Experiments were conducted to study absorption, translocation, and activity of clopyralid and tribenuron in perennial sowthistle. Absorption and translocation were compared in perennial sowthistle at the rosette and bud stage over a period of 9 d. Both 14C-clopyralid and tribenuron were absorbed slowly by perennial sowthistle leaves with 60 and 30% absorption, respectively, 216 h after application. Limited movement of the absorbed 14C was observed for both herbicides with less than 28% of either herbicide exported from the treated leaf. Primary movement of 14C-herbicide within the plant was acropetal; however, no more than 18% of either herbicide was recovered in the upper shoot or buds. After 216 h, 4.4% of the applied 14C-clopyralid and 2.5% of the applied 14C-tribenuron were recovered in the roots and secondary shoots of plants at the rosette stage. Application of either herbicide to perennial sowthistle resulted in a decrease in net carbon assimilation and leaf conductance. Transpiration was reduced in rosette stage plants by both herbicides; however, transpiration in plants at the bud stage was not reduced until 216 h after treatment. Plants treated with tribenuron had lower net carbon assimilation, leaf conductance, and transpiration rates than plants treated with clopyralid. Application of clopyralid at 150 and 300 g ha-1 resulted in visual injury ratings of 65 and 95%, respectively. Tribenuron applied at 10 and 20 g ha-1 gave similar results. Dry weight accumulation was reduced at all application rates compared to untreated plants. Limited absorption and translocation along with high herbicidal activity suggests that small amounts of either herbicide are sufficient to inhibit perennial sowthistle growth.

Field experiments were conducted in 1987 and 1988 to examine interference from a natural population of perennial sowthistle on soybean and dry edible bean. In 1987, an average of 78 and 90 perennial sowthistle shoots per m2 in 71-cm (wide) crop row spacing reduced soybean and dry edible bean yield by 49% and 36%, respectively. In 1988, a year of less precipitation, an average density of 96 and 88 shoots per m2 reduced soybean and dry bean yield by 87% and 83%, respectively. One cultivation 5 wk after planting increased crop yield and decreased perennial sowthistle density compared with no cultivation. Perennial sowthistle reduced seed weight, germination percentage, and seedling growth rate of seed produced by both crops. In the presence of perennial sowthistle, one cultivation resulted in improved seed quality compared with no cultivation.

Greenhouse and field experiments were conducted to examine foliar absorption and activity of nicosulfuron and primisulfuron by quackgrass with various adjuvants. Foliar absorption of 14C-nicosulfuron and 14C-primisulfuron plus petroleum oil adjuvant (POA) was completed by 4 h after application. Absorption of nicosulfuron plus POA and primisulfuron plus POA increased from 11 and 2% of applied, respectively, to 51 and 12% with the addition of urea-ammonium nitrate liquid fertilizer (UAN). At least 83% of the absorbed 14C from either herbicide penetrated the leaf epicuticular waxes. Absorption of 14C-labeled herbicides was greatest with the following adjuvants: POA + UAN > nonionic surfactant (NIS) + UAN = methylated seed oil. The addition of UAN to either NIS or POA significantly increased 14C-herbicide uptake. In greenhouse studies, nicosulfuron and primisulfuron applied with POA plus UAN provided greater quackgrass control than with POA alone. Despite the differences in foliar uptake in the greenhouse, few differences were observed between these adjuvants in 1989 or 1990 field efficacy trials. Quackgrass control was reduced by the addition of atrazine to nicosulfuron plus POA in 1989 and to primisulfuron plus POA in 1990. Combinations with atrazine did not reduce 14C-nicosulfuron or 14C-primisulfuron uptake by quackgrass.

Greenhouse and laboratory studies were conducted to examine certain characteristics of RPA 201772 and how they may affect its utility. 14C-RPA 201772 was used to determine the stability of RPA 201772 in various pH spray solutions over time. After 24 h, degradation of RPA 201772 was 20, 36, and 93% in spray solutions adjusted to pH 4.0, 7.0, and 10.0, respectively. The major metabolite was diketonitrile (DKN), which is herbicidally active. In addition, 9 and 15% of the RPA 201772 had degraded into an inactive benzoic acid derivative at pH 7.0 and 10.0, respectively. The differences in herbicidal activity of RPA 201772 and its metabolite DKN were also evaluated. Corn seeds and velvetleaf seeds readily imbibed RPA 201772, but only low levels of DKN were imbibed. Preemergence (applications of RPA 201772 and DKN were equally active on corn and velvetleaf. Further studies determined that the principal sites of uptake of RPA 201772 by corn was roots and seeds for four different corn hybrids. Another study determined that light was required for RPA 201772 activity. Corn shoots from seeds germinated under 14 h of light exhibited rate-dependent injury, while corn germinated and grown in the darkness was not injured.

