In the driest parts of the Northern Great Plains, moisture is usually the most limiting factor to crop production. In these areas, the combination of direct seeding and chemical fallow represents a viable option to conserve scarce moisture. It also protects the soil from water and wind erosion and eliminates the expenses of tillage operations. Without tillage, growers necessarily rely on herbicides to manage weeds. However, the success of a fallow period goes beyond multiple herbicide applications and encompasses different aspects of the cropping systems.
An effective chemical fallow starts several months before harvest. Indeed, the most efficient tool to suppress weeds is a well established and competitive crop. Weeds that emerge during the spring and summer may be too large to be managed during the fallow period. As a crop matures, it becomes less competitive as it intercepts less sunlight and uses less moisture and nutrients. Weeds allowed to grow during this period may have no significant impact on crop yield, but could take a toll on moisture and nutrients. Moreover, through seed production, these weeds may increase dockage and replenish the seedbank compromising future crops. Thus, proper weed management during the crop phase is essential for successful chemical fallow systems.
At harvest, it is very important to properly distribute crop residue across the field. If crop residues are allowed to accumulate in windrows behind the combine, inadequate weed control may occur during fallow due to uneven herbicide interception. Also, the cooler and wetter conditions of windrows may favor weed germination. These problems can be avoided by checking the residue distribution pattern of the combine. If crop residue distribution is uneven, consider using a residue spreading attachment such as a straw and chaff spreader.
The early post-harvest period also presents its own weed management challenges. Weeds are usually easier to control when they are small. Thus, if rain allows germination of weeds and volunteer crop during the early post-harvest period, a timely application of Roundup (glyphosate), Aim (carfentrazone), Exrpess (tribenuron) or 2,4-D will control this vegetation before it becomes too large. Also, the use of Roundup in conjunction with soil a residual herbicide such as Spartan (sulfentrazone) can enhance control of large individuals while providing extended weed control.
A weed-free fallow period is essential to maximize moisture storage and nutrient retention. However, weed management during fallow presents several difficulties. First, weeds that have escaped control during the early-fallow period are usually too large or metabolically stressed due to hot weather and water limitation to be controlled with herbicides. Second, herbicides used during the summer fallow period compromise the crops that could be planted during the next growing season. To avoid crop injury due to herbicide carryover, it is essential to pay special attention the herbicide degradation rates and the rotational restrictions. Third, cold temperatures may compromise the effectiveness of herbicides. Finally, late germinating weeds such as kochia, Russian thistle, and prickly lettuce may become a nuisance during this period. Periodic scouting of the fields and timely herbicide application may help minimize the impact of these late weeds.
There are many herbicides than can be used in a chemical fallow system. Among them, Aim, a non residual product to control actively growing broadleaves including ALS resistant kochia; Curtail (clopyralid + 2,4-D), a broad spectrum herbicide to manage broadleaf weeds including Canada thistle, wild buckwheat and common cocklebur; Gramoxone Extra (paraquat) to control emerged annual grass and broadleaf weeds of less than 6 inches height; Roundup for annual and perennial grass and broadleaf control; and Spartan to control several small seeded broadleaves including kochia and Russian thistle. A complete list of the herbicides available and their application rates can be found at the Weed Management Handbook published by the Montana, Utah and Wyoming Cooperative Extension Services.
As always, precautions should be taken to maximize the effectiveness of herbicides. For example, Roundup is one of the favored products for chemical fallow. However, it does not control all weeds equally and it should be applied when plants are less than 12 inches tall. When dealing with larger and harder to manage weeds such as kochia, wild buckwheat and Canada thistle Widematch (fluroxypyr and clopyralid) and Spartan are favored.
Because herbicides are at the core of direct seeding and chemical fallow systems, it is very important to minimize the risk of selecting herbicide resistant biotypes. If the same herbicide is repeatedly used for several years, the applicator removes all susceptible weeds favoring the establishment of resistant plants. For example, Ally (metsulfuron) and Harmony GT (thifensulfuron) provide good control of numerous annual broadleaf species. However, these two herbicides are sulfonylureas, killing weeds by targeting the same enzyme. Continuous use of Ally and Harmony GT increases the chances of selecting for sulfonylurea resistant biotypes. Proper rotation of herbicides with different site of actions, crop rotation and rotation of management strategies remains the best option to prevent the appearance of herbicide-resistant weeds.
In summary, in the driest parts of the Northern Great Plains there is little tolerance to moisture and nutrient depletion by weeds. In these areas, a season-long chemical fallow is a viable option as it helps growers to replenish soil moisture and reduce soil erosion. Because tillage is eliminated from the production system, herbicides remain a main option to control weeds. However, weed management in chemical fallow systems goes beyond multiple herbicide applications and includes all phases of the cropping system. A successful weed management program in a chemical fallow system can only be achieved through the establishment of competitive crops, proper crop residue management and timely herbicide applications. Because of the heavy reliance on herbicides, herbicide rotational restrictions and prevention of herbicide resistance are essential.
Disclosure. Common chemical and trade names are used in this publication for clarity by the reader. Inclusion of a common chemical or trade name does not imply endorsement of that particular product or brand of herbicide and exclusion does not imply non-approval.
Categories: Weed, Chemical Fallow, Kochia, Canada Thistle, Wild Buckwheat
Date: 04/28/2005