Field experiments were conducted from 1993 to 1995 to compare weed control by the isopropylamine salt of glyphosate at 0.21, 0.42, 0.63, and 0.84 kg ae/ha applied at three stages of weed growth. Weed control by glyphosate applied at these rates alone or with ammonium sulfate at 2.8 kg/ha was also evaluated. In other experiments, potential interactions between glyphosate and acifluorfen, chlorimuron, and 2,4-DB were evaluated. Velvetleaf, prickly sida, sicklepod, pitted morningglory, entireleaf morningglory, palmleaf morningglory, and hemp sesbania were controlled more easily when weeds had one to three leaves compared with control when weeds had four or more leaves. Glyphosate controlled redroot pigweed, velvetleaf, prickly sida, sicklepod, and barnyardgrass more effectively than pitted morningglory, entireleaf morningglory, palmleaf morningglory, or hemp sesbania. Increasing the rate of glyphosate increased control, especially when glyphosate was applied to larger weeds. Greater variation in control was noted for pitted morningglory, palmleaf morningglory, prickly sida, and velvetleaf than for redroot pigweed, sicklepod, entireleaf morningglory, or hemp sesbania. Ammonium sulfate increased prickly sida and entireleaf morningglory control but did not influence sicklepod, hemp sesbania, or barnyardgrass control. Acifluorfen applied 3 d before glyphosate or in a mixture with glyphosate reduced barnyardgrass control compared with glyphosate applied alone. Chlorimuron did not reduce efficacy. Mixtures of glyphosate and 2,4-DB controlled sicklepod, entireleaf morningglory, and barnyardgrass similar to glyphosate alone.

Availability of soybean with dicamba resistance will provide an alternative weed management option, but risk of dicamba injury to sensitive crops from off-target movement and spray tank contamination is of concern. Research conducted at multiple locations and years evaluated soybean injury and yield response to POST applications of the diglycolamine salt of dicamba. Dicamba was applied at the two to three trifoliate stage (V3/V4) at 4.4, 8.8, 17.5, 35, 70, 140, and 280 g ae ha−1(1/128 to 1/2 of the recommended use rate of 560 g ae ha−1). Soybean injury 7 d after application was 20% following dicamba at 4.4 g ha−1and increased to 89% at 280 g ha−1. At 14 d after application, injury for the same rates increased from 39 to 97%. In a separate study, dicamba was applied at first flower (R1) at 1.1, 2.2, 4.4, 8.8, 17.5, 35, and 70 g ha−1(1/512 to 1/8 of use rate). Soybean injury 7 d following dicamba application was 19% at 1.1 g ha−1and increased to 64% at 70 g ha−1. For the same rates of dicamba, injury from 7 to 14 d after application increased no more than 4 percentage points. For dicamba rates in common for the timing studies, soybean injury 14 d after treatment was greatest for application at V3/V4, but the negative effect on mature soybean height and yield was greatest for application at R1. For dicamba at 4.4 g ha−1(1/128th of use rate), soybean yield was reduced 4% when applied at V3/V4 and 10% when applied at R1. For 17.5 g ha−1dicamba (1/32 of use rate), yield was reduced 15% at V3/V4 and 36% at R1. Based on yield reductions for 4.4 and 17.5 g ha−1dicamba, soybean at flowering was around 2.5 times more sensitive compared with vegetative exposure.

