Tolerant Corn Research Articles

Corn Genotypes Show Distinctive Growth Responses to Various Herbicides

ABSTRACT Corn (Zea mays L.) inbreds and hybrids may respond differently to specific herbicides. Screening corn inbreds and hybrids for tolerance to herbicides is essential as this will enable producers to reduce the risk of crop injury by applying appropriate herbicides. A study was conducted to determine the tolerance of corn to herbicides and to compare results in the greenhouse and in the field. Both inbreds and hybrids demonstrated significant differences in their response to herbicides. Hybrids were generally more tolerant than inbreds. Of the herbicides, metazachlor was the most injurious to corn genotypes. Differential recovery from herbicide injury was observed in the field, and some genotypes recovered fully during the growing season while others did not. Some genotypes showed significant reductions in shoot dry mass while their root dry mass was not reduced; the opposite effect was also noted for certain genotype/herbicide combinations. Percentage reduction in grain yield in the field was positi...

Cultural Systems to Aid Weed Management in Semiarid Corn (Zea mays)1

Abstract: Producers in the semiarid Great Plains are including corn (Zea mays) in dryland rotations; however, weed management is difficult because the corn canopy is not competitive with weeds. My objective was to determine if cultural practices can enhance corn's competitiveness with weeds, thus supplementing current weed management strategies. Cultural systems, comprised of different row spacing, plant population, and nitrogen placement, were evaluated for effects on foxtail millet (Setaria italica) growth during three growing seasons. A cultural system comprised of 38-cm row spacing, 47,000 plants/ha, and N banded near the seeds reduced foxtail millet biomass 60% compared with the conventional system of 76-cm row spacing, 37,000 plants/ha, and N broadcast. Narrow rows had the greatest effect on foxtail millet growth. Corn's tolerance to foxtail millet interference also was improved, as yield loss in the system with narrow rows, high population, and banded fertilizer was reduced threefold compared with ...

The Impact of Long-Term Irrigation on Changes of Ground Water Level and Soil Salinity in Northern Tunisia

The irrigated area of Kalaat Landelous in northern Tunisia is an example of the successful reclamation of 3000 ha of soils affected by shallow ground water levels and salinization. Examination of soil conditions at three key periods during reclamation reveals the reasons for this success. Aspects of irrigation management define these periods, during which measurements were taken of the depth and salinity of ground water over 43 years. An overall salt balance for the irrigated areas was made in order to estimate the risk of salinization. The results reveal a lowering of the ground water level (average depth was 1 m and will be 1.6 m), desalinization of the soils (average electrolytic conductivity of the soil was 0.8 Sm -1 and will be 0.55 m -1 ), and dilution of the salt content of the ground water (average electrolytic conductivity of ground water was 3 Sm -1 and will be 1.4 Sm -1 ). A reduction in the variability of saline conditions was also observed. The salt balance shows a large amount of salt exported from the area, between 22,000 and 66,000 Mg, and only small areas of new salinization have appeared as a result of irrigation. Ultimately, the sustainability of the project depends upon the performance of the drainage network and on the abundance of the rainfall. If drainage is efficient, soil salinity will be maintained below wheat, corn, tomato, pepper, and alfalfa tolerance limits.

