herbicide,” says Moloney. “It’s a lot more dollars, tougher to kill, and if you throw in dry weather on top of it, you have a whole other scenario.”


It may be part of a disturbing trend of complacency, or it might be a sign of emerging struggles with time management or a reliance on newer technology, be it seed traits, computer aids or farm equipment. Moloney is concerned more growers are leaving yield behind, largely because of a simple misunderstanding. “What growers were seeing was that 21 days after planting, the beans were just barely out of the ground, so they didn’t want to spray that early because they felt they were going to have escapes with the beans just barely at unifoliate,” he says.

“Well, that may be true, but the problem was that growers already had a six leaf or an eight leaf lamb’s quarter in there and they should be spraying at that time. They’re ignoring the fact that you need to look at weeds as much as you’re looking at the crop.” One of the key issues for Moloney is the reliance on US practices, particularly taking weed height and not weed stage of development into consideration. Spraying at four inches instead of a weed or crop stage sets up growers for potential losses in yield.

Mike Cowbrough agrees that much of the problem with application timing and understanding stems from height. “I understand why that’s done, because it’s simple. And how do you accurately count leaves on a lamb’s quarter when it starts to put out little leaflets here and there?” explains Cowbrough, weed management program lead with the Ontario Ministry of Agriculture, Food and Rural Affairs. He believes much of the problem exists as a result of a particular farm’s size and managing the time to attend to things like spray applications.

“I don’t doubt that growers are out scouting, and let’s say they go out on Tuesday and things are looking as though they are a couple of inches tall. And they will say they’ll be in a position to go in a week, but they get back there in a week and it’s been good weather, and suddenly, these weeds are out of control. Maybe that’s part of it. We don’t appreciate how quickly things progress, particularly weeds.”

Understanding not the issue, time is
During a presentation late in 2007, to demonstrate an efficacy trial in velvetleaf, Cowbrough explained to growers that he had purposely waited until the weeds were six inches or at the eight to 10 leaf stage in development. Growers pointed out that the weeds were too large to get effective control with anything, proving Cowbrough’s contention that growers do understand there is a point where weeds are too far gone to control effectively.

“Part of the presentation showed the tremendous difference in products and when weeds get large,” says Cowbrough. “You can go from 85 to 90 percent control down to 70 percent control, just based on four to six leaves. So I really believe it’s a time management issue because the fields I see are the large acreage growers who might only have time to see that field at planting, at spraying and at harvest.”

Whatever the reason may be, Dr. Peter Sikkema’s concern is that growers are, as he says, ‘leaving money on the table’. Like Cowbrough, he believes growers understand the importance of herbicide application timing. There is, however, a crucial gap in associating that level of understanding to the loss of yield, often on a daily basis.

Sikkema echoes one of Moloney’s observations that growers should pay more attention to timely herbicide application. “As I tell my students, the higher the price of the corn, the earlier you spray,” advises Sikkema, an associate professor, field crop weed management specialist at the University of Guelph’s Ridgetown Campus. “If corn is $4.00 a bushel, it’s worth it to get out there and protect the full yield potential of the crop. If corn is $2.50 a bushel, you have to really be careful in terms of your input costs.”

Weed management will still cost about $20 to $40 per acre, he notes, regardless of the commodity price. If corn is worth just $2.50 per bushel, a grower may try to use a single herbicide application. But if corn is at $4.00 per bushel, Sikkema notes, spraying twice may be the most profitable weed management program. “Every bushel is worth so much that it’s worth your investment,” he says. Sikkema can quote from a wealth of research data that points to yield loss resulting from delays in herbicide application, often on a daily basis. “Depending on the field, yield losses can be as high as 3.0bu/ac per day in corn, just by spraying on Tuesday versus Monday morning. Now that corn is around that $4.00 per bushel mark, you’re losing $12 per acre from spraying on Tuesday versus Monday morning in a competitive environment.”

Late herbicide applications may appear to be doing a sufficient job, however, yield losses have likely occurred. Photo Courtesy Of Robert Moloney, FS Partners.

