Controlling volunteer wheat in summer seeding alfalfa

Mark Renz, Extension Weed Scientist

Large infestations of volunteer wheat have been documented to reduce first cut alfalfa yields by 80% the following spring (Ott et al. 1989) and reduce alfalfa stands the following spring.  Research was conducted at the University of Wisconsin Lancaster research station to better understand if volunteer what can reduce the density of late summer planted alfalfa the following spring and if controlling these plants in the fall will prevent these reductions in alfalfa densities the following spring.

For this  experiment, alfalfa ( variety Golden Harvest 6417) was no-till drilled into fields of wheat stubble at 13 lbs/A on 8/18/2008. Volunteer wheat densities were counted in the fall in all plots and ranged between 12 and 49 plants /ft² Densities were variable at this site, allowing us to evaluate reductions in establishment across a range of densities (see picture 1 for an example of the competition). On 9/19/08 Poast (sethoxydim) was applied at 1.5 pt/A along with 1qt/A COC and 2 qt/A 28% N to control the volunteer wheat in ½ of the plots. At the time of application the wheat was mostly 6-7 inches tall, but several new flushes of seedlings wheat were also visible. The alfalfa had 4-5 trifoliate leaves and was 4-5 inches tall. 

Results found that while fall alfalfa densities were typical for a summer seeding (24 plants /ft²) spring alfalfa density was reduced twice as much in the untreated plots compared to treated plots (see table 1). We are currently trying to determine if volunteer wheat’s ability to prevent the establishment of alfalfa is density dependant. Detailed analyses will hopefully determine if any volunteer wheat density threshold exists, above which we would recommend treatment.   

 Table 1. Response of alfalfa density and yield from volunteer wheat

 

Variable	Untreated	Poast	P-value
Fall alfalfa density (plants/ft2)	24	24	ns
Spring alfalfa density (plants/ft2)	14	19	0.003
Loss in density alfalfa fall-spring (plants/ft2)	10	5	0.008
Yield tons/A	2.28	1.61	0.001

Yield from plots taken in the spring of 2009 showed plots were higher yielding if the volunteer wheat was not controlled, but forage quality was undoubtedly reduced in untreated plots from the higher percentage of wheat and timing of when it was harvested (see Picture 2). Currently we are analyzing the forage quality and percent alfalfa and wheat in plots and expect to also have results this fall.  

Picture 2. Forage differences the following spring between treated and untreated plots.

 These data from this experiment suggest that volunteer wheat should be controlled if high quality forage is desired the following spring. If volunteer wheat is left uncontrolled, while higher yielding plots will be present the following spring, we would expect these alfalfa stands to be less productive in future years than treated fields due to reduced alfalfa densities (from 19 – 14 plants /ft² in treated and untreated plots respectively). 

Other factors besides volunteer wheat density could also determine alfalfa establishment and survival. For example the fall of 2008 was very dry at this location, and limited soil moisture likely was another factor that reduced establishment. Regardless our results from this experiment suggest wheat densities that ranged between 12 and 49 plants /ft² can reduce spring alfalfa density in Wisconsin. This experiment will be repeated in the fall of 2009 to better understand how different environmental conditions affect this relationship.

REFERENCE:

Ott. P.B., J.B. Dawson, and A.P. Appleby. 1989 Volunteer Wheat (Triticum aestivum) in Newly Seeded Alfalfa (Medicago sativa L.). Weed Technology 3:375-380.