Weed Control and Corn (Zea mays) Tolerance from Soil-Applied RPA 201772

Field experiments were conducted in 1996, 1997, and 1998 to evaluate weed control and corn tolerance from soil-applied RPA 201772. Treatments alone and in tank mixtures with other corn herbicides were applied preemergence (PRE) at two locations with conventional tillage and at early preplant (EPP) and PRE application timings in no-tillage corn. RPA 201772 was applied alone and in tank mixtures with one-half the typical field rates of other PRE corn herbicides. In conventional tillage experiments in 1996, 1997, and at one location in 1998, all treatments containing RPA 201772 provided > 90% control of common lambsquarters, redroot pigweed, common ragweed, and velvetleaf. In two no-tillage experiments, common lambsquarters and velvetleaf control was > 90%, regardless of application timing. However, control of redroot pigweed and common ragweed varied among years and application timings. Weed control was more variable from herbicide treatments applied EPP compared with the PRE application timing. Giant foxtail control in both tillage systems was rate, timing, and year dependent. RPA 201772 rates higher than 79 g/ha controlled giant foxtail > 85% at three of five locations. At one location, tank mixtures with RPA 201772 increased giant foxtail control. Corn injury occurred in one of two conventional tillage locations and at the no-tillage location in both 1996 and 1997. Injury was most commonly observed in coarse-textured soils with low clay and organic matter and was more severe with higher rates of RPA 201772. Increased corn injury was also observed when RPA 201772 was combined with acetochlor plus dicloramid or BAYFOE 5043 plus metribuzin. Corn injury from RPA 201772 occurred at application rates above the proposed rate for use on corn. In some cases, severe injury to corn reduced corn yield. Injury to corn from RPA 201772 was not unique to any tillage system and was site, year, and rate dependent.

Evaluation of Glufosinate-Resistant Corn (Zea mays) and Glufosinate: Efficacy on Annual Weeds

Field studies were conducted in 1996 and 1997 at Belleville and Pawnee, IL, to evaluate single and sequential applications of glufosinate on tolerance of glufosinate-resistant corn and annual weed control. Glufosinate caused 0 to 13% corn injury 7 days after treatment (DAT) and 0 to 6% corn injury 28 DAT. Injury was characterized as stunting with glufosinate. Glufosinate at 400 to 1,200 g ai/ha did not reduce final corn height or grain yield. At Pawnee in both years and at Belleville in 1996, a single application of glufosinate at 400 g/ha controlled giant foxtail, velvetleaf, ivyleaf morningglory, and common lambsquarters 85 to 100%. At Belleville in 1997, sequential applications of glufosinate provided greater weed control (87 to 100%) than a single application (0 to 63%) because of weed emergence after application. Weed control with a single application of glufosinate or with nicosulfuron plus bromoxynil was similar at both locations. Height and grain yield of glufosinate-resistant corn were not different from that of glufosinate-susceptible corn (isoline of glufosinate-resistant corn).

Mechanisms of Corn Tolerance and Velvetleaf Suppressive Ability

AbstractCrops that better tolerate weed competition or preempt resources from weeds may reduce dependence on herbicides for weed control and benefit an integrated weed management program. Irrigated field experiments were conducted to identify morphological traits of corn (Zea mays L.) that enhance its tolerance and weed suppressive ability. Two old (circa 1940) and two modern corn hybrids were grown in mixture with five target densities of velvetleaf (Abutilon theophrasti Medik.) in two years. Corn height and leaf area index (LAI) were measured periodically throughout the growing season. Vertical leaf area distribution was measured at corn anthesis. Quantitative estimates of these traits were obtained for each experimental unit using linear and nonlinear regression analysis. Magnitude of the correlation between these trait estimates and corn yield reduction in mixture with velvetleaf or velvetleaf seed capsule production was used as an indicator of the importance of each trait to the enhancement of corn tolerance or velvetleaf suppressive ability. Canopy traits having highest correlation with corn relative yield in 1995 were maximum LAI (r= −0.38) and crop density (r = −0.24). Those with highest correlation in 1996 were crop density (r = ‐0.36), degree days accumulated between emergence and 50% maximum height (r = 0.34) and LAI (r = 0.19), and vertical leaf area distribution (0.49). Corn canopy traits were not correlated with velvetleaf seed production in 1995. However, vertical leaf area distribution (r = 0.60), maximum LAI (r = −0.32), and corn density (r = −0.26) were correlated with velvetleaf seed production in 1996. Results suggest that increasing maximum corn LAI, rate of canopy closure, or height at which leaf area occurs in the canopy can improve corn tolerance and velvetleaf suppressive ability. These traits can be modified by varying cultural practices such as row spacing and population density, or through breeding.

