Abstract
AbstractWeed control in tree nut orchards is a year-round challenge for growers that is particularly intense during winter through summer as a result of competition and interference with management and harvest operations. A common weed control program consists of an application of a winter PRE and POST herbicide mixture, followed by a desiccation treatment in early spring and before harvest. Because most spring and summer treatments depend on a limited number of foliar-applied herbicides, summer-germinating species and/or herbicide-resistant biotypes become troublesome. Previous research has established effective PRE herbicide programs targeting winter glyphosate-resistant weeds. However, more recently, growers have reported difficulties in controlling several summer-germinating grass weeds with documented or suspected resistance to the spring and summer POST herbicide programs. In this context, research was conducted to evaluate a sequential PRE approach to control winter- and summer-germinating orchard weeds. Eight field experiments were conducted in tree nut orchards to evaluate the efficacy of common winter herbicide programs and a sequential herbicide program for control of a key summer grass weed species. In the sequential-application strategy, three foundational herbicide programs applied in the winter were either mixed with pendimethalin, followed with pendimethalin in March, or applied as a split application of pendimethalin in both winter and spring. Results indicate that the addition of pendimethalin enhanced summer grass weed control throughout the crop growing season by up to 31%. Applying all or part of the pendimethalin in the spring improved control of the summer grass weed junglerice by up to 49%. The lower rate of pendimethalin applied in the spring performed as well as the high rate in the winter, suggesting opportunities for reducing herbicide inputs. Tailoring sequential herbicide programs to address specific weed challenges can be a viable strategy for improving orchard weed control without increasing herbicide use in some situations.
Highlights
Orchard crops are grown on over 1.3 million hectares in California, with the tree nuts almond [Prunus dulcis (Mill.) D.A
Tree nuts are primarily produced in the 700-km-long Central Valley of California, where average annual rainfall ranges from 960 mm yr–1 in the north to 180 mm yr–1 in the south (CIMIS 2020)
Season-long chemical weed management is highly desired by growers, when hard-to-control species and/or weed populations that have evolved herbicide resistance are present in the fields
Summary
Orchard crops are grown on over 1.3 million hectares in California, with the tree nuts almond [Prunus dulcis (Mill.) D.A. Weeds are controlled in these intensely managed cropping systems to reduce direct competition with the crop for water and other inputs, both during orchard establishment and throughout the duration of the several-decade lifespan of the orchard. Controlled orchard weeds can interfere with cultural practices, operation of irrigation equipment, or the accurate placement of water, fertilizers, and other pesticides (Belding et al 2004). Walnuts, and pecans are mechanically shaken from the tree, swept into windrows, and picked up from the orchard floor after several days of drying; in these crops, a weed-free and smooth orchard floor is critical to the efficiency of these harvest operations (Company and Gradziel 2017; Micke 1996)
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