Published in Agron J 97:847-853 (2005)
DOI: 10.2134/agronj2004.0203
© 2005 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Production Papers
Weed Interference and Glyphosate Timing Affect Corn Forage Yield and Quality
William J. Cox*,
Russell R. Hahn,
Paul J. Stachowski and
Jerome H. Cherney
Department of Crop and Soil Sciences, 609 Bradfield Hall, Cornell Univ., Ithaca, NY 14853
* Corresponding author (wjc3{at}cornell.edu)
Received for publication July 28, 2004.
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ABSTRACT
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Corn (Zea mays L.) planting and the first harvest of perennial forages overlap in the northeastern USA in some years. The use of glyphosate [N-(phosphonomethyl)glycine]–resistant corn may help dairy producers lessen their workload during this time by allowing for a timely glyphosate application after completion of the first harvest of perennial forages. We evaluated 92 and 103-d hybrids to determine the impact of season-long weed interference and the optimum timing for glyphosate application on corn forage yield and quality. Season-long weed interference vs. weed-free corn in two competitive growing seasons reduced dry matter (DM) accumulation at silking (R1 stage) by 50 to 65%, DM yield by 70 to 75%, and calculated milk yield by 75 to 80%. Season-long weed interference reduced milk per megagram, a forage quality index, by 10% in a dry year by preventing grain formation. Glyphosate application at the three to four leaf stage (V3–V4) vs. weed-free corn resulted in similar DM accumulation at the R1 stage, DM content at harvest, DM yield, forage quality, and calculated milk yield in both years. Glyphosate application at the V5–V6 stage vs. weed-free corn increased milk per megagram by 7% in the dry year, but resulted in 20 to 25% less DM yield in both years. Dairy producers in the northeastern USA should apply glyphosate by the V3–V4 stage in competitive growing conditions, regardless of hybrid maturity, which may overlap with the first harvest of perennial forages in some years.
Abbreviations: CP, crude protein • DM, dry matter • EPOST, early postemergence • IVTD, in vitro true digestibility • LPOST, late postemergence • MPOST, mid-postemergence • NDF, neutral detergent fiber • NDFd, neutral detergent fiber digestibility • PRE, preemergence • R1, silking stage • Vn, nth leaf stage
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INTRODUCTION
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DAIRY PRODUCERS in the northeastern USA typically plant corn from late April until late May and harvest perennial forages from late May until mid-June. Wet spring conditions, however, can delay corn planting until June and warm spring conditions can accelerate the first harvest of perennial forages to mid-May. Consequently, corn planting and perennial forage harvest overlap in some years. Dairy producers in the northeastern USA often delay the application of preemergence (PRE) herbicides until the V3–V4 stage of corn growth (Ritchie et al., 1993) because of the field workload during May and the first half of June. The delay in application of the PRE herbicides may compromise their efficacy. Dairy producers have expressed interest in using glyphosate-resistant corn because the optimum timing for glyphosate application may occur after completion of the first harvest of perennial forages.
Hall et al. (1992) reported that the critical period of weed control in grain corn occurred from the V3 to V14 growth stage under growing conditions in Ontario, Canada, which indicates that glyphosate would have to be applied by the V3 stage. In a multistate study in the northcentral USA, Gower et al. (2003) reported that the optimum timing for an initial application of glyphosate in grain corn generally occurred by the V4 stage, which corresponded to weed heights of 10 cm or less. Dalley et al. (2004) reported that the optimum timing of glyphosate application on grain corn depended on growing conditions in Michigan. In highly competitive conditions (high weed densities and/or below normal precipitation), optimum glyphosate timing occurred by the V4 stage. In less competitive conditions, optimum glyphosate timing occurred as late as the V9 stage. Sequential glyphosate applications did not improve corn yields in the Michigan study. Gower et al. (2002) reported that glyphosate should be applied before weeds reach 15 cm in height to avoid corn grain yield losses in Ohio. They concluded that reinfestation of weeds after an early postemergence (EPOST) application of glyphosate had less potential to reduce yield than by delaying application and allowing weeds to compete with corn for too long a period before removal.
