Weed competition can reduce growth (Patterson et al., 1990; Smith et al., 2002; Wolf and Smith, 1999), yield (Foshee et al., 1997; Patterson and Goff, 1994), and nut quality (Daniell, 1974) of pecan trees. This reduction in growth has been attributed to competition for nutrients (Bould and Jarrett, 1962; Goff et al., 1991; Worley and Carter, 1972) and water (Patterson and Goff, 1994; Ware and Johnson, 1958).
Benefits from improved weed control are well-documented (McEachern and Storey 1984; Norton, 1970; Norton and Storey, 1970; Patterson et al., 1990; Patterson and Goff, 1994). Several studies have shown that eliminating all weed competition dramatically increases early growth and yield of young pecan trees (Foshee et al., 1997; Smith et al., 2002, 2005a; Wolf and Smith, 1999). In addition, mulching has been shown to be effective in increasing growth of young pecan trees, although the application of mulch is labor-intensive and costly (Merwin et al., 1994; Smith et al., 2000).
An optimum vegetation-free area has been established by Smith et al. (2005b) to be a circle 1.83 m in diameter around young pecan trees in bermudagrass [Cynodon dactylon (L.) Pers.] sod. This resulted in maximum tree growth during the first 5 years of growth. Smith et al. (2002) showed similar results in tall fescue [Schedonorus phoenix (Scop.) Holub (syn. Festuca arundinacea Schreb.)] sod with maximum tree growth obtained with a vegetation-free circle 0.91 m in diameter around newly planted pecans after only 2 years.
Wolf and Smith (1999) reported that growth of young pecan trees was reduced dramatically when one selected dicotyledonous weed species [cutleaf evening primrose (Oenothera laciniata Hill) or Palmer amaranth (Amaranthus palmeri S. Wats.)] was allowed to grow near pecan seedlings, possibly as a result of some allelopathic interaction. In a follow-up study, Smith et al. (2001) documented that growth of container-grown pecans was reduced when trees were irrigated with leachate collected from pots of cutleaf evening primrose.
Typical weed control practices in pecan orchards include preemergent and postemergent herbicide applications (Patterson et al., 1990). Either paraquat [1,1′-dimethyl-4,4′-bipyridinium dichloride] or glyphosate [N-(phosphonomethyl)glycine] is used for most postemergent applications (Patterson et al., 1990). Paraquat is a restricted-use pesticide and an Environmental Protection Agency (EPA) Class I pesticide as a result of acute toxicity (Miller, 1996a). Glyphosate, on the other hand, is an EPA Class IV pesticide and is very safe for applicators (Miller, 1996b). Smith et al. (2005a) reported that glyphosate controlled all weed species in a study conducted to determine the effects of temporal weed interference in young pecan trees. Glyphosate has excellent postemergent activity against most dicotyledonous and monocotyledonous weed species. Reynolds et al. (2000) and McClelland and Webster (1998) reported excellent glyphosate activity on cutleaf evening primrose, which is typically not controlled with preemergent herbicides recommended for pecan orchards in the southeastern United States (Wolf and Smith, 1999). Consequently, glyphosate is commonly used in many pecan orchards (Patterson, 1997), and inadvertent herbicide contact with trees through spray drift is a recurring hazard and a concern for growers.
This 4-year study was conducted to evaluate the effects of glyphosate applied to the trunks or foliage of young pecan trees as reflected by mortality, trunk cross-sectional area (as a measure of growth), nut yield, and nut quality (nuts with kernels). Application treatments were selected to simulate situations ranging from minor spray drift to major misapplication.
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