Field days on diagnosis of plant problems were conducted by Extension specialists at Michigan State University. The objective was to expose participants to a variety of plant growth problems. Techniques were developed to simulate crop injury from herbicide and fertilizer misapplication and environmental stress. Twenty demonstration plots each were established for corn (Zea mays L.) and soybeans [Glycine max (L.) Merr.]. A specific problem was demonstrated in each plot. Two field days were held in 1985. One field day was conducted for agribusiness personnel and the other for Extension field staff. Promotion of the field days was a cooperative effort among Michigan Cooperative Extension Service, Michigan Pesticide Association, and Michigan Plant Food Council. Resource people conducting tours of the plots and leading discussions of the problems included a weed scientist, a soil fertility specialist, and a crop production agronomist. An evaluation survey indicated that this event was useful in furthering the understanding of plant growth problems. Based on the survey responses, future plant problem diagnosis field days are being planned in Michigan.

Research was conducted in 1987 and 1988 to identify effective horseweed control strategies in no-tillage soybean production. Paraquat at 560 g ai ha-1 plus metolachlor at 2200 g ai ha-1, linuron at 840 g ai ha-1, and nonionic surfactant at 0.25% (v/v) applied preemergence provided less than 61% horseweed control. Glyphosate at 840 g ai ha-1, 2,4-D ester at 560 g ai ha-1, HOE-39866 at 840 g ai ha-1, or BAS-514 at 70 g ai ha-1 applied early preplant controlled more than 96% when followed by the above preemergence herbicide combination. Substituting either glyphosate at 840 g ha-1 or HOE-39866 at 840 g ha-1 for paraquat in the preemergence application program significantly increased control. Adding BAS-514 at 70 g ha-1 to preemergence treatments containing paraquat significantly improved control. The substitution of metribuzin, metribuzin plus chlorimuron (10:1 ratio), or linuron plus chlorimuron (16:1 ratio) for linuron resulted in greater control and soybean yield. Sequential applications controlled horseweed equal to or better than a single preemergence application at the same total rate.

Greenhouse and laboratory experiments were conducted to determine the physiological basis for differential tolerance of four Zea mays L. hybrids to RPA 201772. Differences in Zea mays tolerance were quantified by determining the herbicide rate required to injure and reduce Z. mays height 50% (GR50). GR50 values indicated that the Z. mays hybrids ‘Pioneer 3751’ and ‘Pioneer 3737’ were less tolerant to RPA 201772 than the hybrids ‘Pioneer 3394’ and ‘Pioneer 3963.’ Experiments using 14C-RPA 201772 were conducted to determine if hybrid sensitivity was due to differential uptake, translocation, or metabolism of the herbicide. Differences in hybrid tolerance were primarily due to differential herbicide metabolism rates. The time required for 50% inactivation (T½) of RPA 201772 was 42 and 52 h for the more tolerant hybrids and 66 and 93 h for the more sensitive hybrids. Increased uptake of RPA 201772 was also a contributing factor to the sensitivity of one of the hybrids.

The WEEDSIM bioeconomic model was developed in Minnesota and was designed to support weed management decisions for both soil-applied and postemergence weed control programs in Zea mays and Glycine max. In this research, we adapted the WEEDSIM weed management model to Michigan by modifying the crop yield loss functions and herbicide efficacy ratings. We then validated the components of the model and determined whether WEEDSIM led to more profitable weed management than recommendations from SOYHERB or CORNHERB, computer decision aids based solely on herbicide efficacy and cost. The crop year significantly influenced the weed-free yield in Z. mays and G. max, but the weed—crop interference function did not change each year. Total weed seed increased in the untreated compared with the weed-free control over the 3-yr period. Weed seed did not increase significantly in WEEDSIM preemergence/postemergence (PRE/POST), WEEDSIM postemergence, or CORNHERB or SOYHERB treatments compared with the weed-free control, although annual grass seedling density at the time of postemergence herbicide application had increased by 1995 in the WEEDSIM postemergence treatment in G. max because of a 2,4-D amine application only in Z. mays in 1994. WEEDSIM PRE/POST and CORNHERB provided excellent weed control in all three years, and WEEDSIM PRE/POST resulted in gross margins over weed control costs equal to or greater than CORNHERB recommendations. In G. max, Chenopodium album and annual grass control was excellent in all three years for WEEDSIM PRE/POST, WEEDSIM postemergence, and SOYHERB treatments. The highest average gross margin for the 3-yr study was from mechanical weed control in 76-cm-wide rows of G. max ($806 ha−1) and from SOYHERB in 38- and 19-cm-wide rows of G. max ($776 and $808 ha−1, respectively). WEEDSIM recommendations controlled weeds and maintained crop yield in both Z. mays and G. max.