Dairy producers in the northeastern USA typically plant corn forage at 85000 to 90000 kernels ha–1, about 10 to 15% greater than plant densities for grain corn (Cox, 1997). Tollenaar et al. (1994) reported grain yield losses, attributable to weed competition, of 26, 17, and 13% at corn densities of 40000, 70000, and 100000 plants ha–1, respectively, in Ontario, Canada. Murphy et al. (1996) reported 30 to 40% less weed biomass at corn densities of 100000 vs. 70000 plants ha–1 in Ontario, Canada. The higher plant densities for corn forage may increase the competitiveness of corn allowing dairy producers in the northeastern USA to delay glyphosate application until after completion of the first harvest of perennial forages.
Most glyphosate and weed control studies in corn have evaluated grain yield and not forage yield losses from weed interference. Furthermore, only one study has evaluated the effect of weed interference on corn forage quality (Mueller et al., 1993). The objectives of this study were to (i) evaluate the impact of season-long weed interference on corn forage yield and quality, (ii) determine the optimum timing of a glyphosate application for corn forage yield and quality, and (iii) determine if hybrid maturity influenced the optimum timing of glyphosate application, allowing for a delay in glyphosate application on longer-season hybrids.
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MATERIALS AND METHODS
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Field experiments were conducted in 2002 and 2003 on a Honeoye silt loam soil (fine-loamy, mixed, mesic Glossoboric Hapludalfs) at a Cornell University research farm near Aurora, NY (42°44' N lat, 76°40' W long). The experimental sites, which were adjacent to each other, were fallowed in the previous year to the experiment. In the fallow year, the experimental site was moldboard plowed in the spring and then left fallow for the remainder of the year. In the experimental year, the site received an application of glyphosate [N-(phosphonomethyl)glycine] and 2,4-D [(2,4-dichlorophenoxy) acetic acid] in late April for control of perennial weeds, primarily dandelion (Taraxacum officinale Weber in Wiggers). The experimental site was then disked and harrow-cultipacked before planting. Soil tests of the experimental sites indicated a pH of 8.0 in 2002 and 7.9 in 2003 with medium concentrations of P and K (Mehlich Test) in both years.
DeKalb brand ‘DKC42-70RR’, 92-d Relative Maturity (RM), and ‘DKC53-33RR’, 103-d RM, were planted on 11 May 2002 and 5 May 2003 with a four-row planter at 0.76 m row spacing and 88000 kernels ha–1 with 225 kg ha–1 of the starter fertilizer, 10–20–20 (N–P–K). All plots received about 110 kg N ha–1 as a 32% (wt./vol.) N solution of urea [(NH2)Co] and ammonium nitrate (NH4NO3) at the V5 stage of corn growth in both years.
Weed control treatments included an untreated control and a weed-free plot that received a PRE mixture of 1.12 kg a.i. ha–1 of atrazine [6-chloro-N ethyl-N'-(1-methylethyl) 1,3,5-triazine-2-4 diamine]) and 1.4 kg a.i. ha–1 of s-metolachlor [2-chloro-N-(2-ethyl-6- methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] followed by hand-weeding. Three glyphosate treatments were applied at 1.14 kg a.i. ha–1 at early, mid, and late postemergence timings (EPOST, MPOST, and LPOST, respectively, Table 1). All herbicides were applied with a tractor-mounted sprayer with flatfan nozzles (80015) at 50-cm spacing. The sprayer was calibrated to deliver 187 L ha–1 at 235 kPa pressure at a ground speed of 3.2 km h–1. Boom height was 51 cm for the PRE application, 56 cm for the early EPOST, 71 cm for the MPOST, and 91 cm for the LPOST herbicide applications.