Field research was conducted during 3 yr to evaluate response of rice and corn to simulated drift rates representing 12.5, 6.3, 3.2, 1.6, and 0.8% of the usage rates of 1,120 g ai/ha glyphosate (140, 70, 35, 18, and 9 g/ha, respectively) and 420 g ai/ha glufosinate (53, 26, 13, and 4 g/ha, respectively). Early-postemergence applications were made to two- to three-leaf rice and six-leaf corn, and late-postemergence applications to rice at panicle differentiation and to corn at nine-leaf stage (1 wk before tasseling). Crop injury was generally greater for the two highest rates of both herbicides when applied early. Little to no reduction in rice or corn height was observed with glufosinate. Glyphosate consistently reduced rice plant height when the two highest rates were applied early, and heading was delayed 2 to 5 d. In 2 of 3 yr, the highest rate of glyphosate reduced rice yield 99 and 67% when applied early and 54 and 29% when applied late. Germination of rice seeds from glyphosate-treated plants was reduced in 1 of 2 yr and for only the highest rate. For glufosinate, rice yield was reduced 30% and in only one year when applied late at the highest rate. Early application of glyphosate reduced corn yield an average of 22 to 78% for the three highest rates, but only for the highest rate at the late timing (33%). Corn yield was reduced an average of 13 and 11% for the highest rate of glufosinate at the early and late timings, respectively. In greenhouse studies, five rice varieties were equally sensitive, as were five corn varieties, to reduced rates of glyphosate and glufosinate.

Transformation of soybean [Glycine max (L.) Merr.] with a gene encoding a glyphosate‐tolerance 5‐enolpyruvylshikimate‐3‐phosphate synthase enzyme from Agrobacterium sp. strain CP4 resulted in the development of glyphosate‐tolerant line 40‐3‐2. Glyphosate (N‐phosphonomethyl glycine) is the active ingredient of Roundup herbicide. Line 40‐3‐2 was yield tested at 17 locations in 1992,23 locations in 1993, and 18 locations in 1994. At those locations, broadcast applications of glyphosate at various rates were made over 40‐3‐2 or its derivatives from early vegetative growth to pod fill. No significant yield reduction was observed as a result of the glyphosate treatment at any of the locations. Development of glyphosate‐tolerant soybean promises to provide the farmer with access to a new weed control system that should result in lower production costs and reliable weed control under a wide range of conditions.

A balanced forage program should include species that provide the highest yields of quality forage throughout the grazing period. In the Northeast, where cool‐season grasses predominate, the short supply of forage during summer limits beef cow‐calf herd size. Warm‐season (C4) perennial grasses are productive in midsummer and may supplement temperate species for grazing and hay. ‘NY 1145’ big bluestem (Andropogon gerardi Vitman) and ‘Blackwell’ switchgrass (Panicum virgatum L.) were grown on soils (Aeric Fragiaquualt and Aquic Hapludalf) low in available P at two locations in Pennsylvania to characterize changes in quality of leaf and stem tissue associated with maturation. Forage was harvested at 10‐day intervals beginning at the 3‐ to 4‐leaf stage in late June and continuing until seed set in early August. Percentage leaf tissue declined similarly with maturation for the two grasses. Leaf dry matter yields were approximately twice those of stems in June but the opposite was true in August. At early head emergence, percentage leaf tissue for big bluestem and switchgrass averaged 34 and 44%, respectively. Averaged over grasses, leaf and stem forage quality estimates at early head emergence, respectively, were: crude protein (CP), 9.7, 4.3% in vitro dry matter disappearance (IVDMD), 60.4, 50.0%; neutral detergent fiber (NDF), 66.0, 75.3%; lignin, 4.7, 7.2%; and phosphorus (P), 0.20, 0.16%. Big bluestem leaves were higher in CP but lower in NDF than switchgrass leaves. Stem tissue of big bluestem was lower in NDF but higher in lignin than that of switchgrass. The decline in leaf and stem CP, IVDMD, and P with maturation was less pronounced in leaf tissue. The increase in NDF and lignin with maturity was greater in stems than in leaves. Fiber accumulation in stem tissue continued after seedheads emerged and was accompanied by decreases in CP and IVDMD. At early vegetative stages, the high percentage of good quality leaf tissue suggest the potential use of big bluestem and switchgrass for ruminants with above maintenance energy requirements. However, at later growth stages (late joint early head), the increase in stem tissue and associated decline in nutritive value suggest using these grasses either for grazing or hay by ruminants to meet only maintenance energy needs.

