11 



Control of Broadleaf Weeds 



At locations with perennial broadleaf weeds, 2,4-D and/or dicamba 

 (3, 6-dichloro-o^-anisic acid) was applied early in the fall preceding 

 the planned no-till seeding. 



Broadleaf weed control from these herbicides was usually very 

 satisfactory, however, at Northwood, where dandelions were abundant, 

 2,4-D + dicamba applied in the fall before seeding failed to provide 

 control due to weed dormancy at time of treatment. This was probably 

 induced by the cool, dry weather and the relatively low soil fertility. 

 These herbicides can also be applied in the spring, allowing 3 to 4 weeks 

 for deactivation in the soil before seeding. While application of 2,4-D 

 one day after seeding gave better control of dandelions than did 

 application of paraquat, 2,4-D was excessively toxic to alfalfa 

 seedlings (Table 7) . 



At Pitts field, the judgment that the number of broadleaf weeds 

 were insufficient to justify application of 2,4-D in the fall proved to 

 be in error. On plots treated in the fall with grass-killing herbicides, 

 such as pronamide and glyphosate, dense growth of perennial broadleaf 

 weeds occurred. Glyphosate, applied in the spring, however, gave excellent 

 control of these weeds. 



Table 7. Effects of spring-applied 2,4-D and paraquat on a 1979 sod 

 seeding of alfalfa at Northwood, N.H. 



Alfalfa Dandelion Alfalfa yield. 



Herbicide seedling yield, spring following 



density Oct. 22 seeding 



2 



No. /ft lbs/acre ■ tons/acre 



Paraquat 6.7 304 0.96 



2,4-D 2.7 232 0.49 



LSD (.05) 2.0 62 0.32 