The Mode of Action of Isoxaflutole: I. Physiological Effects, Metabolism, and Selectivity

Isoxaflutole (5-cyclopropyl isoxazol-4-yl-2-mesyl-4-trifluoromethylphenyl ketone) is a novel herbicide for preemergence control of a wide range of important broadleaf and grass weeds in corn and sugarcane producing characteristic foliar bleaching activity in susceptible species prior to plant death. Following either foliar or root uptake, isoxaflutole is rapidly converted to a diketonitrile derivative (2-cyclopropyl-3-(2-mesyl-4-trifluoromethylphenyl)-3-oxopropanenitrile) by opening of the isoxazole ring. This diketonitrile undergoes degradation to a benzoic acid derivative (2-mesyl-4-trifluoromethyl benzoic acid) in treated plants and the extent of this degradation is correlated to the degree of susceptibility, being most rapid in tolerant corn and slowest in susceptibleAbutilon theophrasti. Following a preemergence application of [phenyl(U)-14C]-isoxaflutole, equivalent to 250 g ai ha−1only 10% of the extracted14C activity was the active diketonitrile in corn, compared to 90% inA. theophrasti. The rate of diketonitrile degradation was intermediate in moderately susceptibleIpomeaspp. The foliar symptoms are associated with an accumulation of phytoene in treated leaves ofBrassica kaber, typical of herbicidal inhibitors of phytoene desaturase. However, neither the diketonitrile derivative nor isoxaflutole inhibits the desaturation of radiolabeled phytoene by a microsomal extract of carrot cell cultures. However, the diketonitrile does inhibit 4-hydroxyphenylpyruvate dioxygenase, preventing the biosynthesis of homogentisate, the precursor of plastoquinone and α-tocopherol. The content of both these prenylquinones in emerging leaves of isoxaflutole-treated plants decreases prior to the accumulation of phytoene and the decrease in carotenoids. The mode of action of isoxaflutole is suggested to be due to an indirect inhibition of phytoene desaturase resulting from the absence of plastoquinone, an essential cofactor for the desaturase.

Differential Tolerance of Corn (Zea mays) Inbreds to Four Sulfonylurea Herbicides and Bentazon

Nine corn inbreds were selected from a test on 250 inbreds based on distinctly different response to herbicides. These inbreds showed up to 50,000-fold differences, which varied by inbred and herbicide. Only the inbred GA209 was sensitive to bentazon and the four sulfonylurea herbicides. GA209 and B90 were the most sensitive inbreds to the sulfonylurea herbicides. F2R was generally the most tolerant inbred. The pattern of herbicide tolerance varied greatly. For example, the relative tolerance of F2 to thifensulfuron was 12-fold more than to rimsulfuron while Mo17 was 56-fold more tolerant to rimsulfuron than thifensulfuron. F2 was fivefold more tolerant of chlorsulfuron than chlorimuron, whereas B80 was 100-fold the reverse. These inbreds provide a small subset of varieties to evaluate the large and inherent differences in herbicide tolerance of corn.

Carbon dioxide assimilation efficiency of maize leaves under nitrogen stress at different stages of plant development

Sub‐optimal nitrogen (N) affects the N‐rich carbon dioxide (CO2) assimilation enzymes which can limit maize (Zea mays) production. The status of the carboxylation system is closely correlated to the Assimilation Efficiency Index (AEI) which is the initial slope of the CO2 assimilation rate versus intercellular leaf CO2 concentrations. Experiments were undertaken to ascertain the effect of soil N nutrition on the AEI, determine genotypic variability for AEI under N‐deficiency, determine how leaf and plant development affect treatment differences, and examine correlations between the AEI and plant development. Studies were conducted in the field and greenhouse on five maize genotypes on leaves of different ages at three stages of plant development. Field studies were conducted on a fine, silty mixed, mesic cumulic Hapludoll (1.2 g N kg‐1), and high and low N treatments were imposed in the greenhouse. Quantum yield of emerging and mature leaves was determined. Results indicated that emerging and fully mature leaves had the greatest AEI values compared to other expanding leaves. Low N availability reduced the AEI of younger leaves but increased the AEI on the oldest leaf. The AEI increased until tasseling and then declined. Correlations were established between the AEI and leaf N concentrations and with CO2 assimilation. Grain yield was correlated to the AEI during grain fill. Quantum yield of the mature leaf was greater with low N than with high N availability indicating that the energy capture or transfer mechanism was less affected by N levels than was the CO2 trapping mechanism. There were pronounced gsnotypic differences in the AEI at tassel emergence but not in leaf N concentrations intimating differences in the distribution of N to enzymes and other compounds important for CO2 assimilation. Internal N distribution was also dependent upon available N. The study demonstrated that the ability of a plant to maintain high carboxylation activity under N stress may be a valuable selection criteria for obtaining tolerance of corn to low soil N.

