Response of Eight Sweet Maize (Zea mays L.) Hybrids to Saflufenacil Alone or Pre-Mixed with Dimethenamid-P

Saflufenacil is a new herbicide for use in field maize (Zea mays L.) and other crops that may have potential for weed management in sweet maize. Tolerance of eight sweet maize hybrids to saflufenacil and saflufenacil plus dimethenamid-p applied preemergence (PRE) were studied at two Ontario locations in 2008 and 2009. Saflufenacil applied PRE at 75 and 150 g·ha and saflufenacil plus dimethenamid-p (pre-mixed) applied PRE at 735 and 1470 g·ha caused minimal (less than 5%) injury in Cahill, GH4927, Harvest Gold, Rocker, BSS5362, GG236, GG447, and GG763 sweet maize hybrids at 1 and 2 weeks after emergence (WAE). Saflufenacil or saflufenacil plus dimethenamid-p applied PRE did not reduce plant height, cob size, or yield of any of the sweet maize hybrids tested in this study. Based on these results, saflufenacil and saflufenacil plus dimethenamid-p pre-mixed applied PRE at the doses evaluated can be safely used for weed management in Cahill, GH4927, Harvest Gold, Rocker, BSS5362, GG236, GG447, and GG763 sweet maize under Ontario environmental conditions.


Introduction
Sweet maize (Zea mays L.) is one of the most important field grown vegetables in Ontario [1].In 2009, nearly 112,000 tonnes of sweet maize was produced on approximately 9000 hectares with a farm-gate value of $36 million, and ranked as the second largest field grown vegetable crop in Ontario in terms of farm-gate value [1].Weed control is critical in sweet maize production to maintain quality and yield and be competitive in the global market place.More research is needed to identify herbicide options that can effectively control grass and broadleaved weeds in sweet maize production.
Saflufenacil is a pyrimidinedione that inhibits protoporphyrinogen-IX-oxidase (PPO).Susceptible weeds to saflufenacil show injury symptoms within a few hours and die in 1 to 3 days [6].Saflufenacil has both contact and residual activity against susceptible weeds and is mainly translocated in the xylem [6].Saflufenacil is applied at relatively low doses and has low environmental, toxicological and eco-toxicological impact with minimal residual carryover and persistence in the soil [6].The proposed dosage for sweet maize in Ontario is 75 g a.i.ha -1 .Saflufenacil provides a novel mode of action (PPO inhibitor) for sweet maize that is different than currently used broadleaved herbicides reducing potential for the selection of herbicide resistant weed biotypes [6,7].
Saflufenacil is also compatible with residual herbicides that control grasses.BASF has developed saflufenacil plus dimethenamid-p premix (BAS781) for use in maize and other crops [6].Dimethenamid-p is a chloroacetamide herbicide that in susceptible plants inhibits very long chain fatty acid synthesis [7].Dimethenamid-p can provide season long control of a broad spectrum of grass and broadleaved weeds such as barnyardgrass (Echinochloa crusgalli), autumn panicum (Panicum dichotomiflorum), giant foxtail (Setaria faberi), green foxtail (Setaria viridis), yellow foxtail (Setaria glauca), large crabgrass (Digitaria sanguinalis), smooth crabgrass (Digitaria ischaemum), witchgrass (Panicum capillare), redroot pigweed (Amaranthus retroflexus), American black nightshade (Solanum americanum), and eastern black nightshade (Solanum ptycanthum) [2,7].Dimethenamid-p at the registered application doses has been shown to cause little or no injury in field maize [8,9].Saflufenacil plus dimethenamid-p can provide an effective broad spectrum herbicide option for the control of troublesome species in sweet maize.
Saflufenacil and saflufenacil plus dimethenamid-p are desirable compliments to the current weed management programs in sweet maize because of its low dosage; broad-spectrum weed control, environmental safety, and new mode of action that will help reduce selection for herbicide resistant biotypes.There is no published information on the sensitivity of sweet maize hybrids to the PRE application of saflufenacil or saflufenacil plus dimethenamid-p.If tolerance is adequate, registration of saflufenacil and saflufenacil plus dimethenamid-p will provide sweet maize growers with an additional option for annual weed control.Sensitivity of sweet maize to herbicides is dependent on the application dose, hybrid, and environmental conditions.Sweet maize hybrid sensitivity has been documented for foramsulfuron [10], bentazon [11], prosulfuron [12], mesotrione [13], nicosulfuron [14,15], primisulfuron [16], isoxaflutole [17], and thifensulfuron-methyl [18].
The objective of this study was to determine the sensitivity of Cahill, GH4927, Harvest Gold, Rocker, BSS5362, GG236, GG447, and GG763 sweet maize to saflufenacil and saflufenacil plus dimethenamid-p applied PRE under Ontario environmental conditions.

