A rainfall simulator was used to deliver the equivalent of 1.3 cm of water in 0.25 h at 0, 3, 6, 12, 24, and 48 h after POST applications of asulam to johnsongrass in greenhouse and field studies. Johnsongrass control responses were similar when asulam was applied with either a crop oil concentrate or a commercially blended organosilicone/crop oil concentrate premix. Rainfall 24 h after asulam application reduced johnsongrass control in greenhouse studies. Maximum visual johnsongrass control of 80, 69, and 69% was obtained in field studies when rainfall occurred 20, 14, and 8 h after asulam application, respectively. Based on reductions in johnsongrass dry weight, rainfree periods needed to insure maximum performance with asulam in field studies ranged from 3 to 16 h. Variation in critical rainfree periods was related to plant growth stage when asulam was applied and environmental conditions and is indicative of the inconsistency in johnsongrass control commonly observed with asulam.

In two separate field experiments conducted over time, at-planting preemergence (PRE) applications of sulfometuron at 110 to 150 g ai/ha followed by March applications of sulfometuron at 30 g/ha or metribuzin at 2,580 g ai/ha at the start of the initial (plant cane) growing season controlled more bermudagrass and johnsongrass emerging in sugarcane from rhizome and stolon pieces than standard at-planting followed by March applications of atrazine at 2,240 g ai/ha (no control), metribuzin at 2,580 g ai/ha, or pendimethalin at 2,240 g ai/ha. Treatments containing at-planting applications of imazapyr at 280 g ai/ha or thiazopyr at 560 g ai/ha did not provide levels of control similar to sulfometuron. Crop injury in excess of 10% occurred only when imazapyr at 280 g/ha (18 to 42%) or sulfometuron at 30 g/ha (11 to 32%) was applied in March to emerging sugarcane. Sugarcane stalk numbers, stalk heights, and ultimately cane and sugar yields were increased when compared to the standards with all sulfometuron treatments and when thiazopyr was applied at planting and again in March.

Soil- and foliar-applied herbicide treatments were evaluated for the control of seedling johnsongrass in the interim between row formation and the planting of fallowed sugarcane fields approximately 90 d later. Soil-surface applications of metribuzin at 1,680 g ai/ha, pendimethalin plus atrazine each at 2,240 g ai/ha, terbacil at 1,680 g ai/ha, and sulfometuron at 35 and 70 g ai/ha and an incorporated application of trifluralin at 2,240 g ai/ha followed by a surface application of atrazine at 2,240 g/ha did not consistently control seedling johnsongrass until the crop was planted. Rhizome johnsongrass populations originating from seedling johnsongrass that escaped the fallow treatments were lowest in the newly planted crop when sulfometuron at 140 to 280 g/ha was applied to the soil surface and when glyphosate was applied POST at 2,240 g ai/ha, particularly as a sequential treatment alone or as a spot treatment at 2% by volume following applications of metribuzin, terbacil, and pendimethalin or trifluralin with atrazine. Sugarcane shoot populations in the fall after planting and sugar yields at the end of the crop's first growing season were highest where fallow treatments minimized johnsongrass development. These treatments also provided broad spectrum control of other seedling weeds, the residues of which influenced crop development.

Asulam applied POST alone and in mixtures with residual herbicides and with DSMA and MSMA was evaluated for johnsongrass control. Johnsongrass control following treatment with asulam at 3.7 kg ai ha−1 was not improved by tank mixtures with the residual herbicides atrazine, metribuzin, pendimethalin, and terbacil compared with asulam applied alone. Johnsongrass injury symptoms appeared sooner when asulam was applied in mixtures with either DSMA or MSMA, but johnsongrass control 4 wk after treatment was similar to asulam applied alone. As a result of the johnsongrass control obtained with early POST treatments of asulam applied alone, cane and sugar yields were increased by 56 and 58%, respectively, when compared with the weedy check. Additional cane and sugar yield increases were not obtained by any of the asulam mixtures because johnsongrass control was not improved. Asulam applied again in May to johnsongrass regrowth improved late-season control in two of the three studies over a single early POST application, but cane and sugar yields were not increased.

