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Hitting the best post-emergence spray timing

Use growing degree days to assess timing.

March 5, 2008
By Gordon Leathers

Figure 1. Critical time of weed removal. Source: MAFRI.

If all the different weeds emerged at exactly the same time, it would make post-emergence spraying a lot easier. Weed control would be more effective, with fewer escapes to impact yield or in returning seeds to the seedbank.

Of course, in the real world, weeds do not work that way. Some, like kochia, downy brome or Japanese brome, emerge very early. Others, like redroot pigweed or green and yellow foxtail, will sneak out later. This may necessitate more than one pass with the sprayer. However, under the right conditions and with a little luck it is still possible to have effective one-pass weed control, according to weed specialist, Bruce Murray with Manitoba’s Department of Agriculture, Food and Rural Initiatives, and Gary Martens, staff agronomist at the University of Manitoba.

“One of the questions I’m getting more and more from producers is ‘when is the right time for me to go in and spray if I only want to spray once?’” Murray says. “It comes down to what we call the critical time of weed removal.”

When lilacs start to flower, about 50 percent of wild oats have probably emerged. Photo By Bruce Barker.

Spraying too early means that weeds emerging later will cut into the yield. On the other hand, spraying too late means that competition from early emerging weeds has already taken its toll and the yields will be reduced no matter what a grower does. “So timing is everything.” Martens explains, “You want to hit the early emerging ones before they become competitive and bite into the yield, and you want to delay long enough for the crop to fill in and get on top of any later emerging weeds.”

The way researchers evaluate the impact of early emerging weeds is to determine the critical time of weed removal. To do this, they will set up a number of plots and vary the timing of weed control. Weeds are sprayed at a specific date and then the plot is kept weed free for the rest of the season. By doing this, only the weeds that have emerged prior to this date affect crop yield. The next plot is sprayed a few days later and then is kept weed free for the rest of the season, and so forth for the remaining plots. This produces the critical time of weed removal, which is the amount of time that weed control can be delayed before yields start to suffer (see Figure 1).

The other side of the issue is how long it takes for the crop to get sufficiently large enough to out-compete any later emerging weeds without a yield hit. This is called the critical weed free period. To determine this, researchers keep the plots free of weeds from the start and then initiate weed control at staged intervals. This helps determine when the crop is big enough to out-compete late emerging weeds. Stopping weed control operations before this time would result in a yield hit (see Figure 2).

Figure 2. Critical weed free period. Source: MAFRI.
Figure 3. Critical time of weed control. Source: MAFRI.

By combining the two graphs, researchers are able to come up with the critical timing of weed control, which is the period of time between the critical weed free period and the critical timing of weed removal. Therefore, based on this hypothetical example, a producer would need to keep this crop clean from the three to six week period (see Figure 3).

It should be noted that in these trials, Roundup Ready crops were used for convenience. These curves should also reflect weed management in non-herbicide tolerant crops, as well as with mechanical control operations like tillage.

Many factors affect the critical timing of weed control duration. With a competitive crop like barley, wheat or canola, the critical timing of weed control will be shorter. With a non-competitive crop like beans, corn or sunflowers, the duration will be longer. The type of weeds and the density of weeds present will affect this curve, as will weather conditions.

Using growing degrees days to estimate timing
To target this critical timing of weed control, weed species identification and time of their emergence is crucial. There is no calendar date for this, though, because weed and crop emergence is dependent on the number of accumulated Growing Degree Days (GDD). GDD are calculated by adding the maximum daily temperature to the minimum, dividing the sum in half and then subtracting the base temperature. This has to be done daily with each day’s number added to the previous totals.

Martens has a simpler way of doing this. He has developed a GDD calendar based on the behaviour of indicator plants that one would find in almost any farmyard, road allowance or woodlot. “We know that at 460 GDD, lilacs will start flowering and that first hint of purple is going to show up,” Martens explains. “And at 460 GDD, we also know that 50 percent of the wild oats have probably emerged.” Martens has compiled this type of information onto a web site. To try the GDD calendar, go to

“It helps farmers save money by giving them the theory behind when to spray, so they can make a correct decision every year,” Martens says. -end-