Field studies were conducted in 1992 and 1993 to determine the effect of atrazine and pendimethalin on velvetleaf competitiveness in corn. The three factors included in this research were herbicide, velvetleaf density (zero or nine plants per meter of row), and treatment (treated with herbicide or untreated). Herbicide treatments consisted of atrazine (0.6 and 1.1 kg/ha) or pendimethalin (0.6 and 1.1 kg/ha) applied preemergence. In 1992 and 1993, both atrazine and pendimethalin applied at 1.1 kg/ha greatly reduced velvetleaf growth and seed production and prevented velvetleaf from reducing corn yield. Both atrazine and pendimethalin applied at 0.6 kg/ha reduced velvetleaf growth and seed production in 1993, but only atrazine prevented yield loss in 1992. In the absence of velvetleaf, corn yield was not affected by either herbicide. These results indicate that sublethal exposure to atrazine or pendimethalin can significantly reduce velvetleaf competitiveness in corn.

The recent introduction of glufosinate-resistant and glyphosate-resistant crops provides growers with new options for weed management. Information is needed to compare the effectiveness of glufosinate and glyphosate on annual weeds. Greenhouse trials were conducted to determine the response of barnyardgrass (Echinochloa crus-galli), common lambsquarters (Chenopodium album), common ragweed (Ambrosia artemisiifolia), fall panicum (Panicum dichotomiflorum), giant foxtail (Setaria faberi), large crabgrass (Digitaria sanguinalis), and velvetleaf (Abutilon theophrasti) to glufosinate and glyphosate. The response of velvetleaf and common lambsquarters was investigated at multiple stages of growth. Glufosinate and glyphosate were applied to each weed species at logarithmically incremented rates. The glufosinate and glyphosate rates that provided a 50% reduction in aboveground weed biomass, commonly referred to as GR50 values, were compared using nonlinear regression techniques. Barnyardgrass, common ragweed, fall panicum, giant foxtail, and large crabgrass responded similarly to glufosinate and glyphosate. Common lambsquarters 4 to 8 cm in height was more sensitive to glufosinate than glyphosate. In contrast, 15- to 20-cm tall-velvetleaf was more sensitive to glyphosate than glufosinate.

Field studies were conducted in Michigan, Minnesota, and Wisconsin to explore interactions among DPX-V9360 herbicide applied postemergence, terbufos insecticide applied as an in-furrow treatment, and the environment. Field corn (‘Pioneer 3751’) and sweet corn (‘Jubilee’) were planted with and without an in-furrow application of terbufos. DPX-V9360 was applied postemergence when the corn was in the 4- to 6-leaf stage at 0, 35, 70, 140, and 280 g ai ha-1 with nonionic surfactant and 28% N fertilizer solution. Crop response to DPX-V9360 was similar at all three locations, varying only in magnitude of injury. Crop injury was greater with Jubilee sweet corn than with Pioneer 3751 field corn. Injury to both hybrids increased as DPX-V9360 application rate increased. Application of terbufos increased injury from DPX-V9360 to both hybrids. Significant yield reduction did not occur with either hybrid when DPX-V9360 was applied at rates of 140 g ha-1 or less and no terbufos was applied.

Soybean-based [Glycine max (L.) Merr.] and petroleum-based crop oil concentrates were equally effective as spray adjuvants for control of quackgrass [Agropyron repens (L.) Beauv. #3 AGRRE] with the butyl ester of fluazifop {[±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} and sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one}. With favorable growing conditions at application in 1983 and 1985, an adjuvant concentration of 1% (v/v) with these herbicides was adequate for quackgrass control. Unfavorable growing conditions at application in 1984 resulted in unacceptable quackgrass control, and spray volume did not affect control. However, increasing the adjuvant concentration to 4% (v/v) with either herbicide applied in a spray volume of 70 L/ha nearly doubled quackgrass control. In 1983 and 1985, equal or greater quackgrass control with each herbicide was achieved with spray volumes as low as 35 L/ha compared to 280 L/ha.

