Growth, Survival and Physiological Effects of Hexazinone and Sulfometuron Methyl Applied Overtop of Longleaf Pine Seedlings

Abstract

A field study was conducted in Santa Rosa County, FL to evaluate the effects of two herbicides, hexazinone and sulfometuron methyl, on longleaf pine seedling (Pinus palustris Mill.) survival, growth, and physiological functions over a 2 yr period (2001-2002). An old-field site was planted with 1-yr-old container-grown seedlings in January 2001, and herbicides were band-applied overtop of the seedlings in April 2001. Hexazinone rates were 0.56 and 1.12 kg ai/ha, and the sulfometuron methyl rates were 0.21 and 0.42 kg ai/ha. Volumetric soil moisture was also measured in May, July, August, and October in each treatment. At the end of the first growing season (2001), survival was not improved by any of the herbicide applications. However, root collar diameter (RCD) increased for hexazinone applied at 0.56 and 1.12 kg ai/ha, and sulfometuron methyl applied at 0.42 kg ai/ha at the end of the first growing season compared to the control. At the end of the second growing season, RCD was 28.7, 30.9, 30.5, 32.1, and 33.3 mm for the control, sulfometuron methyl applied at 0.21 and 0.42 kg ai/ha, and hexazinone applied at 0.56 and 1.12 kg ai/ha, respectively. Also, the percentage of seedlings out of the grass stage at the end of the study was 72, 74, 79, 81, and 89% for the control, sulfometuron methyl applied at 0.21 and 0.42 kg ai/ha, and hexazinone applied at 0.56 and 1.12 kg ai/ha, respectively. Foliar responses, such as net photosynthesis, stomatal conductance, and transpiration were measured in October 2001. There was no evidence of herbicide injury on foliar physiological functions relative to the control seedlings 6 months after treatment. During 2001, soil moisture was related to hexazinone and sulfometuron methyl application rates in May and July, but not in August or October. Averaged across May and July, soil moisture increased from 5.4% in the control to 7.6 and 7.5% in the highest hexazinone and sulfometuron methyl treatments, respectively. Based on seedling survival, growth, and herbicide costs, the most effective treatment was hexazinone applied at 0.56 kg ai/ha.