Field studies were conducted in 1997 and 1998 to evaluate the efficacy of imidazolinone weed management systems and crop tolerance of imidazolinone-tolerant (IT) corn to imazapic. Imazapic (36 and 72 g/ha) was evaluated when applied PRE; early postemergence (EPOST), when corn was at the two- to three-leaf stage; and late postemergence (LPOST), when corn was at the six- to eight-leaf stage. Imazapyr imazethapyr EPOST and metolachlor atrazine followed by (fb) primisulfuron LPOST were evaluated as commercial standards. Imazapic at 36 g/ha EPOST controlled johnsongrass, Texas panicum, smellmelon, and ivyleaf and entireleaf morningglory at least 93% when adequate rainfall occurred. Devil's claw was controlled at least 85% with imazapic EPOST or LPOST at either rate. Imazapic at 36 g/ha EPOST and LPOST controlled eclipta 88 and 91%, respectively. Yellow nutsedge was controlled >91% with imazapic LPOST, which was superior to imazapic applied EPOST. Broadleaf signalgrass was controlled 94% with imazapic at 72 g/ha LPOST, which was significantly higher than other herbicide treatments. Imazapic at 36 and 72 g/ha applied PRE under moisture stress resulted in reduced weed control; but when adequate rainfall occurred, weed control was generally similar to that of EPOST and LPOST applications. Imazapic crop response at 72, 105, 140, and 211 g/ha applied at EPOST and LPOST was evaluated in two IT corn hybrids. Crop response varied with soil and environmental conditions and application timings. Imazapic at 72, 105, and 140 g/ha EPOST resulted in crop injury 33 to 55% at 6 wk after planting (WAP) in a coarse soil; however, crop injury decreased to <20% at 12 WAP. Low crop injury (<9%) was observed at 12 WAP in a clay soil. Imazapic applied LPOST resulted in lower crop injury than EPOST application. Corn plant height and yield were not affected by any imazapic treatment.
Nomenclature: Imazapic; imazapyr; imazethapyr; atrazine; metolachlor; primisulfuron; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. #3 IPOHE; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray # IPOHE; smellmelon, Cucumis melo (L.) var. dudaim Naud. # CUMMD; Texas panicum, Panicum texanum Buckl. # PANTE; johnsongrass, Sorghum halepense (L.) Pers. # SORHA; devil's-claw, Proboscidea louisianica (Mill.) Thellung # PROLO; eclipta, Eclipta prostrata L. # ECLAL; yellow nutsedge, Cyperus esculentus L. # CYPES; broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash # BRAPP; corn, Zea mays L., ‘Garst 8326IT’, ‘Garst 8396IT’, ‘Cargill 7994IT’, ‘Garst 8222IT’.
Additional index words: Crop response, application timings.
Abbreviations: EPOST, early postemergence; fb, followed by; IR, imidazolinone-resistant; IT, imidazolinone-tolerant; LPOST, late postemergence; TAES, Texas Agricultural Experiment Station Field Laboratory; TP, TP Farms; WAP, weeks after planting.