Field studies were conducted over four years to determine sugarcane response to clomazone applied POST at 1.1 and 2.2 kg ai/ha. Sugarcane leaves present when clomazone was applied were chlorotic 2 WAT. Injury 4 WAT ranged from 0 to 38% following single applications in March and April. Chlorosis 4 wk after the May application of a March plus May sequential treatment was only greater than for the single March application in 1992. The number of harvestable sugarcane stalks following treatments with clomazone was intermediate between those produced with standard applications of either metribuzin at 2.6 kg ai/ha (least injurious) or terbacil as a single treatment at 1.8 kg ai/ha in March and April or as a 1.8 plus 0.9 kg/ha March plus May sequential treatment. Sugar recovery as an average of all herbicide treatments was lower for the sequential applications than for the single applications. Averaged over single and sequential treatments, theoretically recoverable sugar levels with clomazone at both rates was intermediate between the recoveries obtained with standard applications of metribuzin (highest) and terbacil. Gross cane and sugar yields following all applications of clomazone at 1.1 kg/ha and following single applications of clomazone at 2.2 kg/ha in March and April were similar to the metribuzin standard. Sugar yields following the sequential treatment of clomazone at 2.2 kg/ha, although lower than the metribuzin standard, were similar to the standard terbacil sequential treatment.

Sulfometuron at 17 g ai/ha in the planting furrow (13% of the anticipated fallow-field application rate) inhibited sugarcane emergence and development and ultimately reduced sugar yields in the initial production year by 13% when compared to a weed-free control that contained no herbicide in the planting furrow. Residual levels of metribuzin in the planting furrow representing 100% of the standard fallow-field application rate of 1,680 g ai/ha had no adverse effect on sugarcane development or sugar yield. When applied only to the soil surface immediately after planting, sulfometuron did not injure sugarcane, and sugar yields were equivalent to standard, at-planting, preemergence (PRE) applications of either metribuzin at 2,020 g/ha or a mixture of pendimethalin plus atrazine each at 2,240 g ai/ha. To minimize sugarcane injury, sulfometuron should be kept out of the germinating zone of lateral buds along planted sugarcane stalks.

Field studies were conducted over 3 yr to evaluate rhizome johnsongrass control with asulam applied POST at 3.7 kg ai/ha 3 d before fertilization (DBF) or 0, 3, 7, 10, or 14 d after fertilization (DAF). Liquid fertilizer (18-6-12) was applied with injector knives at 0, 112, or 224 kg N/ha 5 cm on each side of a 60-cm line of rhizome johnsongrass present on 1.8-m-wide conventional sugarcane beds. Johnsongrass response to timing of asulam application after fertilization varied among years but was not affected by fertilizer rate. Johnsongrass control with asulam applied 3 DBF was greater than applications made 0, 3, or 7 DAF. With one exception, differences in johnsongrass control following fertilization and asulam application times were also reflected in the biomass of treated johnsongrass and its regrowth harvested 6 and 10 wk after asulam application, respectively. Reduced johnsongrass control associated with asulam application following fertilization was related to stress from root/rhizome injury during fertilizer placement rather than the quantity of fertilizer applied.

The impact of POST applications of fluazifop-P on sugarcane grown in Louisiana was determined for 3, 6,11, 22, and 44% of the lowest rate recommended for the control of johnsongrass in soybean (100 g ai ha−1). In field studies, sugarcane injury 4 WAT increased as the rate of fluazifop-P increased. Injury in excess of 80% was observed when fluazifop-P at 44 g ha−1 was applied in May and June and 54% when applied in August Sugarcane stalks treated with fluazifop-P were fewer in number, shorter and lighter, and immature at harvest This resulted in lower Brix and sucrose levels, higher fiber content, and ultimately lower sugar yields. Reductions in sugar yields associated with an application of fluazifop-P closely paralleled visual injury 4 WAT. Injury from application of fluazifop-P was also evident in the subsequent ratoon crop where reductions in spring shoot counts of 14 to 25% were obtained even when fluazifop-P had been applied at 6 g ha−1 the previous year. Greenhouse studies indicated that the response observed in the field with CP 70-321 sugarcane was indicative of the major cultivars grown, with the possible exception of CP 74-383.