A growth room experiment compared seedling growth after nine weeks of two wheat genotypes in pure cultures and mixtures with Italian ryegrass at a low phosphorus (P) level and the P level recommended by soil testing. At the recommended P level in both pure and mixed culture, Mit, a semi-dwarf genotype, had a greater height, leaf area, tiller number, and dry weight of leaves, stems and roots than did Kharkof, a landrace. These results reflected the visual selection criteria for seedling vigor and tillering used in the initial development of the semi-dwarf cultivars. Field experiments compared total dry matter, calories, and percent crude protein and acid detergent fiber of oats, wheat, and ryegrass in pure culture and mixtures at four locations in central Texas at first clipping, second clipping, and unclipped. Although, forage yield was lower for the first clipping than the second clipping or the unclipped treatment, the first clipping had the best forage quality. There was a strong environmental influence on yield and quality. The best forage was pure wheat at College Station, pure ryegrass at Marlin, the wheat-ryegrass mixture at Temple, and pure oats at Thrall. Since Temple had the greatest yield of any site, the wheat-ryegrass mixture demonstrated the highest yield potential. The soil adsorption characteristics of halosulfuron was examined using six soils. Soil adsorption of halosulfuron appeared to be a function of organic matter. Low Kf,ads values indicated that a relatively high plant availability of halosulfuron could be available for plant uptake. Sorghum produced in soils with a low organic matter content may experience injury because of the high plant availability.

Management of volunteer horseradish is a major challenge in fields when horseradish is included in the crop rotation. Three field studies were conducted: 1) to evaluate the growth habit and density of volunteer horseradish, 2) to evaluate the efficacy of fall herbicide applications for control of volunteer horseradish, 3) to evaluate the tolerance of subsequent horseradish crops to persisting soil residues of halosulfuron. One year following horseradish production, field infestations of volunteer horseradish ranged from 0.08 to 6.60 plants/m2 with the majority of plants emerging from established roots left below the depth of horseradish harvest and tillage. Fall herbicide applications provided effective control of volunteer horseradish compared to the nontreated control. Combinations of 2,4–D tank–mixed with glyphosate, iodosulfuron, halosulfuron, or rimsulfuron:thifensulfuron achieved the greatest control of volunteer horseradish and reduced volunteer horseradish densities to less than 1 plant /m2, while glyphosate alone provided the least control and density reduction. In addition to volunteer horseradish control, combinations of 2,4–D with glyphosate, iodosulfuron, halosulfuron, or rimsulfuron:thifensulfuron also provided 97% or greater control of winter annual weeds (henbit, common chickweed, and Carolina foxtail). Although the herbicide halosulfuron is very effective at controlling volunteer horseradish, it has the potential to injure future horseradish crops due to soil persistence. Field studies showed that after 4 months following a June halosulfuron application, horseradish can be replanted with no visual injury or reduction in root biomass. Tillage may provide temporary removal of volunteer horseradish plants, however, herbicide applications (e.g., 2,4–D with halosulfuron, iodosulfuron or rimsulfuron:thifensulfuron) provide the most consistent, complete control of volunteer horseradish.

Cucumber (Cucumis sativus L.), pumpkin (Cucurbita maxima Duch. ex Lam.), and summer squash (Cucurbita pepo L.) are economically important crops in Virginia. Only a few herbicides are registered for weed control in these crops. Halosulfuron is a sulfonylurea herbicide which controls several broadleaf weeds and yellow nutsedge (Cyperus esculentus L.). Cucurbit crops have some tolerance to this herbicide. The efficacy of halosulfuron for control of several weed species and tolerance of four vine crops to halosulfuron were investigated in field and greenhouse studies in 1999, 2000, and 2001. In the field, halosulfuron was applied to cucumber, pumpkin, zucchini squash, and yellow summer squash at 4, 9, 18, and 27 g ai/ha preemergence (PRE) and postemergence (POST) in combination with clomazone at 174 g ai/ha plus ethalfluralin applied PRE at 630 g/ha. Crop injury, weed control, and crop yield was collected from the field studies. Weed control by halosulfuron was dependent upon application method. Halosulfuron applied preemergence controlled only common ragweed (Ambrosia artemisiifolia L.) and smooth pigweed (Amaranthus hybridus L.). Postemergence halosulfuron controlled common ragweed, smooth pigweed, morningglory species (Ipomoea spp.), yellow nutsedge, and rice flatsedge (Cyperus iria L.). All four crops treated with halosulfuron produced yields equal to or higher than the crops receiving clomazone and ethalfluralin alone or the hand-weeded check. In the greenhouse, tolerance of cultivars of each crop to halosulfuron was investigated with the same rates applied in the field. Cultivars responded similar to postemergence halosulfuron applications with respect to fresh and dry weights in all four crops. The response of several populations of acetolactate synthase inhibiting (ALS) resistant smooth pigweed to postemergence halosulfuron was also investigated. Halosulfuron activity against ALS-inhibitor resistant smooth pigweed was population dependent. In the greenhouse, postemergence halosulfuron at the same rates used in the field studies controlled yellow nutsedge. / Master of Science

Herbicide combinations are common in corn production in the United States to control broadleaf and grass weed species. Studies were conducted in 1995 and 1996 to: (1) investigate the interactions of 2,4-D and dicamba with halosulfuron-methyl on common lambsquarters and common ragweed control in corn, (2) determine the effect of 2,4-D on the foliar absorption, translocation, and metabolism of 14C halosulfuron-methyl in common lambsquarters, (3) examine the interactions of 2,4-D, dicamba, and ALS-inhibitor herbicides with rimsulfuron plus thifensulfuron-methyl (RT) and with sethoxydim on giant foxtail, common ragweed, and common lambsquarters control in corn. Combinations of halosulfuron-methyl with 2,4-D or dicamba were generally additive in their effects on common lambsquarters and common ragweed control, and were occasionally synergistic on common lambsquarters. Synergistic herbicide interactions in the greenhouse were observed with 2,4-D (17 g/ha ) and halosulfuron-methyl (18 g/ha) and 2,4-D (70 g/ha ) in combination with halosulfuron-methyl at 4.5 and 36 g/ha, respectively. Absorption and translocation of 14C-halosulfuron-methyl were not influenced by the addition of 2,4-D, with absorption increasing with time. Three unknown halosulfuron-methyl metabolites (M1, M2, and M3) with Rf values of 0.0, 0.97, and 0.94, respectively, were isolated. The addition of 2,4-D increased the level of M3 at the 18 g/ha halosulfuron-methyl rate, which may contribute to common lambsquarters phytotoxicity. Antagonism on giant foxtail control was observed with all combinations of RT and 2,4-D. Tank mixtures of RT with flumetsulam plus clopyralid plus 2,4-D, atrazine, 2,4-D, and dicamba plus atrazine controlled giant foxtail £ 78% 65 (DAT). RT mixed with flumetsulam plus clopyralid plus 2,4-D injured corn 26%, and yields were reduced 34% when compared to RT alone. Giant foxtail control from sethoxydim tank-mixed with bentazon plus atrazine with urea ammonium nitrate (UAN), or with ALS-inhibiting herbicides except halosulfuron-methyl in combination with 2,4-D was 24% lower when averaged over treatments. Yields of sethoxydim-resistant (SR) corn treated with sethoxydim mixed with combinations of sulfonylurea herbicides plus 2,4-D were low, with the exception of the combination halosulfuron-methyl with sethoxydim and 2,4-D. These studies indicate that thoroughly understanding postemergence (POST) corn herbicide tank mixtures is crucial for effective weed management. / Ph. D.