Weeds - Season 6
Early spring germinating weeds such as kochia, slimleaf and common lambsquarters, Russian thistle, common sunflower, etc., are tall growing weeds that are generally not affected by wheat stands. However, taller varieties have less of a problem with these weeds than medium and shorter varieties. Scattered early germinating weeds, if not controlled, will grow taller in shorter varieties and may cause problems at harvest, necessitating a harvest-aid herbicide.
Weeds - Season 6
Late germinating summer annual weeds can be a problem in wheat fields that have poor wheat stands. However, some weeds, like wild buckwheat, can be a problem even in good stands of wheat. Since it is a vine, it grows up the wheat plants and causes yield loss and harvest problems. Timing is important for controlling wild buckwheat and wild vetch. Apply herbicides later than optimum for winter annual weeds but before the wheat canopy covers the ground. Best results are obtained after these weeds have germinated and before the wheat in in the joint stage. With the increase in plantings of semi-dwarf wheat, we've seen an increase in the amount of wheat acres treated with herbicides. Shorter winter wheat varieties are not as competitive with weeds as taller varieties.
If a grower is going to spray wheat stubble after harvest for ecofallow, many problems can be reduced by controlling broadleaf weeds in spring with a herbicide. Kochia, slimleaf lambsquarters, Russian thistle and common sunflower are the tall weeds that may interfere with harvest and intercept the herbicide before it reaches smaller weeds after harvest. Also, if they are cut off, little leaf area is available to intercept the herbicide for control.
Growers can reduce herbicide inputs by scouting their fields and identifying areas needing treatment. Surveys taken after winter wheat harvest have shown that some fields had a poor stand of winter wheat in certain areas of the field. These areas include waterways, hilltops, or areas where the snow had blown off the fields. Wheat stands in these areas generally had fewer than 280 stems persquare yard at harvest. These areas should be sprayed with a herbicide so that competition from weeds is reduced.
Planting an adapted winter wheat at the proper time improves the wheat's ability to compete with weeds. Apply fertilizer during the prewheat-fallow period or as a starter when the wheat is planted. Delaying all the fertilizer application until spring gives the weeds an advantage. Surveys have shown that fields only receiving spring-applied fertilizer have more and bigger weeds than fields fertilized the previous summer or fall.
Field experiments were conducted at nine locations in Texas and Georgia in 2005 and 2006 to evaluate peanut tolerance to lactofen. Lactofen at 220 g ai/ha plus crop oil concentrate was applied to peanut at 6 leaf (lf), 6 lf followed by (fb) 15 days after the initial treatment (DAIT), 15 DAIT alone, 6 lf fb 30 DAIT, 30 DAIT alone, 6 lf fb 45 DAIT, 45 DAIT alone, 6 lf fb 60 DAIT, and 60 DAIT alone in weed-free plots. Lactofen caused visible leaf bronzing at all locations. Yield loss was observed when applications were made 45 DAIT, a timing that would correspond to plants in the R5 (beginning seed) to R6 (full seed) stage of growth. At all locations except the Texas High Plains, this application timing was within the 90 d preharvest interval. Growers who apply lactofen early in the peanut growing season to small weeds should have confidence that yields will not be negatively impacted despite dramatic above-ground injury symptoms; however, applications made later in the season, during seed fill, may adversely affect yield.
Peanut (Arachis hypogaea L.) production decreased from 582,000 hectares in 1968 to a forecasted 451,170 hectares in 2011 (Anonymous, 2011a). Peanut yield nearly doubled over this period in part due to use of more effective herbicides, peanut genetics with improved disease resistance and greater yield potential, improved irrigation efficiencies, and more efficient cultural practices. However, weeds continue to be a major problem in all peanut growing regions. Weeds can reduce peanut yield 60 to 80% through competition and reduced harvest efficiency (Buchanan et al., 1982; Wilcut et al., 1995).
Peanut has several unique features that contribute to challenging weed management. First, peanut produced in the United States require a fairly long growing season (140 to 160 d), depending on cultivar and geographical region (Henning et al., 1982; Wilcut et al., 1995). Consequently, soil-applied herbicides do not provide season-long control and mid-to-late season weed problems are common. Secondly, peanut has a prostrate growth habit, a relatively shallow canopy, and is slow to shade inter-rows allowing weeds to be more competitive with the peanut plant (Walker et al., 1989; Wilcut et al., 1995). Additionally, peanut fruit develops underground on pegs originating from branches that grow along the soil surface. This prostrate growth habit and pattern of fruit development restricts cultivation to an early season control option (Brecke and Colvin, 1991; Wilcut et al., 1995). With conventional row spacing (91 to 102 cm), complete ground cover may not be attained until 8 to 10 wk after planting. In some areas of the U.S. peanut growing region, complete canopy closure may never be attained.
