Improving Weed Management

BOZEMAN - Apr 4/05 - SNS -- Crop producers are faced with many critical crop and weed management decisions in the spring, says Montana State University Extension Service Cropland Weed Specialist Fabian Menalled.

To help, Menalled outlined the following 10 tips to help producers successfully manage weeds and secure a good harvest. His tips are:

1. Establish a healthy crop.

A well-established and healthy crop that develops a uniform stand is perhaps the most effective tool you have against weeds. Use the best available agronomic practices to get a good and early start and a close crop canopy. More and more studies are showing that a highly competitive crop shades out late emerging weeds, reducing the need for rescue treatments and the overall cost of herbicides.

2. Know your weeds and match specific problems to specific solutions.

Periodic scouting will allow you to know what weeds are present across your crop acreage. Be aware that "windshield scouting" will not give you a precise assessment of the various weed species that grow in your fields, their size, location and density. You need to walk or four-well drive your fields in a systematic pattern. Take a field map with you and trace the location of the different weed patches. This map will allow you to spot treat a field instead of using a blanket application.

3. Time your management practices.

Another benefit of periodically scouting your fields is that you will be able to better determine the most appropriate time to implement a weed management practice. If a weed community is dominated by early-emerging species such as prickly lettuce, common lambsquarters, field pennycress or common sunflower; postponing planting will allow you to eliminate these seedlings with tillage or herbicide. On the other hand, early planting is a reasonable approach to managing late-emerging species such as the morning glory, yellow foxtail, and common cocklebur. The early planting lets the crop to get ahead of the weeds. Be aware that if you postpone your postemergence treatments too much with the goal of controlling late emerging weeds, you may need to use higher herbicide rates. Moreover, you may have significant yield reductions due to early season competition.

4. Calibrate your sprayer and check each nozzle individually.

Usually, growers calibrate their equipment to make sure they apply the correct amount of gallons per acre. However, individual nozzle output may vary. For example, nozzles may over-apply by 10 to 30 percent due to poor calibration and worn tips. Although checking each individual nozzle takes time, doing so will result in more accurate applications and reduce the risk of weed escapes or crop injuries.

5. Check the weather.

An actively growing weed is a key factor to getting good control, whatever the herbicide being used, and it may be necessary to wait until temperature and moisture conditions are suitable to achieve good control. As a general rule, burn-down herbicides should not be applied when temperatures are in the low 50s or upper 70s. Likewise, do not spray postemergence herbicides when there has been a prolonged dry period or temperatures are in the high 90s. Usually this causes excessive crop damage and is not very effective at controlling weeds.

6. Know your soils.

Soil-applied preplant incorporated and preemergence herbicides are valuable tools to manage early season weeds and, if they are residual, can provide season-long weed control. However, miscalculations could cause crop injury or fail to control weeds. Among the many factors that determine the concentration and persistence of soil-applied herbicides are soil factors such as texture, acidity (pH), moisture and organic matter. As general rule, soils high in organic matter or clay may require higher rates or more frequent herbicide applications than sandy and coarse soils. Also, the risk of herbicide carryover varies with soil pH.

7. Avoid cosmetic control practices.

The main goal of a weed management program should be to secure crop yields. Before managing your weeds, take into account that factors such as soil moisture, weather, and crop and weed stage will influence how competitive a weed is. For example, late emerging weeds tend to have little or no impact once the crop has formed a closed canopy. Thus, postemergence control practices should be concentrated during the early stages of the crop development and late emerging plants could be disregarded. Yet, extremely large and heavy infestations should not be tolerated. This is particularly true in when dealing with highly competitive and difficult to control weeds such as jointed goatgrass, Canada thistle and field bindweed.

8. Work against herbicide resistance.

Weeds, like any other living organism, are variable. Some of them have an innate ability to survive and reproduce after a treatment with a dose of herbicide that would normally be lethal. Every time you apply a herbicide you are killing the susceptible plants and selecting potential herbicide resistant individuals. If during several years you use the same herbicide over and over again, you are increasing the selection pressure and speeding development of herbicide resistant weeds. To reduce the risk of selecting herbicide resistant biotypes, you should rotate among herbicides with different site of action, applied either as tank mixes, premix formulations or sequential applications. Also, you should rotate management practices, such as the incorporation of timely cultivation. Finally, crop rotation is an excellent tool to reduce the selective pressure on herbicide resistant weeds.

9. Minimize weed seed production.

There is an old proverb that says "one year's seeding, seven years weeding." Weeds can produce anywhere from a couple of hundred to many thousands of seeds per year. Thus, preventing seed production is a key component in the development of a successful long-term weed management program. This is particularly true when dealing with weed species that have a persistent seedbank such as field pennycress and common lambsquarters.

10. Be a good neighbor.

Building a strong relationship with your neighbors, local Extension agents, ag chem dealers and seed representatives can help you make sound decisions and save money. For example you could benefit from early pay and bulk discounts. Also, they are an invaluable source of information on new alternatives to manage your weeds.

In summary, a good weed management programs starts with a healthy crop. Integrating practices is essential to increase the success of a weed management program and secure high yields. In doing so, you need to combine cultural, mechanical, and chemical control practices to limit the introduction and spread of weeds, help the crop compete with weeds, and make it difficult for weeds to adapt to the specific growing conditions of crop fields.