Weed Management  Myths and Excuses

“We can eradicate weeds.” On a field scale, we will never eradicate weeds, we can only suppress them. Eradication may be possible with intense efforts on isolated infestations of specific weeds, such as a newly introduced noxious weed.

“Weed seed live in the soil for over 100 years.” Most weed seeds either sprout or decay within several years, especially grass weed seed. It is well documented that weed seed survival increases the deeper placed in the soil. However, in artificial burial studies, a few weed seeds have remained viable for several decades and these results have contributed to this myth.

“Herbicides are the only way to control weeds.” Competition exerted by the crop is often overlooked and under-appreciated. Shading by crops can be very effective -- just look at most any skip in a field. Crop competition combined with other weed management techniques constitute useful tools.

“The herbicide I used grew a good crop.” The herbicide controls weeds but soil, hybrid, variety selection, and crop management imparts yield potential. The herbicide aids in protecting crop yield. More herbicide does not increase yield.

“Scout for weeds? Why? The herbicide has a guarantee!” Also, “I have the same weeds that my neighbor does.”  Scouting is important to match the right herbicide or herbicide combination with weeds present in the field. If it is true that you have the same weeds as your neighbor, do they know which weeds they have?

“I’ll wait until all the weeds are up before I spray.” This strategy may not protect your crop yield because of early season weed competition. The yield of most crops like corn, wheat, sunflower, dry beans are affected more by early season competition. Soybean are more resilient and can tolerate some weeds early. Some weeds exert more competition than others.

“But, the weeds were only four inches tall when they were sprayed!”  Sometimes this can be a hard one to believe until you see the actual size of the dead weeds in the field. Weeds can end up being taller than expected because they grow so fast (rather than people using defective rulers).

“There was no wind, it couldn’t have drifted!” Wind inversions and even small amounts of wind in combination with herbicides and a nearby sensitive plant or crop can result in crop injury or death. With calm conditions, wind currents can change directions several times during an application and move herbicide particles in unexpected directions. A light breeze is actually more desirable because the direction of any drift is more predictable. If there is a sensitive site downwind, the application can be delayed until the wind is blowing away from the site.

“Any crop injury is from herbicide.” Herbicides may or may not be involved in injury symptoms exhibited. All factors including environment, fertility, disease, compaction, insects or combinations of any of these must be examined to determine the cause.

“I always rinse out my spray tank/There wasn’t much left in the tank.” Tank contamination is a common cause of herbicide injury. Some herbicides need more than a quick rinse to remove herbicide residues. Plus, a little chemical residue can go a long way in contaminating the next load.

“Rotational restrictions are on the label only because of lawyers.” It may be true that lawyers approve label wording and labels are legal documents. However, many rotational restrictions exist to protect sensitive crops from injury. This is especially important for the many minor crops grown in N.D. There may be restrictions for other rotational crops because tests have not been conducted to determine extent of residue in the crop. In either case, directions on the label for rotational crops must be followed.

“Sprayer calibration is not that critical.” Ever try spreading a stick of butter (4 oz) on a slice of bread equivalent to 1 acre (43,560 sq ft)? That is the same difference with what we do with herbicides. Sprayer calibration is the only way we can do this successfully. Proper calibration can save money, protect against crop injury and ensure effective weed control.

“If a little bit is good, more must be better.” Sometimes it seems that herbicide applications are made out of retaliation and retribution to punish weeds for existing. Cost of crop production is high enough without making weeds deader. Late applications on large, somewhat tolerant weeds force rates at the high end if a rate range is given, but any time rates exceed label risk of crop injury increases.

—Richard Zollinger, NDSU extension weed specialist

Perennial Weeds: 10 Things Not To Do

1. Ignore small patches -- The infestation is much less trouble to control before the patch spreads.

2. Misidentification -- The first commandment in successful control is knowing the real identity of the enemy.

3. Let plants go to seed -- Perennials can expand by seed and vegetatively.

4. Manage in conventional crops -- The advent of glyphosate resistant crops allows effective and economic control of perennial weeds.

5. Let them have the fence rows -- That’s where many bad infestations start, so monitor them close.

6. Use tillage before planting -- This is not an effective control strategy in a glyphosate resistant crop. Tillage resets the development clock (plants will reach the bud to flower stage later) and may result in erratic emergence than when left undisturbed (except burndown). If perennials are present -- think no-till.

7. Apply to young perennials -- With in-crop applications, apply when perennials are larger and more shoots have emerged. The Hunter-method of Canada thistle control should be used if at all possible -- see ND Weed Guide (online at www.ag.ndsu.nodak.edu/cropprod.htm ).

8. Disregard cultivation -- Perennial weeds are set back considerably by tillage. Cultivation used in combination with herbicides results in better control.

9. Spread propagules with tillage equipment -- Root segments, tubers, and rhizomes can be carried from field to field without proper cleaning and prevention. The rush to get across the fields may cause people to overlook the sanitization of equipment.

10. Forget Mapping -- Fall is an excellent time to map the weed patches and areas in fields where perennials are established. This is better than guessing next spring after you have been out of the field for a while.

—Richard Zollinger, NDSU extension weed specialist

How Weeds Can Affect Wheat

• Ten wild oats or wild mustard plants per square foot will reduce wheat yields 10-20 bushels per acre or 35%.
• Two to three Kochia plants per square foot can reduce yields 30%.
• Canada thistle patches often reduce yields by 60%.
• Green foxtail can reduce yields 10-15% when wheat is planted late.

Source: NDSU

Grass Weed Competition in Wheat

Weeds/ sq. yard Foxtail % wheat Wild Oat yield reduction 10 0 8-9% 50 4-5% 18% 75 6-7% 25% 100 8-9% 34% 150 15% 40%