Materials and Methods
Field experiments were conducted at the University of Guelph, Ridgetown Campus, Ridgetown, Ontario and the Huron Research Station, Exeter, Ontario in 2008 and 2009.The soil at the Ridgetown location was a Watford/ Brady loam composed of 51% sand, 32% silt, 16% clay, and 5.5% organic matter with a pH of 7.2 in 2008 and 49% sand, 34% silt, 17% clay, and 9.2% organic matter with a pH of 7.2 in 2009.The soil at the Exeter location was a Brookston clay loam composed of 34% sand, 36% silt, 30% clay, and 3.6% organic matter with a pH of 8.0 in 2008 and 39% sand, 37% silt, 24% clay, and 4.3% organic matter with a pH of 7.9 in 2009.Seedbed preparation consisted of moldboard plowing in the fall and cultivation in the spring.Fertilizer was broadcast and incorporated prior to seeding based on soil tests and local recommendations.
There were two experiments established side by side at each site (one evaluating saflufenacil and the other evalu-ating saflufenacil plus dimethenamid-p).The experiments were arranged in a split-plot design with four replications.The main plots were herbicide dose, and the subplots were sweet maize hybrids.Selection of herbicide doses was based on the manufacturer recommended use dose and twice the manufacturer recommended dosage.
Treatments consisted of a non-treated check and two doses of saflufenacil (0, 75 and 150 g a.i.ha -1 ) or saflufenacil plus dimethenamid-p (0, 735 and 1470 g a.i.ha -1 ) representing the untreated control and the 1X and 2X of the proposed label dose, respectively.Eight of the most commonly grown processing sweet maize hybrids in southwestern Ontario encompassing a range of endosperm genotypes were selected: Cahill (su), GH4927 (su), Harvest Gold (su), Rocker (su), BSS5362 (sh 2 ), GG236 (su), GG447 (su), and GG763 (su) sweet maize.Each of the main plots was 6 m wide by 8 m long at Ridgetown and 6 m wide by 10 m long at Exeter.The subplots each consisted of a single row of each sweet maize hybrid with rows spaced 75 cm apart.The sweet maize was thinned to 50,000 plants ha -1 shortly after emergence.The plots were then kept weed-free using inter-row cultivation and hand hoeing as required.
Herbicide treatments were applied PRE four to eight days after planting using a CO 2 -pressurized backpack sprayer calibrated to deliver 200 L aqueous solution at 241 kPa.The boom was 1.5 m wide with four ULD120-02 nozzles (ULD120-02 nozzles tip; Spraying Systems Co., Wheaton, IL.) spaced 0.5 m apart.
Crop injury including stand reduction was evaluated visually comparing the non-treated hybrid to the respective treated hybrids on a scale of 0 to 100% at 1 and 2 weeks after emergence (WAE).A rating of 0% was defined as no visible effect of the herbicide and 100% was defined as plant death.Average maize height (based on ten random plants per subplot) was measured for each subplot 3 WAE.The height of the plant was defined as the maximum height from the soil surface with the leaves fully extended.At maturity, each subplot was harvested by hand and cob size, marketable yield (a cob greater than 5 cm in diameter) and total yield were recorded.Because the results of the statistical analyses for total and marketable yields were similar, only marketable yield is reported.
All data were subjected to analysis of variance (ANO-VA).Tests were combined over locations and years and analyzed using the PROC MIXED procedure of SAS (Statistical Analysis Systems Institute, Cary, NC, USA).Variances of percent crop injury at 1 and 2 WAE, plant height, cob size, and yield were partitioned into the fixed effects of herbicide treatment, hybrid, and herbicidehybrid interaction and into the random effects of siteyear, block (site-yr), site year-treatment, site year-hybrid and site year-hybrid-treatment.Significance of random effects was tested using a Z-test of the variance estimate and fixed effects were tested using F-tests.Error assumptions of the variance analyses (random, homogeneous, normal distribution of error) were confirmed using residual plots and the Shapiro-Wilk normality test.To meet the assumptions of the variance analysis, visual injury at 1 and 2 WAE were subjected to an arcsine square root transformation and cob size data were log transformed.No transformation was required for plant height or yield.Treatment means were separated using Fisher's protected LSD test.Means of percent injury and cob size were compared on the transformed scale and were converted back to the original scale for presentation of results.Type I error was set at P ≤ 0.05 for all statistical comparisons.