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Table 1. Date, height, and leaf stage (Vn) of corn, and height of the major weed species at the time of early (EPOST), mid-(MPOST), and late (LPOST) postemergence applications of glyphosate, and density of the major weed species in the untreated control plots at the V4 stage of corn growth in 2002 and V5 stage in 2003.
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The experimental design was a randomized complete block in a split-plot arrangement with five replications. Hybrids were main plots and weed control treatments were the subplots. Main plots measured 12 by 15 m and subplots measured 12 by 3 m. The inner 7.5 m of the two center rows of each subplot was the harvest area. The outer 2.25 m of the two center rows was the sampling area for weed density and corn growth.
Weed densities, by species, were counted within a 0.23-m2 quadrant in the untreated control plots at the V4 stage in 2002 and the V5 stage in 2003 (Table 1). Five corn plants were selected at the V8 growth stage (3 July in both years) and the R1 growth stage (29 July in 2002 and 28 and 31 July in 2003). Green leaves were measured with an LI-3100 leaf area meter (LI-COR, Lincoln, NE) and then placed with the remaining plant parts in a forced-air dryer and dried at 60°C to constant moisture. Total DM accumulation was calculated on a land-area basis determined by final plant densities in each subplot (average of 75643 plants ha–1 in 2002 and 81375 plants ha–1 in 2003).
The center two rows of the inner 7.5 m of each plot were harvested by hand for forage yield once the PRE weed-free treatment attained about 350 g kg–1 DM content (27 Aug. 2002 and 3 Sept. 2003). Ten plants were randomly selected from the harvest plants to estimate DM content and forage quality characteristics. The ears were shelled after drying to determine grain concentration. Stover, cob, and grain were then reassembled into a whole plant sample and ground sequentially through a hammer mill and a Wiley mill (Thomas Scientific, Swedesboro, NJ). Samples were then passed through a splitter, reduced to 50 g, and further ground through a Cyclone mill (Udy Corp., Ft. Collins, CO) fitted with a 1-mm screen.
Subsamples (0.5 g each) were analyzed by wet chemistry for neutral detergent fiber (NDF), using the ANKOM system (ANKOM Technology, Fairport, NY) according to procedures by Van Soest et al. (1991), and for total N using a Leco FP528 N analyzer (LECO Corp., St. Joseph, MI) with Dumas combustion (Tate, 1994; Wiles et al., 1998). The crude protein (CP) concentration was calculated by multiplying total N by 6.25. Subsamples (0.25 g each) were also analyzed for in vitro true digestibility (IVTD) according to Stage 1 of the procedure described by Marten and Barnes (1980), using a 48-h incubation period at 39°C in 5 mL of buffered rumen fluid containing 20 mL of the Kansas State buffer supplemented with 0.5 g L–1 urea. In vitro neutral detergent fiber digestibility (NDFd) was determined according to Cherney et al. (1997), using the rumen buffer described by Marten and Barnes (1980) and using the Daisy II200/220 in vitro incubator (ANKOM Technology, Fairport, NY) and the ANKOM200-220 fiber analyzer. The buffer contained urea. Ruminal fluid inoculum was obtained from a nonlactating, rumen-fistulated Holstein cow, offered a medium quality orchardgrass (Dactylis glomerata L.) hay diet for ad libitum intake. Digestibility samples (0.25 g) were incubated in duplicate for 48 h at 39°C, and undigested residues were treated with neutral detergent solution.
Subsamples (1.0 g) were also analyzed for ash content by combustion at 510°C for 4 h. Subsamples (0.1 g) were analyzed for starch by Dairy One (DHI Forage Testing Lab, Ithaca, NY). The subsamples were pre-extracted for sugars, then a glucoamylase enzyme was used to hydrolyze starch to dextrose. The subsamples were then injected into a YSI 2700 SELECT Biochemistry Analyzer, where dextrose is oxidized to hydrogen peroxide and lactose. Hydrogen peroxide is detected by an electrode, and current at the electrode is directly proportional to hydrogen peroxide concentration, which is directly related to dextrose and starch concentrations.