Field research was conducted for a period of 2 yr to evaluate the response of soybean and cotton to simulated drift rates representing 12.5, 6.3, 3.2, 1.6, and 0.8% of the usage rates of 1,120 g ai/ha glyphosate (140, 70, 35, 18, and 9 g/ha, respectively) and 420 g ai/ha glufosinate (53, 26, 13, 7, and 4 g/ha, respectively). Early-postemergence applications were made to 2- to 3-trifoliate soybean and 2- to 3-leaf cotton, and late applications to soybean at first flower and cotton at early bloom. A mid-postemergence application was also made to cotton at pinhead square (first flower bud development). Soybean and cotton injury and height reductions occurred in most cases for only the two highest rates of the herbicides with variation noted between years. Soybean height was reduced by no more than 11%, regardless of herbicide rate or timing. On the basis of visual injury, soybean was more sensitive to glyphosate than to glufosinate when applied early in 1998, but sensitivity was equal for both the herbicides in 1999. When herbicides were applied late, soybean was more sensitive to glufosinate in the first year. Cotton was more sensitive to glufosinate 7 d after application in both years, regardless of timing, but by 28 d differences between herbicides were less apparent. Cotton maturity was not delayed by either herbicide, on the basis of days to first square or flower and nodes above white flower. Both crops were able to recover rapidly from herbicide injury, and yields were not affected negatively.

Seed production and seedling emergence of three broadleaf weed species were evaluated following glyphosate application at initial seed set, mid seed fill, and physiological maturity. In greenhouse experiments averaged across glyphosate rates of 0.42, 0.63, and 0.84 kg ae ha−1, Xanthium strumarium 100-bur weight and burs per plant were reduced at least 69 and 70%, respectively, for application at initial fruit set compared with later applications, and seedling emergence was 3% of the nontreated check. Glyphosate application at initial seed set reduced Sesbania exaltata 100-seed weight 73%, seed per plant 86%, and seedling emergence 94%. Senna obtusifolia 100-seed weight, seed per plant, and seedling emergence were reduced 46, 83, and 66%, respectively, when glyphosate was applied at initial seed set. In field experiments, X. strumarium and S. exaltata seed production were reduced only when glyphosate was applied at initial seed set. Compared with the nontreated check, seedling emergence following initial seed set application was reduced 82% for X. strumarium and 94% for S. exaltata. S. obtusifolia response in the field was inconsistent with no reductions in seed per plant or seedling emergence observed the first year. The second year, initial seed set application reduced seed per plant 88% and seedling emergence 72%.

Field research conducted over 3 yr evaluated the utility of preemergence (PRE), soil-applied herbicides at half- and full-label rates in glyphosate-resistant soybean. Soil-applied herbicide treatments at full-label rates included pendimethalin plus imazaquin (0.84 + 0.14 kg ai/ha), pendimethalin (1.12 kg/ha), metolachlor (1.68 kg ai/ha), dimethenamid plus imazaquin (1.0 + 0.14 kg ai/ha), sulfentrazone plus chlorimuron (0.22 + 0.04 kg ai/ha), and metribuzin plus chlorimuron (0.36 + 0.06 kg ai/ha). Weed density and growth were reduced when PRE herbicides were used, and in many cases for broadleaf weeds, half-label rates were as effective as full rates. None of the herbicides provided complete control of all weeds. Sulfentrazone plus chlorimuron reduced ivyleaf morningglory density an average of 90%. For hemp sesbania, metribuzin plus chlorimuron reduced weed emergence over 3 yr at least 95%. The initial glyphosate application was made when the largest weeds, barnyardgrass or hemp sesbania, reached 10 cm. In 1998 all soil-applied herbicide treatments extended the time period of glyphosate application by 3 to 5 d when compared with the nontreated control. In 1999 the full rate of metribuzin plus chlorimuron delayed the application of glyphosate by 6 d, and an extension of 7 d was noted for the full rates of sulfentrazone or metribuzin plus chlorimuron in 2000. When soil-applied herbicides were used each year, only a single application of glyphosate was needed. A second glyphosate application was needed in only 1 yr when soil-applied herbicides were not used. Even though differences in weed control were observed among the herbicide treatments, soybean yield was the same.