Results and Discussion
Statistical analysis of the data on visible injury, plant height, cob size and yield showed that the random effects of location, year, year by location and interactions with treatments were not significant.Therefore, data were pooled and averaged over years and locations (Tables 1-4).a Abbreviations: su = sugary; sh 2 = shrunken endosperm mutant genotype; b Results are averaged for both locations and years; means followed by the same letter within a row for each treatment are not significantly different according to Fisher's Protected LSD test (P ≤ 0.05).

Crop Injury
Visible injury symptom observed was leaf speckling.
Results are similar to those reported in field maize.Soltani et al. (2009) [19] found 1% or less injury in maize with saflufenacil applied PRE at 50, 100 and 200 g a.i.ha -1 .Moran (2010) [20] also found no injury in field maize with saflufenacil applied PRE at 75 and 150 g a.i.ha -1 or saflufenacil plus dimethenamid-p applied PRE at 735 and 1470 g a.i.ha -1 .Little visible injury seen in different sweet maize varieties evaluated in this study is similar with previous studies on clopyralid [21] and to a Abbreviations: su = sugary; sh 2 = shrunken endosperm mutant genotype; b Results are averaged for both locations and years; means followed by the same letter within a row for each treatment are not significantly different according to Fisher's Protected LSD test (P ≤ 0.05).

Plant Height
No reduction in plant height was observed for any of the eight sweet maize hybrids treated with saflufenacil or saflufenacil plus dimethenamid-p applied PRE at doses evaluated (Table 2).Plant height was similarly unaffected by increasing herbicide doses.In other studies, Soltani et al. (2009) [19] reported no adverse effect in field maize height with saflufenacil applied PRE in field maize at dose up to 200 g a.i.ha -1 .Lack of any height reduction between sweet maize hybrids evaluated in this study with saflufenacil and saflufenacil plus dimethenamid-p is similar to those found with other herbicides such as clopyralid, halosulfuron and topramezone [21,26].

Cob Size
Saflufenacil and saflufenacil plus dimethenamid-p applied PRE at doses evaluated caused no decrease in cob size of Cahill, GH4927, Harvest Gold, Rocker, BSS5362, GG236, GG447, and GG763 sweet maize (Table 3).Results in these trials are similar to findings with other herbicides such as halosulfuron which did not caused any negative impact on cob size at 1X or 2X of the proposed label dose for any of the sweet maize hybrids studied [26].However, other studies have shown that cob size of susceptible hybrids can be reduced up to 67% with clopyralid or thifensulfuron-methyl [18,21].

