Broom snakeweed control and seed damage after herbicide applications

Abstract

Broom snakeweed (Gutierrezia sarothrae [Pursh] Britt. Rusby) is a major weed problem in the southwestern U.S. because it is toxic to livestock and suppresses forage productivity. In this study, broom snakeweed control, seed production and viability were determined after broadcast spraying in 1997 and 1998 with metsulfuron ({2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino]carbonyl]amino]sulfonyl]benzoic acid}; 0.03 kg a.iha-1) and picloram ((4-amino-3, 4, 6-trichloro-2-pyridine-carboxylic acid); 0.28 kg a.iha-1). In 1997, plants were sprayed every 2 weeks beginning 1 October when snakeweed was in mid-flower and seed fill and continued until 15 December when seeds were being dispersed. Broom snakeweed control was not different by spray date and averaged 98% with picloram and 77% with metsulfuron. Only plants sprayed on 1 October 1997 with either herbicide had significantly less seed viability than nonsprayed plants, but seed production was not different. In 1998, herbicide applications were repeated at 2 week intervals for 6 weeks beginning on 1 September when snakeweed was in early-flower and seed development. Broom snakeweed control with picloram (average 88%) was consistently high across all spray dates, whereas, control with metsulfuron (average 25%) was always poor. Both herbicides reduced seed production by an average of 99, 95, and 38% when applied on 1 and 15 September 1998 and 1 October 1998, respectively, but seed production was not different among sprayed and nonsprayed plants after these dates. In the spring of 1999, broom snakeweed seedlings were common in all areas previously sprayed in 1997, but few seedlings established in plots sprayed in 1998. In the spring of 2001, the number of newly emerged broom snakeweed seedlings observed in nonsprayed and herbicide-treated areas was the same, irrespective of spray year, herbicide type or date applied. Data indicate that herbicide applications made at flower when seed is in early fill can provide satisfactory plant control and lower seed production. Spraying after seed has reached physiological maturity does not affect seed production or viability. In this study, results were inconclusive for determining if timed herbicide applications in autumn can be used to minimize later broom snakeweed establishment.