Tolerance of Early Maturing Corn (Zea mays) Hybrids to DPX-79406

The effect of DPX-79406 was evaluated on eight corn hybrids in 1993 and on 18 hybrids in 1994. Experiments were conducted each year at Charlottetown, Prince Edward Island, and Stewiacke, Nova Scotia. Corn tolerance was similiar between sites but varied from year to year with certain hybrids. Generally, tolerance was hybrid dependent. Most hybrids displayed chlorosis and stunting 7 days after treatment (DAT) but recovered quickly. Injury at 7 DAT ranged from 8 to 64%. Grain yield indicated that ‘Pickseed 2444,’ ‘Pioneer 3921,’ ‘Pioneer 3979,’ and ‘DK 302’ were tolerant to 25 g ai/ha of DPX-79406. Pioneer 3979 and Pickseed 2444 grain yields were reduced with DPX-79406 at 50 g/ha in 1 yr. ‘DK 291’ and ‘DK 235’ were severely injured both years and did not recover appreciably from early injury. Responses of ‘Pioneer 3984’ and ‘Pickseed 2477’ varied from year to year, and they were considered sensitive to DPX-79406. ‘Pioneer 3947,’ ‘Pioneer 3962,’ ‘Pickseed 2290,’ ‘Pickseed 2299,’ ‘DK 220,’ ‘G-4011,’ and ‘N0565’ were tested only 1 yr and were tolerant to both rates of DPX-79406.

Weed Control and Crop Injury in Ecofallow Corn (Zea mays) Using Imazethapyr

Imidazolinone-resistant and -tolerant corn hybrids give growers a new weed control option. Field experiments were conducted in 1993 and 1994 to evaluate imazethapyr for controlling weeds in no-till corn planted into winter wheat stubble in a winter wheat–ecofallow corn rotation. Imidazolinone-tolerant (IT) and imidazolinone-resistant (IR) corn were protected genetically from injury by imazethapyr that was applied preplant, preemergence, or postemergence to ‘Pioneer Brand 3417,’ ‘Pioneer Brand 3417 IR,’ ‘ICI Seeds 8532,’ and ‘ICI Seeds 8532 IT’ hybrids. No difference in corn injury occurred between IR and IT corn. Imazethapyr applied to resistant or tolerant corn hybrids could be used to control weeds in ecofallow corn. Imazethapyr at 35 or 70 g ai/ha controlled triazine-resistant kochia better than the standard herbicide treatment of metolachlor plus cyanazine.

Preemergence Broadleaf Weed Control and Crop Tolerance in Imidazolinone-Resistant and -Susceptible Corn (Zea mays)

Field studies were conducted in 1994 and 1995 at Dekalb and Urbana, IL, to evaluate preemergence broadleaf weed control and crop tolerance in imidazolinone resistant (IR) and susceptible (non-IR) corn using atrazine, imazethapyr, AC 263,222, CGA-152005, MON 12000 with and without MON 13900 (a safener), and flumetsulam + clopyralid. When sufficient rainfall occurred within 28 d of application to insure herbicide absorption, the IR corn variety was more tolerant than the susceptible variety to imazethapyr, AC 263,222, CGA-152005 at 40 and 80 g/ha, and MON 12000 with and without MON 13900. Overall crop tolerance of IR corn was equal to that of corn treated with atrazine for all herbicide treatments except CGA-152005, which injured IR corn. Control of velvetleaf, common lambsquarters, Pennsylvania smartweed, tall morningglory, and jimsonweed for all herbicide treatments was equal or superior to that of atrazine at 1.7 kg/ha. However, control of common cocklebur was significantly greater with atrazine compared to imazethapyr and the low rate of CGA-152005.