Appearances are deceiving
Good weed management, continues Sikkema, goes beyond mere appearances in the field. A clean field at harvest may look good, but it could be an indication of lost yield potential. “There’s still this perception among some growers that they want to make sure every last weed is up before they spray on their glyphosate, just so they don’t have to spray twice,” he says, calling it a bad management practice to start. “Some growers have the perception that if they spray at the seven leaf stage of corn or the third or fourth trifoliate in soybeans, and it’s perfectly clean at harvest, they’ve done a good job of weed management. For all the appearances, it looks like they’ve done a good job, but they have lost money by spraying too late.”

The answer comes back to two things: scouting and recognizing the specific nature of any field. Moloney contends that growers could alleviate much of the uncertainty of weed size and staging by walking their fields. “If they do the ‘drive by’ while those soybeans are cracking through the ground, it’s only been 21 days and the grower says, ‘There’s nothing coming’, but there is, because that little half or one inch lamb’s quarter that you don’t see has six or eight leaves on it,” says Moloney. “You can’t tell that when you drive by a field.”

“Weed management is field specific,” says Sikkema. In experiments conducted in fields at Ridgetown Campus, where there are 250 acres of test and research plots, results can vary dramatically within a distance of two miles. “It’s all because of differences in soil type, weed species composition and weed density,” says Sikkema. He adds that $30 per acre spent on weed management on all parts of a farm may not be the best investment. “If you use the same weed management program on every acre, chances are you’re not spending enough on weed control on some fields and spending too much money on the others, and you’re losing money on both fields because of it.” -end-

Points to remember on weeds and application timing

The following is a list of tips worth considering when planning spray application timing for corn or soybeans. Each year, Dr. Peter Sikkema, an associate professor at the Ridgetown Campus of the University of Guelph, shares these with his students and Top Crop Manager thanks him for sharing them with our readers.

  1. *Weed species composition. In general terms, broadleaf weeds are more competitive than grasses. With a lot of broadleafs, growers must spray earlier than with annual grasses which are not as competitive.
  2. *Weed density. In short, the heavier the weed pressure, the earlier a grower must spray.
  3. *Relative time of weed and crop emergence. If weeds come up early in the development of the crop, those weeds will be extremely competitive and a grower has no choice but to spray early to avoid substantial yield losses. In contrast, a corn or soybean crop with no weeds for the first four to six weeks after planting is not likely to lose a lot of yield due to weed competition.
  4. *Environmental conditions. In a dry year, there is not enough moisture for both the crop and the weeds, which makes early spraying an easy decision to make.
  5. *Fertility level. Weeds are more competitive in soils with poor fertility; like soil moisture, there are not enough nutrients for the crop and the weeds. Early herbicide application is advised.
  6. *The cost of the herbicide. In this case, glyphosate is a relatively inexpensive herbicide. Even if it requires a second later application to control late flushes, it may be cost effective. In contrast, imazethapyr + bentazon (Clean Sweep) at $30 per acre requires more attention to detail on the part of a grower, and places a greater emphasis on accurate application timing.
  7. *The price of the commodity. As a general rule of thumb, the higher the price of corn or soybeans, the earlier they should be sprayed: a grower wants to protect every last bushel out there. -end-

The Bottom Line

We are not driven by crop price to change our methods: you still have to go for the best possible yield. We always try for the earliest application we can. We may start a little too early, like spraying at the two leaf stage instead of waiting until the three leaf stage, because we never know when we might not be able to get into a field due to conditions. If someone is concerned about a clean looking field, they are still better to go early then spray again later if needed. Looking at growth stage of the weeds is a very valid point. Lennie Aarts, Wainfleet, Ontario.

Excellent article. I see far more problems resulting from weed control timed too late than too early. Time management is definitely a factor in a busy season, but delayed weed control usually cost you yield and often results in more total dollars spent on herbicides to get the job done. I consider timely weed control a high priority in herbicide tolerant crops and an absolute necessity in conventional crops. Alan McCallum, Iona Station, Ontario.

The last decade or so has brought us several excellent post-emergence herbicides, pushing aside the ‘soil applied’ chemistries that dominated the 1970s and ‘80s. However, just because it is now possible to control eight inch quackgrass in corn, or have soybeans magically appear from a forest of broadleaves, doesn’t mean these applications maximized profit. It isn’t the size of the weeds or crop staging the day we start spraying that matters, but the day we finish.

Cool, backward springs are exceptionally deceiving; weeds germinating close to the surface will rapidly advance on ‘marginal days’, while the crop planted at one to 11⁄2 inches barely moves. Ken Nixon, Ilderton, Ontario.


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