Pasture Herbicide Residue Effects on Crimson Clover.

To extend the grazing season and provide high quality cool season forage, southern forage producers frequently overseed their warm season perennial grass pastures with legume crops in autumn.  Herbicides are regularly used by producers in the summer months to control broadleaf weeds in grass pastures.  Often those herbicides, or components of them, have considerable residual activity in the soil such that they cause production issues with the fall-planted legumes.  In this study, four replicates of each Chaparral (2 oz/A and 2.5 oz/A), GrazonNext HL (1.5 pt/A and 2.1 pt/A), and Grazon P+D (2 pt/A) were applied to a mixed common/Coastal bermudagrass pasture on April 15, May 15, June 15, and July 15, 2013.  Crimson clover was overseeded across all treatment plots on October 15, 2013.  Herbicide treatment had no apparent detrimental effect on seedling germination or emergence, however, seedling phytotoxic effects were observed for most herbicide treatments applied in May, June and July.  Clover density in February, 2014 was also significantly reduced in all May, June and July herbicide treatments except Grazon P+D, when compared to the control treatment and clover in nearly all herbicide treatments for all months exhibited at least some degree of phytotoxicity in February compared to the untreated control.  The exception was Grazon P+D, which caused less phytotoxic effects than the other herbicides treatments applied in April, May and June.  Under conditions similar to those under which this study was completed, residues of Chapparal and GrazonNext  are likely to cause reduced clover performance if applied less than five months in advance of the anticipated clover planting date.  If a later herbicide application was needed or desired, Grazon P+D would be less likely to contribute to reduced crimson clover performance.  While there are likely differences among clover species regarding their level of herbicide residue tolerance, it may be wise to avoid Chapparal and GrazonNext for summer weed control where fall-planting of legumes is practiced.