Managing Corn Rootworms: (Coleoptera Chrysomelidae) on Dairy Farms: the Need for a Soil Insecticide

Field trials were conducted at 20 locations in New York during 1993 and 1994 to evaluate the effects of using manuring practices and soil insecticide on rootworm injury, plant height, lodging, and yield of corn. All fields had adult densities of western corn rootworm, Diabrotica virgifera virgifera LeConte, exceeding 1 beetle per plant during the year preceding the test. The following 4 treatments were tested: (1) inorganic fertilizer, (2) inorganic fertilizer plus soil insecticide, (3) manure, and (4) manure plus soil insecticide. Root injury was significantly reduced by soil insecticides during both years but was unaffected by manure treatments. Both manure applications and soil insecticides increased plant heights during June and July. The effect of treatments on yields differed by year. In 1993, soil insecticides significantly increased silage yields an average of 3.05 metric tons (MT)/ha but had no effect on grain yield. In contrast, manuring did not significantly affect either silage or grain yield. In 1994, silage yields were 3.45 MT/ha higher in manured treatments compared with nonmanured treatments. Soil insecticides increased silage yield 2.3 MT/ha in nonmanured treatments but did not affect silage yield in manured treatments. Grain yields were not affected by either insecticide or manure treatments in 1993 but were significantly higher (4.7 quintals/ha) in insecticide treatments in 1994. The effect of manure on corn tolerance to western corn rootworm seemingly is greater during years when corn is not under moisture stress. Multiple regression models, incorporating soil characteristics, nutrients in fertilizers, and planting information, were developed to predict crop yield and change in yield expected with the addition of a soil insecticide. Yield models also suggest the need for more controlled experiments to characterize directly the effects of drainage, potassium and phosphorus availability, pendimethalin herbicide injury, soil pH, and rainfall on soil insecticide benefits.

Nicosulfuron Tolerance in Sweet Corn (Zea mays) as Affected by Hybrid, Rootworm Insecticide, and Nicosulfuron Treatment

Sweet corn tolerance to nicosulfuron as affected by interactions between nicosulfuron applied postemergence (POST), sweet corn hybrid, and rootworm insecticide applied at planting was determined in 1992 and 1993 field experiments. Sweet corn vigor was reduced as nicosulfuron rate increased from 35 to 140 g ai/ha. Sweet corn vigor was reduced 4% more by nicosulfuron with oil emulsifier mixture than nicosulfuron with non-ionic surfactant one week after treatment (WAT). Adjuvant selection did not influence nicosulfuron injury to sweet corn 2 WAT. Nicosulfuron applied postemergence directed (PDIR) caused less sweet corn injury than nicosulfuron applied broadcast POST. ‘Jubilee’ sweet corn was more sensitive to nicosulfuron and insecticide treatments than ‘Excellency’ or ‘Green Giant 40.’ Nicosulfuron reduced sweet corn vigor twice as much when terbufos was applied at planting compared to no insecticide at planting. Neither chlorpyrifos or chlorethoxyfos at planting increased sweet corn injury due to nicosulfuron compared to no insecticide. Excellency and Green Giant 40 yields were not reduced by any treatment. Yields were reduced only when nicosulfuron was applied to Jubilee treated with terbufos. Jubilee yield reduction from nicosulfuron and terbufos was prevented by applying nicosulfuron PDIR.

Effect of Manure and Corn Hybrid on Survival of Western Corn Rootworm (Coleoptera: Chrysomelidae)

