Mark Renz, University of Wisconsin Extension Weed Scientist
Forage quality can be reduced by weeds in alfalfa. While this response can be seen at any time during the life of an alfalfa stand, effects are most evident in the first harvest of establishing alfalfa. This loss in forage quality from weeds, while common, can vary widely from field to field and year to year. This is due to weed species, weed density, and the timing of the harvest. Although few believe it, broadleaf weeds can have high forage quality if harvested before or during flowering. But as fields are harvested based on alfalfa development, broadleaf weeds are past flowering and have low forage quality. Annual grasses are also a threat to forage quality, but these weeds are typically not a major problem in the first harvest of spring planted alfalfa as they are still small and don’t contribute much biomass.
Controlling weed species can prevent reductions in forage quality. Research conducted in 2012 across seven locations in Wisconsin demonstrates this concept. In this research glyphosate or imazamox was applied to Roundup Ready alfalfa when weeds were small (4” tall) or 1-2 weeks later (12” tall). When summarizing across locations forage quality was improved whenever an herbicide was applied (see table 1). While glyphosate applied to small weeds had the greatest forage quality (170 RFQ) differences existed between locations. Analysis of each location separately revealed that only four of the seven sites had improved forage quality from any of the treatments (p<0.10) (Table 2). This is likely due to the differences in weed species and density among sites. While glyphosate applied to the small weeds was highest or statistical similar to the highest RFQ at each location, imazapyr applied at the same timing only performed worse at one location (Spooner). While delayed timing of application did reduce RFQ at some locations, this was only at locations with heavy weed populations (Door, Clark, Spooner) (Table 2).
So what do these results mean? They confirm that weeds CAN reduce forage quality, but not always. Reductions in RFQ were closely linked to weed biomass at five of the seven sites. So if you require high quality alfalfa in your establishing fields of alfalfa this spring, you should consider treating fields. Treatments increased RFQ anywhere from 0-46%. While we only studies two herbicides, results suggest that herbicides should be selected that are the most effective on the target species as RFQ was directly related to weed biomass. Please consult Pest Management in Wisconsin Field Crop (A3646) for more information.
For information on other aspects of this project please visit page 85 of the Wisconsin crop management conference 2013 proceedings: http://www.soils.wisc.edu/extension/wcmc/proc/2013_wcmc_proc.pdf