A herbicide efficacy trial in winter wheat conducted near Havre, was a perfect example of how a competitive crop is the best weed management tool available to producers. The trial included 36 herbicide treatments and 2 non-herbicide treatments. At the time of treatment, the wheat was at the 5 to 6-leaf stage, kochia was 1 inch tall, mustard species were 3 inches or more, and pigweed was in the cotyledon stage. By harvest time, at the end of the trial, all of the weeds were gone -- even in the non-treated areas. All herbicide treatments were completely effective. However, the absence of weeds at the end was due to the crop smothering out the weeds. The crop will not always provide complete weed control, but herbicides will be less effective in the absence of a competitive crop.
What makes a crop competitive? A competitive crop utilizes resources before they are available to the weeds. The essential steps of developing a competitive crop begin with good stand establishment. Components of establishing a good crop stand include seeding rate, seed quality (good germination percentage), seeding depth, moisture at seeding, good soil to seed contact, fertilizer placement, and seed treatment.
By properly managing the factors influencing stand establishment, the vigor and growth of new seedlings improves. Failure to properly manage these components leads to poor germination, weak seedlings with poor growth and vigor, increased winter kill and increased susceptibility to disease and insects. This ultimately leads to open crop canopies and inefficient use of water and nitrogen – opening the door to weed invasion. Furthermore, improperly managing stand establishment factors leads to increased reliance on herbicides for protecting crop yield and preventing weed seed production.
A vigorously growing crop is also important towards establishing a competitive stand. Crop growth and vigor (resource consumption) can be influenced by stand density, row spacing, seeding band width, fertilizer rate and placement, disease management, insect management, variety selection, relative emergence time of crop and weed, soil moisture, crop seeding date, crop rotation and crop type.
Factors that impact crop growth impact the ability of the crop to use resources such as light, nitrogen, and most importantly water. By influencing available resources, the crop interacts with weeds, thereby influencing weed reproduction. In contrast, resource use by weeds can impact crop yield. Injury from disease, insects, poor soil moisture or fertilization management, or other factors that impact crop growth can lead to inefficient use of resources. Again, opening the door for weed invasion and competition, increasing reliance on herbicide use for weed management.
Crop rotation is an effective means of accomplishing disease, insect, moisture and fertilization management with minimal reliance on inputs such as fungicide, insecticide, herbicide or fertilizer. Several studies have been initiated across Montana to document benefits of crop rotation as well as the best methods of rotating crops.
Research weed scientists and agronomists at MSU have invested efforts into studying methods to improve crop competitiveness relative to weeds.
Current recommendations for seeding rates were established in the mid-1950's, in small plots, under weed free conditions. Recommended seeding rates were established in non-weed conditions and set at the minimum crop density that resulted in the maximum yield. Add wild oat to the situation and seeding should be increased 25 to 50 percent over the recommended 60 to 70 lb/A to maximize yields and minimize weed seed set. The best way to increase seeding rate is to narrow row spacing, but even at 12" row spacing, seeding at 90 lb/A improved yields in weedy conditions. Under drought conditions increasing seeding rate may decrease crop yield. Thus, during times of drought seeding rate should not exceed 80-90 lb/A to avoid yield loss.
Bruce Maxwell, weed ecologist at MSU in Bozeman, and Bob Stougaard, weed scientist at NWARC-Kalispell, studied wild oat competition with barley. They observed that increasing the seeding rate of barley two-fold significantly decreased wild oat seed production (Table 1). Further, the increased seeding rate decreased barley yield loss as well.
|
Barley |
Barley |
Wild Oat |
|
Seeding rate density (lbs/acre) |
Yield loss (%) |
Seed production (seed/ft2) |
|
0 |
-- |
2000 |
|
60 |
20 |
1600 |
|
120 |
10 |
1300 |
A.J. Bussan, cropland weed specialist, studied Persian darnel by wheat competition. The first part of Bussan’s study examined the effect of density on the seed set of Persian darnel and yield. The second part involved the effect of Persian darnel on reproductive tillers. Increasing seeding rate increased wheat tiller production when competing with Persian darnel, thereby minimizing yield loss due to competition.
Bob Stougaard and Gregg Carlson, agronomist at NARC-Havre, researched crop density by band width effects of spring wheat on wild oat. They found that increases in seeding band width of spring wheat increased spring wheat population density, head numbers, and yield when oat was present (Table 2). Increasing the band width also decreased wild oat dry weight and seed yield. Further, increases in spring wheat planting density and subsequent population density increased spring wheat dry weight, head numbers, and grain yield as well as decreased wild oat dry weight and seed yield. Lastly, Stougaard and Carlson also measured the light penetrating the spring wheat canopy and, as expected, found that it decreased under both the wider band width and the higher planting densities. This suggests increasing seeding rate and band width increased the light (resource) capture of spring wheat, decreasing the light available for the oat. Possible added benefits of this method that were not measured, could include decreased evaporation because the extra shading by the wheat canopy would have kept the soil cooler and protected from the wind, thereby conserving moisture.
|
Seed band width |
Spring wheat yield (bu/A) |
|
|
1998 |
1999 |
|
|
Narrow |
25 |
33 |
|
Medium |
27 |
34 |
|
Wide |
28 |
35 |
Bussan conducted a study near Sun River that examined the effect of fertilizer rate on wild oat management. Zero, 20, 40, 60, 80 and 100 lb/A of N was applied per year for 3 years in a field of continuous spring wheat. In the second year, the 0 and 100 lb/A treatments of N had twice as much wild oat as the other treatments. In the third year, the 100- lb/A of N had the most wild oats of all the nitrogen treatments including areas treated with full label rates of Puma (2/3 pint) and Achieve (0.60 lbs). This suggests wild oat can take advantage of excess nitrogen. Furthermore, the third year of the zero lb/A of N had no wild oat in any herbicide treatment and the fewest wild oat in non-treated areas.
This is not a recommended management strategy. But limiting the availability of nitrogen to wild oat can decrease growth and improve management. In addition, poor nitrogen fertility can stress wheat and potentially increase weeds other than wild oat.
One method of limiting the availability of nitrogen to wild oat while still fertilizing the crop is by deep banding nitrogen. More research is needed in order to continue improving fertilizer rate and placement as well as the interaction with weed management.
Luther Talbert, spring wheat breeder at MSU, studied spring wheat competitiveness among varieties. Spring wheat varieties varied in their impact on wild oat with some varieties having a larger impact on wild oat seed set than others. Taller varieties tended to have a larger negative impact on wild oat than shorter varieties. In addition, highest yielding varieties were best at suppressing weed growth. Impacts were not significant enough to use variety selection as a weed management tool - yet. Research needs to be conducted to determine how to use different spring wheat varieties in combination with increased seeding rate, narrower row spacing, and herbicides to most effectively manage weeds.
Weed management has become increasingly reliant on herbicides for successful crop production. However, by implementing best management strategies for crop production and by focusing on increasing the competitive effect of the crop on weeds, the overall production may be increased. Crop competition, may or may not provide sufficient weed management alone. However, by combining increased crop competitiveness with herbicides targeted at specific weed problems producers will improve weed control in the current year and weed problems will be diminished over the long-term.
This is a brief introduction into factors that effect the competitiveness of crops (specifically small grain) and research activities at MSU to give insight into better weed management methods.
Categories: Weed, Integrated Weed Management
Date: 04/28/2005