Control of annual grasses and small-seeded broadleaf weeds can be achieved with a dinitroaniline herbicide applied preplant incorporated (PPI) (Wilcut et al., 1994a); however, control of larger-seeded weeds such as ivyleaf morningglory (Ipomoea hederacea (L.) Jacq.) must occur with the addition of other herbicides such as imazethapyr and imazapic (Cole et al., 1989, Grey et al., 1995, Grichar et al., 1994, Wilcut et al., 1991a, 1994c, 1995). Imazethapyr applied postemergence (POST) provided broad-spectrum and most consistent control when applied within 10 d of weed emergence (Cole et al., 1989; Grey et al., 1995; Grichar et al., 1992; Wilcut et al. 1991a, b; 1994b, c). Imazapic applied POST controls the same spectrum of weeds as imazethapyr (Nester and Grichar, 1993; Grichar et al., 1994; Wilcut et al., 1993, 1994c, 1995). In addition, imazapic provided control and suppression of Florida beggarweed [Desmodium tortuosum (S.W.) D.C.] and sicklepod [Senna obtusifolia (L.) Irwin & Barneby], which are not adequately controlled by imazethapyr (Grey et al., 2001). The limiting factors on the use of imazethapyr and imazapic are the rotational restriction (18 mo.) to crops such as cotton (Gossypium hirsutum L.) and sorghum (Sorghum bicolor L. Moench) and the potential development of weeds resistant to the ALS-inhibiting class of herbicides (Grey et al., 2005; Matocha et al., 2003; Wilcut et al., 1995; York and Wilcut, 1995).
Lactofen was registered for use POST in peanut in 2005 for control of several annual broadleaf weeds including annual morningglory. Lactofen is classified as a diphenyl ether (cell membrane disruptor) which interferes with protoporphyrinogen IX oxidase synthesis and causes accumulation of protoporphyrin IX (Duke et al., 1991). Protoporphyrinogen IX is a potent photosensitizer that generates high levels of singlet oxygen in the presence of molecular oxygen and light, leading to light-induced oxidative breakdown of cell constituents (Duke et al., 1991). In general, herbicides classified as cell membrane disruptors (contact inhibitors) must be applied to small weeds. In previous weed control studies, lactofen controlled 99% Palmer amaranth (Amaranthus palmeri S. Wats.) in two of three yrs, while controlling this weed 80% in the third yr (Grichar, 1997). Grichar (1994) reported that lactofen at 0.28 kg/ha controlled 92% spiny amaranth (Amaranth spinosus L.) when applied early postemergence (EPOST), but control was less than 80% when applied late postemergence (LPOST); however, Jordan et al., (1993) reported that lactofen applied LPOST controlled prickly sida (Sida spinosa L.) and morningglory species (Ipomoea species) as well as lactofen applied EPOST. Lactofen controlled 86% common lambsquarters (Chenopodium album L.) when applied at ground-crack (GC), but control declined to 34% when applications were delayed 2 wks (Wilcut, 1991). Lactofen controlled 87 to 95% prickly sida and 83 to 86% morningglory species when applied at GC and 2 wks after GC, respectively, but control declined to 0% when applications were made at 4 wks after GC. Wilcut et al. (1990) reported that broadleaf weed control systems containing sequential lactofen systems control common lambsquarters 100% and morningglory species 86%.
Early-season lactofen applications (6 lf) caused peanut stunting and foliar injury up to 11%, whereas the 45 DAIT treatment injured peanut 8% when evaluated late August in 2006 (Table 3). Lactofen had no effect on the incidence of tomato spotted wilt virus in either year (data not shown).
Negligible visual injury was observed at this location over the course of the growing season (Table 3). Similar transient peanut injury with lactofen has been previously reported at Plains, Georgia for POST applications (Moore et al. 1990).
Pollen counting began in the Great Basin region at the start of the 2009 tree season in mid-January. Counts started at the low range mid-January and were observed in the high range by mid-February. Very high counts for trees became noted briefly in April and returning to the high range from May to end of June. Moderate levels of trees are noted in the Great Basin mid-July and fell to low counts noted mid-August. The tree season ended completely mid-November.
Grass season started with low counts in mid-April, moderate counts at the start of May and achieved briefly high counts at the end of May. Moderate counts in the 2009 season persisted to mid-July, when counts entered the low range consistently. Grass season ended at the end of September 2009.
Weed season started in the low range mid-May of 2009. Moderate counts were noted at the end of July. Weed pollen measurements dropped to the low range mid-Octorber and persisted in this range until the end of counting season in mid-November. 041b061a72