The interactions between manure and hybrid tolerance and their subsequent effects on the survival of the western corn rootworm, Diabrotica virgifera virgifera LeConte, and other soil arthropods were investigated in 1992 and 1993. To evaluate alternative management strategies for the western corn rootworm, treatment combinations included 2 corn hybrids ('Pioneer hybrid 3733' and 'Cornell 281'), 4 manure rates (0, 45, 90 and 135 metric ton/ha) and 3 western corn rootworm egg densities (1992: 0, 500, 700 eggs per 30.5-cm row; 1993: 0, 400, 800 eggs per 30.5-cm row). Pioneer hybrid 3733 has been rated superior to Cornell 281 in resistance to stalk and root lodging. Plots of Pioneer hybrid 3733 contained fewer western corn rootworm larvae on 6 July 1992, but the number of larvae and adults did not differ between hybrids on any other date. Although western corn rootworm larval counts did not differ by manure rate in either year, adult emergence was reduced an average of 45% in manured plots compared with nonmanured plots in 1992. Wet soil conditions probably contributed to the reduced larval counts and adult emergence found in all plots in 1992 compared with 1993 and could have interacted with manure to increase mortality in treated plots. Adult emergence increased with egg density, but the ratio of emergence to egg density decreased as egg density increased. Numbers of mesostigmatid mites significantly increased in manured plots during both years. No evidence of enhanced predation was reflected in larval counts. Although previous studies have reported increased western corn rootworm survival in tolerant corn hybrids with large root systems, manure applications and the subsequent increase in root mass did not lead to increased western corn rootworm populations.

Interactions Between SAN 582H and Selected Safeners on Grain Sorghum (Sorghum bicolor) and Corn (Zea mays)

The efficacy of the safeners naphthalic anhydride (NA), CGA-92194, and CGA-133205 as protectants of grain sorghum and of NA, R-29148, CGA-154281, and dichlormid as protectants of corn against injury from the chloroacetamide herbicide, SAN 582H, was evaluated under greenhouse conditions. SAN 582H applied alone reduced the shoot growth of ‘Funk G522DR’ and ‘Funk G623’ sorghum, but had no effect on the growth of the ‘GP-10’ genotype of sorghum. Corn (‘Northrup-King 9283’ hybrid) was considerably more tolerant than grain sorghum to SAN 582H. The rate of SAN 582H causing 50% reduction of corn shoot height (GR50) was 5.1 kg/ha as compared to 1.1 kg/ha for grain sorghum. NA applied as seed dressing at 5 or 10 g/kg seed, provided moderate protection to ‘G522DR’ and ‘G623’ sorghum and no protection to corn against SAN 582H injury. Dressing of sorghum seeds with CGA-133205 provided moderate protection to G522DR sorghum, whereas seed-applied CGA-92194 provided moderate protection to G623 sorghum against SAN 582H. The dichloroacetamide safeners R-29148, dichlormid, and CGA-154281 were all effective as protectants of corn against SAN 582H injury. These results demonstrate that, as with other chloroacetamide herbicides, the tolerance of corn and grain sorghum to SAN 582H can be enhanced with the use of safeners of the dichloroacetamide type and oxime ether type, respectively.

Interactions of Tribenuron with Four Safeners and Piperonyl Butoxide on Corn (Zea mays)

The efficacy of the safeners naphthalic anhydride (NA), R-29148, benoxacor, and dichlormid as protectants of corn against injury from the methyl ester of tribenuron was evaluated under greenhouse conditions. Tribenuron applied alone reduced the root length of both ‘Pioneer 3180’ and ‘Northrup-King 9283’ corn. The concentration of tribenuron causing 50% reduction of corn root length (GR50) was 1 μM. NA applied as a seed dressing at 0.5 and 1.0% (w/w) was the most effective of the four safeners tested and provided excellent protection to both corn hybrids against 1, 2, and 4 μM tribenuron. Soaking corn seeds in an aqueous solution of R-29148 containing 0.5,1, or 2 mM protected ‘Pioneer 3180’ against 1 μM of tribenuron. The safening efficacy of R-29148 against tribenuron injury to ‘Northrup-King 9283’ corn was limited. Dichlormid and benoxacor, were less effective as tribenuron safeners and offered partial protection to both corn hybrids. Individual applications of the synergist piperonyl butoxide (PBO) at 0.1 and 0.2 mM did not affect the growth of corn seedlings. PBO synergized the activity of all concentrations of tribenuron, but only on the ‘Northrup-King 9283’ hybrid. These results demonstrate that the tolerance of corn to tribenuron can be enhanced with the use of safeners such as NA and R-29148, and that oxidative reactions may be involved in the metabolism of tribenuron by corn.

