WATER-QUALITY IMPACTS ASSOCIATED WITH PREEMERGENCE AND POSTEMERGENCE APPLICATIONS OF ATRAZINE IN MAIZE

Herbicide contamination in surface runoff varies greatly depending on the herbicide management alternatives used. This study was conducted to determine if herbicide runoff is influenced by the timing of herbicide application with respect to maize (Zea mays L.) emergence. Preemergence (PRE) and postemergence (POST) applications of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] were compared during 1988 and 1989 on a field with no prior history of atrazine use on a Barnes loam (fine-loamy, mixed Udic Haploboroll) soil at the Swan Lake Research Farm near Morris, MN. A rainfall simulator applied precipitation events of 50 min with an intensity of 64 mm h-1 on Days 1,7, and 30 after PRE or POST applications of atrazine. Rainfall 1 d after application resulted in 32 g ha-1 of atrazine (less than 2% of the amount applied) in the runoff water and sediment for PRE treatments compared with 16 g ha-1 (less than 1% of the atrazine applied) for POST treatments. Similarly, 7 d after the herbicide applications, 20 g ha-1 was lost with PRE treatments and 1 g ha-1 for POST treatments. At 30 d, PRE treatments resulted in 8 g ha-1 being transported in the water and sediment compared with 1 g ha-1 for POST treatments. The rate of surface runoff, rather than the concentration of herbicide in the runoff, was the critical factor responsible for the reduction of the quantity of atrazine in the surface runoff with the POST treatments as compared with the PRE treatments.