Yield
Saflufenacil applied and saflufenacil plus dimethenamidp applied PRE at doses evaluated caused no adverse effect on yield of Cahill, GH4927, Harvest Gold, Rocker, BSS5362, GG236, GG447, and GG763 sweet maize (Ta-ble 4).Yield was similarly unaffected by increasing herbicide doses in all sweet maize hybrids evaluated.In other studies, Soltani et al. ( 2009) [19] reported no adverse effect in yield with saflufenacil applied PRE in field maize at dose up to 200 g a.i.ha -1 .Moran (2010) [20] also found no yield reduction in field maize with saflufenacil applied PRE at 75 and 150 g a.i.ha -1 or saflufenacil plus dimethenamid applied PRE at 735 and 1470 g a.i.ha -1 .Yield response with saflufenacil and saflufenacil plus dimethenamid-p are similar to yield response in other herbicides, such as clopyralid [21], topramezone [22] and halosulfuron [26] which were not adversely affected when the herbicide was applied at the label dose.However, other studies have reported significant injury in some sweet maize hybrids with certain herbicides.Diebold et al. (2003Diebold et al. ( , 2004) ) [10] reported up to 94% reduction in yield with formsulfuron in sweet maize.Similar yield reduction were reported with mesotrione [13], nicosulfuron [14], foramsulfuron [10] and nicosulfuron plus rimsulfuron [24,25] in some sensitive sweet maize hybrids.The potential for and level of crop injury from use of nicosulfuron, mesotrione, and foramsulfuron on any specific sweet maize hybrid is conditioned largely by CYP alleles at the nsf1/ben1 locus on the short arm of chromosome 5 [27].However, the sensitivity of sweet maize to other herbicides is controlled by other gene loci.Bentazon metabolism, for example, is controlled by ben1, as well as two independent genes, Cr1 and Cr2 [28].It is hypothesized that sweet maize tolerance to saflufenacil is also conditioned by alternate alleles of the above genes and/or different gene loci, which have, as yet, not been determined.

Conclusion
Based on this study, the sweet maize hybrids Cahill, GH4927, Harvest Gold, Rocker, BSS5362, GG236, GG447, and GG763 are tolerant to saflufenacil and saflufenacil plus dimethenamid-p applied PRE at doses evaluated.Saflufenacil and saflufenacil plus dimethenamid-p applied PRE to eight sweet maize hybrids had no negative effect on sweet maize injury, height, cob size, or yield.As the dose of saflufenacil or saflufenacil plus dimethenamid-p was increased from 1X to 2X of the proposed label dose, there was no negative effect on any sweet maize hybrid.This study shows that saflufenacil and saflufenacil plus dimethenamid-p can be safely applied to these eight sweet maize hybrids at the proposed label dose.The registration of saflufenacil alone or premixed with dimethenamid-p would provide Ontario sweet maize producers with a new, broad-spectrum herbicide that controls selected annual grass and broadleaved weed species.Furthermore, if used in a diversified, integrated weed management program it would reduce the selection intensity for herbicide resistant weeds.

Table 1 . Injury at 1 and 2 weeks after emergence (WAE) of eight sweet maize hybrids treated prior to emergence with saflufenacil at 0, 75, and 150 g•ha -1 or saflufenacil plus di- methenamid-p at 0, 735, and 1470 g•ha -1 at Exeter, ON, and Ridgetown, ON, in 2008 and 2009. Injury 1 WAE Injury 2 WAE Treatment/Hybrid a 75/735 b 150/1470 b 75/735 b 150/1470 b _________________%_________________
a Abbreviations: su = sugary; sh 2 = shrunken endosperm mutant genotype; b Results are averaged for both locations and years; means followed by the same letter within a row for each treatment are not significantly different according to Fisher's Protected LSD test (P ≤ 0.05).