The influence of adjuvants, inorganic salts, and herbicide formulations on the antagonistic effect of Na-bentazon on sethoxydim absorption was studied in quackgrass. Adjuvant BCH 815 00, at 2.4 L ha−1 alone or at 1.2 L ha−1 combined with 1.1 kg ha−1 of ammonium sulfate, added to the sethoxydim and Na-bentazon spray mixture overcame the antagonism on 14C-sethoxydim absorption in quackgrass. Crop oil concentrate at rates up to 9.6 L ha−1 did not overcome the antagonism. Adding ammonium phosphate or ammonium nitrate to the spray solution was as effective as adding ammonium sulfate in overcoming the antagonism caused by reduced absorption of sethoxydim even from the combination of Na-bentazon plus Na-acifluorfen. Ammonium salts and NH4-bentazon overcame the antagonism of Na-bentazon by forming readily absorbed NH4-sethoxydim.

The effect of atrazine rate and subsequent tillage on seedling alfalfa injury from atrazine carry-over was examined at two locations in Michigan. Atrazine concentration and alfalfa injury 1 yr after treatment were greater in the northern location than in the central Michigan location. At both locations, alfalfa injury was significantly greater under no-tillage than moldboard plowing. With no-tillage, alfalfa was injured more in the area between the previous corn rows than within the rows. Analysis of soil samples collected from within corn rows and between rows showed higher atrazine concentration between the corn rows.

The antidotes dichlormid, MON-4660, CGA-154281, R-29148, and MON-13900 were tested in the greenhouse to protect Zea mays L. (corn) against RPA 201772 injury. High rates of RPA 201772 injured four Z. mays hybrids > 30%. R-29148 and MON-13900 were the most effective of the five antidotes evaluated. R-29148 applied at rates ⩾ 45 g ha−1 provided excellent protection against RPA 201772 injury and also prevented injury to Z. mays from diketonitrile, the active metabolite of RPA 201772. In laboratory studies, R-29148 did not alter absorption of 14C-RPA 201772 from soil; however, R-29148 significantly enhanced the rate of RPA 201772 metabolism and inactivation in Z. mays. The mixed function oxidase inhibitor piperonyl butoxide (PBO) increased RPA 201772 injury on all hybrids. These results demonstrate that Z. mays tolerance to RPA 201772 can be enhanced with the use of antidotes such as R-29148 and MON-13900, that R-29148 protects Z. mays from RPA 201772 and diketonitrile by the enhancement of metabolism, and that oxidative reactions may be involved in the metabolism of RPA 201772 in Z. mays.

Field studies on a sandy loam soil determined the influence of application method and rate and subsequent tillage on corn response to imazaquin and imazethapyr residues remaining in the soil. Imazaquin was applied preplant incorporated (PPI) and preemergence (PRE) at 70, 140, and 280 g ai/ha and postemergence (POST) at 70 and 140 g/ha. Imazethapyr was applied PPI and PRE at 70, 105, and 140 g ai/ha and POST at 70 and 105 g/ha. Subsequent tillage included fall moldboard plowing followed by spring field cultivation, fall chisel plowing followed by spring field cultivation, and no tillage. Herbicide dissipation was determined by analyzing soil cores taken in the spring at the time of corn planting and by measuring corn height and grain yield. Imidazolinone residues were detected in only 1 of 2 yr. Imazaquin was detected more frequently than imazethapyr. Imazaquin concentrations in the upper 10 cm of soil 11 mo after a PPI application of 280 g/ha were 5, 6, and 7 ng/g of soil in moldboard plow, chisel, and no-tillage systems, respectively. Imazaquin (5 ng/g) was also detected in the 10–18-cm soil depth in the moldboard plow system. Corn height and grain yield were not reduced from imazaquin or imazethapyr, regardless of application method or rate in any tillage system.

