by Fabian Menalled, MSU Crop Weeds Specialist, Department of Land Resources and Environmental Sciences


Figure 1: Photo of a wheat field with mountains in the distance

Figure 1: Wheat field. Photo courtesy of USDA-ARS.

Many times we think of crop harvesting and weed management as two independent tasks. Yet harvesting provides a great opportunity to improve weed management. First and foremost, while in the combine farmers have a unique opportunity to reflect on the season’s successes and failures. Systematically traveling across the fields provides a great chance to reflect on approaches for the next crop’s weed management program by carefully checking locations of which species thrived this year. The next step is to find out what went wrong and what can be improved. The following is a list of some of the many things farmers could think about at harvest.

Weeds Occur in Patches

In general, weeds are not distributed uniformly across fields but in patches of high densities. Several causes could be responsible for these patches. Is it possible that you have selected herbicide resistant weed biotypes? Did you get bad crop establishment during the summer that resulted in a less competitive canopy at the site of the weed patch? Is there any underlying nutrient or moisture characteristic at that site that could have resulted in an increased weed survivorship and growth? Carefully considering these and other potential mechanisms responsible for the success of the weed population you detect in a particular field can help adjust the management approach to prevent the growth of these patches.

Post-Harvest Weed Management

Weed species such as kochia are difficult to control once they have been cut by the combine because they drop their seeds in one spot. Because kochia seedlings can emerge at any time during the winter, they can produce dense clumps of seedlings which are very hard to control as their mass impedes effective herbicide coverage. Post-harvest treatments with glyphosate (Roundup and other generic names) and paraquat applied late August to early September when kochia plants are actively growing and have produced enough leaf tissue for herbicide absorption can help to substantially reduce seed production. However, post-harvest herbicide options should not be based or planned solely on the weed species currently in the field, but also take into account the spring planting intentions.

Impact of Harvesting on Weed Communities

Weed species are differently influenced by crop harvesting operations and their strength and selectivity depend on timing and technique. For example, repeated growing of the same crop harvested at similar times and by similar methods selects specific weed species. The timing of harvest and stubble height are decisive selective factors for which weed species will produce seeds, the amount of seeds being produced, as well as dispersal patterns. For example, high stubble can lead to higher seed production of species such as common mallow or prostrated knotweed. Variations in timing and methods of harvesting and diversified crop sequences can help avoid selecting for specific and difficult to manage weed species.

Figure 2: Photo of a tractor harvesting a barley field, rolling hills in the background

Figure 2: Barley harvest. Photo courtesy of USDA-ARS.

Managing Winter Annual Species

Winter annual species such as cheatgrass and jointed goatgrass germinate and emerge in late summer, become semi-dormant and overwinter, resume growth in early spring, and flower and complete their life cycle the following summer. Farmers increasing acreage of winter crops shouldn't be surprised that winter annual weeds become a widespread management issue. Managing winter annual weeds starts in fall when they are more susceptible to weed control practices and scouting for their presence can give a head start on management.

Harvest the Weediest Field Last and Carefully Clean the Combine

Leaving the worst fields for last is a simple approach to minimize the spread of weed seed and has been shown to be economically effective. Carefully cleaning equipment is another simple approach to minimize the transfer of weed seeds between fields. These simple steps can help farmers minimize the spread of weeds, including herbicide resistant biotypes.

Fall Is the Time to Manage Perennial Weeds

As fall temperatures cool, growers have an opportunity to manage perennial weeds. Cooler temperatures trigger the movement of food reserves down to the root systems, enhancing movement of herbicides to the plant’s root system and improving control. However, farmers should be aware that perennial species vary in sensitivity to frost, and the application window differs between species. For example, Canada thistle can survive light frosts and is effectively controlled with relatively late fall herbicide applications. Other perennial weeds such as hemp dogbane and common milkweed complete their life cycles by late summer and do not tolerate frost well, so fall herbicide applications should not be delayed when controlling these species. Finally, although fall application will not guarantee excellent control of field bindweed, late control practices can be effective, provided there is re-growth of this species.

These are just a few fall season considerations farmers can take into account to develop effective integrated weed control programs.

Further Information

Contact Weed Ecology and Management specialist Fabian Menalled with questions or comments about weed management during crop harvest.

This article was originally published in the Fall 2014 issue of the Montana IPM Bulletin.


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