ALTERNATING ACTIVE INGREDIENTS IN SEQUENTIAL APPLICATIONS OF PREEMERGENCE CRABGRASS HERBICIDES. A.J. Patton*, D.V. Weisenberger, and Z. Reicher, Purdue University, W. Lafayette, IN (85)
Crabgrass (Digitaria spp.) is a problematic weed for many turf managers. The traditional approach of applying one application of a preemergence herbicide in the spring does not always provide adequate season-long control. Therefore, many turf managers will apply half the label rate in the spring prior to crabgrass germination and then make a sequential application of the same preemergence herbicide approximately 60 days later in order to extend the window of herbicide activity. Turf mangers, especially lawn care operators, often need increased flexibility in their preemergence herbicide applications due to the number of customers they have and the large application date window that they use. For most consistent crabgrass control, previous research and practical experience suggests the initial and sequential application should consist of the same active ingredient. However, that research was reported in 1991 and active ingredients and/or recommended rates of active ingredients have changed since then. Therefore, the objective of this research was to compare the effectiveness of various sequential applications of dithiopyr, prodiamine, and pendimethalin when the preemergence applications were followed by the same or a different herbicide for the sequential application. A four by four factorial was used with four herbicide treatments (dithiopyr at 0.28 kg/ha, prodiamine at 0.42 kg/ha, pendimethalin at 1.68 kg/ha, and untreated) used for the initial application at half label rates followed by the same four herbicide treatments used for the sequential application at half label rates. Additionally, full rates of dithiopyr at 0.56 kg/ha, pendimethalin at 3.36 kg/ha or prodiamine at 0.73 kg/ha were included for comparison in both 2009 and 2010 in West Lafayette, Indiana. Separate but adjacent experimental areas were used each year, and the experimental design was a randomized complete block with three replications. As expected, sequential applications of all nine herbicide combinations (omitting the untreated) resulted in less crabgrass cover by August (<21%) compared to single spring applications at the full rate (>25% cover) for dithiopyr, pendimethalin, and prodiamine. Because of the postemergence crabgrass control from dithiopyr, dithiopyr as the sequential June application following an untreated March application consistently provided better crabgrass control than when prodiamine or pendimethalin were used as a sequential application following no herbicide in March. There were no significant differences in crabgrass cover when the same or different active ingredients were used for the initial and sequential applications. This suggests that turf managers can switch from one active ingredient to another when making sequential applications To support these findings, this study will be repeated in 2011 in West Lafayette, Indiana and at two locations near Lincoln, Nebraska.