Weed Populations and Herbicide Residues in Apple Orchards after 5 Years

Abstract

Weed population shifts occurred when simazine [2-chloro-4,6-bis(ethylamino)-s-triazine], diuron [3-(3,4-dichlorophenyl)-1,1-dimethyl-urea], terbacil (3-tert-butyl-5-chloro-6-methyluracil), and dichlobenil (2,6-dichlorobenzonitrile) were applied annually for 5 years to an apple orchard. The number of brambles (Rubus sp.) and Virginia clematis (Clematis virginianaL.) plants increased with terbacil. usage. Bramble populations increased with the high rate of dichlobenil and goldenrod (Solidago sp.) increased at the low rate. The major vegetation on plots receiving amitrole (3-amino-s-triazole) was not altered from that of a mowed check. However, amitrole reduced growth of shallow planted trees. Postemergence applications of paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) four times during each growing season resulted in more bare soil than was in the mowed checks at the end of the fifth growing season. Yields of apples were higher in the simazine, terbacil, and dichlobenil-treated plots than in the mowed checks. Simazine and terbacil residues did not accumulate in the upper 60 cm of soil during the study; a slight accumulation of dichlobenil appeared to occur in the surface 0 to 15-cm.