Safening Influence of LAB 145 138 on Nicosulfuron, Terbufos and Bentazon Interactions in Sweet Corn (Zea mays)

LAB 145 138 (LAB) was evaluated as a safener to improve sweet corn tolerance to nicosulfuron applied POST alone or with terbufos applied in the planting furrow or bentazon applied POST. To ensure enhanced injury for experimental purposes, nicosulfuron was applied at twice the registered rate alone or mixed with bentazon at the six- to seven-leaf growth stage of corn previously treated with the highest labeled rate of terbufos 15 G formulation. LAB applied as a seed treatment (ST) or POST at the two- to three-, four- to five-, or six- to seven-leaf growth stages reduced height reduction and yield loss from nicosulfuron applied POST in combination with terbufos applied in-furrow. LAB applied POST at the four- to five-leaf growth stage was most effective in preventing injury from this treatment, with yield reduced only 8% compared with 54% from the nicosulfuron and terbufos treatment. LAB applied POST at the eight- to nine-leaf growth stage did not alleviate injury. With the nicosulfuron, terbufos, and bentazon combination, LAB applied POST at the three- to four- or six- to seven-leaf growth stages decreased height reduction and yield loss caused by this combination, with LAB at the three- to four-leaf growth stage being most effective.

Nicosulfuron and Primisulfuron Selectivity in Corn (Zea mays) and Two Annual Grass Weeds

Research was conducted to elucidate mechanism(s) of nicosulfuron and primisulfuron selectivity in corn, woolly cupgrass, and shattercane. Corn absorbed less than one half the nicosulfuron and primisulfuron that woolly cupgrass and shattercane absorbed. Acetolactate synthase (ALS) isolated from three species responded similarly to nicosulfuron and primisulfuron. Values for 50% (I50) inhibition of ALS for nicosulfuron were 36.9, 37.2 and 29.5 nM for corn, woolly cupgrass and shattercane, respectively. Primisulfuron I50values were 13.8, 11.5 and 10.0 nM for corn, woolly cupgrass and shattercane, respectively. Shattercane's I50was different from corn's but not from that of woolly cupgrass. Corn rapidly metabolized nicosulfuron and primisulfuron, with a half-life of less than 4 h. Shattercane metabolized the herbicides more slowly, with a half-life greater than 72 h for nicosulfuron and 36 h for primisulfuron. Nicosulfuron and primisulfuron half-lives were greater than 72 h and less than 4 h, respectively, in woolly cupgrass. Therefore, selectivity with nicosulfuron and primisulfuron is likely based on metabolism to nonphytotoxic compounds. Corn tolerance to nicosulfuron and primisulfuron was also attributed to reduced herbicide penetration and translocation below the treated leaf.

Safeners Reduce Corn (Zea mays) Chloroacetanilide and Dicamba Injury Under Different Soil Temperatures

The results of two experiments involving the effects of safeners on chloroacetanilide and dicamba injury in corn are reported here. The first experiment was conducted to determine whether MON-4660, CGA-154281, dichlormid, and MON-13900 would increase corn tolerance to alachlor, metolachlor, and acetochlor, respectively, over a range of soil temperatures (12 to 37 C). All three herbicides, particularly acetochlor, reduced corn shoot lengths at temperatures less than 32 C. The safeners effectively increased corn tolerance to their respective chloroacetanilide herbicide at all soil temperatures. However, acetochlor injury was not completely overcome by dichlormid or MON-13900 when temperatures were less than 27 C. The second experiment was conducted to determine whether these safeners would alleviate injury caused by chloroacetanilides applied with dicamba in sensitive (Pioneer 3320) and tolerant (Cargill 7567) corn hybrids at soil temperatures ranging from 17 to 27 C. Chloroacetanilides (4.5 kg ai/ha) plus dicamba (0.6 kg ae/ha) reduced shoot lengths in both hybrids, with acetochlor/dicamba combinations causing the greatest reductions. All four safeners increased corn tolerance to chloroacetanilide/dicamba tank-mixtures equally.