a brand of
 

Thwart herbicide-resistant weeds with alfalfa

Thwart herbicide-resistant weeds with alfalfa

By Robin Newell

This article appeared in the February 2024 issue of Hay & Forage Grower on pages 28-31. Click to read the original.

Alfalfa offers a great opportunity to combat the spread of herbicide-resistant weed variants on your farm for two main reasons. First, alfalfa is a forage crop with multiple cuttings per year. The frequent harvest schedule for alfalfa greatly reduces the opportunity for weeds to thrive and produce viable seed. A disc mower or cutterbar does not discriminate between “normal” weeds versus weed variants that have developed resistance to various herbicide groups.

Secondly, alfalfa provides a good opportunity to rotate herbicides by applying different herbicide groups for weed control than you would normally apply to other crops in rotation. The practice of herbicide rotation can be used to break the cycle of herbicide-resistant weed selection brought on by repeated applications of the same herbicide group in the same fields.

Start early with alfalfa

The key period for weed control in alfalfa is during the establishment phase, when alfalfa seedling growth is in competition with emerging weeds. Once alfalfa is established, it’s difficult for weed species to thrive within a productive stand of alfalfa, let alone set seed and produce progeny.

It’s best to plant alfalfa into a clean seedbed free of weeds no matter how small they are. This can be accomplished with tillage and/or burndown herbicides prior to planting and seedling emergence. Preplant herbicide selection can be made to provide residual weed control during alfalfa emergence and early establishment. Scout fields soon after emergence to determine whether an early post-emergence herbicide application will be needed for seedling weed control. The best control will usually be obtained when weeds are very small.

Planting alfalfa with a nurse crop can also be a strategy for suppressing weeds while breaking repeated herbicide use cycles. Small grain nurse crops like oats, barley, or triticale can be planted in early spring along with alfalfa to get a jump on some of the problematic weeds that may emerge later. The nurse crop suppresses weed growth, but it also suppresses seedling alfalfa. Nevertheless, young alfalfa plants begin establishing a taproot during growth under a nurse crop and can typically compete better with weeds following the nurse crop harvest. Harvesting the nurse crop in the boot stage sets up the alfalfa for regrowth following cutting and takes weed biomass off the field before it can set seed.

Fields with a lot of broadleaf weed pressure may still benefit from herbicide application for broadleaf weed control during nurse crop growth. If that becomes necessary, consider a herbicide option that is labeled for both alfalfa and the nurse crop, being cognizant of harvest interval requirements. Any herbicide applied during this phase should ideally be of a different group than herbicides you typically apply to other crops in rotation, especially if your aim is to hit hard against resistant weed variants.

Late-summer planting can be another strategy worth considering, especially for alfalfa seedings following a small grain crop. Emerging weed pressure is reduced versus spring planting, making a late-summer seeding advantageous for clear seeding alfalfa. Ideally the new seeding would have at least 60 days before a killing freeze, giving alfalfa adequate growth and taproot development to become well established. This will allow the new stand to come on strong with good forage production the following spring.

Robust competition is an ally

Once established, weed control in alfalfa takes on a different perspective. As a perennial crop, well-managed alfalfa fields can provide multiple years of robust competition against annual weed species, as well as many perennial weeds. The relatively fast regrowth of alfalfa competes strongly against most weeds, and the routine cutting schedule puts additional stress on weeds that may be present. Herbicide applications for hay or haylage harvested for on-farm use is often considered unnecessary for these reasons; however, commercial hay growers may wish to control weeds for the sake of appearance in the bale, even if weeds are only a minor portion of the harvested plant material. Spiny or toxic weed species always need to be controlled if possible. If these or other problematic weeds are limited to smaller portions of the field, consider spot treatment as a cost-effective control measure.

An often overlooked opportunity for residual weed control can be found in dormant alfalfa herbicide applications. There are several herbicides labeled for application to dormant alfalfa, and most of these herbicides provide residual weed control into the following season. While this can be advantageous from an application timing standpoint, be aware that some of these herbicides have longer rotation intervals as well. You’ll want to be confident in your decision to keep the alfalfa stand through the rotation interval period. Several publications provide information for evaluating potential alfalfa productivity based on stem counts. That information, coupled with your field observations, can aid in decision making.

What about aging alfalfa stands with declining stem counts?

While it might be tempting to keep an old, thinning stand one more year, maintaining adequate weed control can be more challenging. Older alfalfa fields with thinning stands can open up the canopy and provide an opportunity for weed growth. Consider rotating to another crop when older stands begin to thin and lose productivity. Timely rotation not only avoids re-establishing a weed population within thinning, older stands, it also takes advantage of alfalfa’s fixed nitrogen in follow-up rotation crops. The body of research shows that alfalfa can supply all the nitrogen needed for a following corn or sorghum crop in most circumstances. Growing these crops after a good stand of alfalfa has been shown to provide a rotational yield boost that exceeds the nitrogen benefit as well.

Herbicide rotation opportunities

Herbicide rotation has become an important and critical component of herbicide resistance management. Likewise, including herbicides from two or more site of action (SOA) classification groups has been shown to slow the development and proliferation of resistant weed variants. Site of action represents the classification of herbicides into numbered groupings where the herbicides within a group affect a similar site or biochemical process within plants. Most commercial herbicides are effective in controlling a given spectrum of weed species by blocking or disrupting a specific biochemical process in plant function. Unaffected species or herbicide-resistant variants either don’t have the affected pathway, have an alternate biochemical pathway to maintain plant function, or may even have a way to block herbicide absorption or movement into the site of action.

Too much reliance on any one herbicide class can hasten the development of weed resistance in your fields. Herbicidal control strategies that utilize two or more herbicide classes within a given crop, as well as application of different herbicide classes across crops in rotation, are generally recommended.

Herbicide resistance can develop when an individual plant expresses a mutation that enables survival against the specific biochemical action of herbicides within a given SOA class. If a mutant plant survives to produce progeny, and if the weed population is subjected to repeated use of the same herbicide across multiple generations, including other herbicides of the same SOA, resistance can be enhanced.

If plants or seeds of the resistant variant are carried to other fields by equipment, manure, wildlife, wind, and so forth, then a resistant variant can become widespread, especially on farms and in regions where the specific herbicide use occurs over and over again for many generations of selection. We now have many examples of this occurring.

As always, make sure your herbicide application plans follow label directions. Pay attention to harvest intervals and rotational restrictions, especially when considering herbicides that you haven’t used before and may be unfamiliar with. Herbicide labels can be found online, but you may have to search a bit.

Alfalfa offers options

As a perennial crop, alfalfa offers an opportunity to rotate some different herbicide classes not commonly used in typical grain and cereal crop rotations. Good weed management stewardship includes determining the herbicide classes you’ve been using across all your crops in rotation, then striving to use different herbicide classes when you rotate to alfalfa.

The table that follows can serve as a reference for herbicides labeled for alfalfa use, their application timing, herbicide classification group (shown in the table as SOA), and a few key considerations or restrictions. Again, read and follow herbicide labels for herbicide use requirements. Some herbicides have supplemental labels for individual crop practices or additional state requirements.