Greenhouse studies were conducted to determine the response of six corn hybrids and two soybean varieties to acetolactate synthase (ALS) inhibitor herbicides applied with terbufos and/or piperonyl butoxide (PBO), a mixed function oxidase (MFO) inhibitor. Field experiments also were conducted to determine the response of six corn hybrids to the combination treatments, terbufos plus ALS inhibitor herbicides and/or PBO and/or antidote. PBO at 0.33 kg/ha tank-mixed with nicosulfuron and primisulfuron injured the Northrup King 9283 corn hybrid. Great Lakes 584 corn was less sensitive than Northrup King 9283 to these combination treatments. Pioneer 3377 IR corn hybrid was resistant to the combination of nicosulfuron or primisulfuron plus PBO at 2 kg/ha and also to the combination treatments of imazethapyr herbicide plus PBO even though terbufos was previously applied. ICI 8532 IT, ICI 8532, and Pioneer 3377 hybrids were injured by the combination of nicosulfuron or primisulfuron and/or terbufos and/or PBO at 2 kg/ha. ICI 8532 IT corn hybrid was not injured by the combination treatment of imazethapyr or thifensulfuron and terbufos. In the field studies, Pioneer 3377 IR and Ciba 4393 RSC hybrids were resistant to sulfonylurea and imidazolinone herbicides even when applied with PBO regardless of the presence of terbufos. All treatments of chlorimuron plus terbufos caused considerable injury to ICI 8532 IT, ICI 8532, Pioneer 3377, and Ciba 4393, but not Pioneer 3377 IR and Ciba 4393 RSC. The combination of thifensulfuron with PBO injured Elgin ‘87 soybean, but the W20 soybean was tolerant to this combination treatment. Combination of imazethapyr with PBO did not affect the growth of Elgin ‘87 soybean.

Wild carrot is becoming a serious weed problem in Michigan continuous no-tillage crop production. Greenhouse and field research was conducted to identify effective management strategies for wild carrot control in no-tillage cropping systems. In the greenhouse, PRE applications of acetochlor plus dichlormid (5.8:1 w:w), cyanazine, linuron plus chlorimuron (18:1), and metribuzin plus chlorimuron (10:1) and POST applications of bentazon, CGA-152005, clopyralid, cyanazine, and MON 12000 provided the greatest control of wild carrot seedlings. In the field, PRE and POST treatments containing chlorimuron consistently controlled overwintered wild carrot greater than 71% at 30 d after the POST application in no-tillage soybean. Atrazine, MON 12000, nicosulfuron, and primisulfuron applied POST consistently controlled overwintered wild carrot greater than 78% at 30 DAT in no-tillage corn. Glyphosate at 0.84 or 1.68 kg ae/ha applied in October to established wild carrot provided greater than 74% control the following spring. Early preplant (EPP) applications of glyphosate at 0.84 kg/ha in no-tillage soybean gave 95 and 24% control of overwintered wild carrot in St. Clair and Lenawee Counties, MI, respectively, at 70 DAT. PRE applications of glyphosate at 0.84 kg/ha controlled overwintered wild carrot less than 69% at 58 DAT in no-tillage corn and soybean. Fall applications of 2,4-D ester at 1.12 kg ae/ha provided 18 and 88% control of wild carrot the following spring in Clinton and Lenawee Counties, respectively. EPP applications of 2,4-D ester at 1.12 kg/ha provided 7 and 72% control of overwintered wild carrot in St. Clair and Lenawee Counties, respectively.

The effect of methylated seed oil (MSO), the organosilicone adjuvant DC-X2-5394, and ammonium sulfate on the efficacy, absorption, and spray retention of primisulfuron applied alone or with atrazine, dicamba, and bentazon to shattercane and giant foxtail was evaluated. Primisulfuron efficacy on both species was reduced by the three tank-mix combinations. Atrazine antagonism was not explained by decreases in foliar absorption and/or spray retention. Reductions in primisulfuron absorption and/or foliar spray retention appeared to cause bentazon antagonism on both weeds and dicamba antagonism on shattercane. MSO, DC-X2-5394, and ammonium sulfate completely reversed dicamba and bentazon antagonism on shattercane and partially reversed bentazon antagonism on giant foxtail by increasing foliar absorption and/or spray retention. Compared with non-ionic surfactant, MSO and DC-X2-5394 consistently increased giant foxtail control with primisulfuron by increasing foliar absorption and/or spray retention.

Field experiments were conducted in Michigan, Minnesota, and Wisconsin in 1990 to explore interactions between nicosulfuron applied POST and terbufos insecticide at 0.06 or 0.11 g ai/m of row applied in-furrow on ‘Pioneer 3751’ field corn and ‘Jubilee’ sweet corn. Nicosulfuron at 0, 70, and 140 g ai/ha plus nonionic surfactant and 28% nitrogen fertilizer was applied to both corn types. Field corn response to nicosulfuron and terbufos was similar at all locations, whereas sweet corn injury varied with location. Nicosulfuron injured field corn more when applied at the four-leaf than the three-leaf stage. Injury to both corn types increased as nicosulfuron rate increased or when applied following terbufos. Nicosulfuron at 140 g/ha without terbufos did not reduce yield of either corn type; however, corn previously treated in-furrow with terbufos reduced yield.