[Edited_by_A._Ciancio,_C.N.R.,_Bari,_Italy_and_K.(Bookos.org)

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J.D. DUTCHER ET AL.
crop. Density studies have determined the effect of increasing weed density in
season-long competition with the crop. Period of competition studies have
determined the length of time the crop can tolerate competition from a specific weed
at a specific density without incurring yield loss; or conversely, how long the crop
must be maintained free of this weed before it can produce an optimum crop.
Different types of information can be obtained with these studies. The critical period
of weed control (CPWC) has been defined by Knezevic, Evans, Blankenship, Van
Acker, and Lindquist (2002), as the time interval between two measured crop-weed
competition components: (i) the maximum amount of time early-season weed
competition can be tolerated by the crop before a yield loss occurs, and (ii) the
weed-free period required from planting to obtain optimum yield (independent of
other confounding factors, i.e. late season drought, insects, diseases, etc.). This
period has been shown to vary from zero up to several weeks in work done with
annual crops and weeds (Knezevic et al., 2002). In layman terms this answers the
questions; how long must I maintain weed control in the crop to obtain optimum
growth and yield, or how late can I wait to initiate and maintain weed control
without incurring yield loss? Because most of the weed-competition studies were
conducted using annual crops grown in narrow spacings (< 1 m) competing with
annual weeds, weed control measures were generally applied on a “broadcast” basis.
This is generally not the case with weed management in pecan orchards.
Current weed management in pecan orchards involves weed control within a
narrow strip centered on the pecan tree row. Also, many of the most troublesome
weeds in pecan orchards are perennials. The CPWC has never been determined for
pecan and would probably take several years research to determine. Smith (1999)
determined that substantial reduction in growth of pecan trees occurred by weed
competition from cutleaf eveningprimrose (Oenothera laciniata), a cool-season
species, implying that the presence of weeds results in reduced growth of pecans
even in cool months.
Although several methods of orchard floor management have been used in the
past, including mowing and cultivation (Ark, 1954), the use of registered herbicides
is currently the most common method used by far (Alabama Cooperative Extension
System, 2007; Georgia Cooperative Extension Service, 2007; Smith & Carroll,
2004). Herbicidal active ingredients used to maintain weed control in pecan orchards
include oryzalin, pendimethalin, diuron, simazine, norflurazon, fluazifop,
sethoxydim, clethodim, flumioxazin, halosulfuron, paraquat, glufosinate,
glyphosate, and carfentrazone. These herbicides can be used at different times and
for specific weeds in orchard floor management. Some are restricted to non-bearing
trees only, and some to trees established at least 2–3 years in the orchard. Most
orchard floor management programs use a combination of these products to
maintain season-long control in a weed-free strip centered on the tree row.
Combinations of registered herbicides used in both preemergence and
postemergence programs can provide optimum growth and yield (Faircloth,
Patterson, Foshee, Nesbitt, & Goff, 2007).
The area between tree rows is traditionally maintained in grass sod to facilitate
movement of spray equipment that is used for fungicide and insecticide applications
during the growing season. Consequently, this is known as the “sod-strip” method of