Potential milk yield indices were then estimated from the spreadsheet, Milk 2000 (Schwab and Shaver, 2001). Milk Mg–1 (kg milk Mg–1 corn forage), a forage quality index, was calculated from NDF, NDFd, CP, ash, and starch concentrations. Milk yield (kg milk ha–1 corn forage) was calculated as the product of milk Mg–1 and DM yields.
We conducted separate statistical analyses for each year because timing of weed control treatments and weed species and densities differed for each growing season. We used the Shapiro-Wilk statistic in the PROC CAPABILITY: NORMAL TEST option of the SAS statistical package, version 7.0 software (SAS Inst., 1998) to test for normalcy. All data tested normal so transformation of the data was not necessary. Hybrids and weed control treatments were considered fixed and replications were considered random in the analysis of variance. We used the General Linear Model (GLM) procedures of the SAS statistical package, version 7.0 software (SAS Inst., 1998) to conduct analyses of variance for DM accumulation at the V8 and R1 growth stages, DM content at harvest, DM yield, starch, NDF, NDFd, IVTD, CP, milk per megagram, and calculated milk yield. All effects were considered significant at 
= 0.05. Fisher's protected LSD ( = 0.05) was used to separate means when main effects tested significant. Also, Fisher's protected LSD was used to separate means of weed control treatments within a hybrid when significant hybrid x weed control interactions were observed with procedures outlined by Little and Hills (1978).
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RESULTS AND DISCUSSION
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A wet May and June with normal temperatures, followed by a very dry July and August with warm temperatures, characterized the 2002 growing season (Table 2). Moderate densities of common ragweed (Ambrosia artemisiifolia L.), yellow nutsedge (Cyperus esculentus L.), common lambsquarters (Chenopodium album L.), and green foxtail [Setaria viridis (L.) Beauv.] were counted in the untreated control at the V4 stage in 2002 (Table 1). A cool May and June with normal precipitation, followed by a cool July with very wet conditions and a warm August with dry conditions, characterized the 2003 growing season. High densities of common lambsquarters and green foxtail and moderate densities of wild mustard [Brassica kaber (D.C.) L.C. Wheeler] and common ragweed were counted in the untreated control at the V5 stage in 2003. The very dry conditions in July and August of 2002 and the very high weed densities in 2003 resulted in competitive growing conditions for corn in the untreated control in both years. Visual observations at the R1 stage indicated similar weed control for the glyphosate and PRE weed-free treatments in both years.
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Table 2. Monthly precipitation and growing degree days (GDD, 30–10°C system) during the 2002 and 2003 growing seasons at Aurora, NY.
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Weed control treatments affected DM accumulation at the V8 and R1 stages, and there were no hybrid x weed control treatment interactions in either year of the study (Table 3). The untreated control and the LPOST glyphosate treatment vs. the PRE weed-free treatment averaged 45% less DM accumulation in 2002 and 75 to 80% less DM accumulation in 2003 at the V8 stage. At the R1 stage, both treatments averaged 50% less DM accumulation in 2002 and 60 to 65% less DM accumulation in 2003 vs. the PRE weed-free treatment. Tollenaar et al. (1994) reported about 20% less DM accumulation at the V8 and R1 stages, and Evans et al. (2003) reported 25% less DM accumulation at the V9 stage and 50% less at the R1 stage in the untreated control vs. the weed-free treatment. Total weed densities in the untreated control in both of those studies averaged less than 200 weeds m–2. Total weed densities in the untreated control in our study approached 400 weeds m–2 in 2002 and 1300 weeds m–2 in 2003, which resulted in more competitive growing conditions. The EPOST and PRE weed-free treatments averaged the same DM accumulation at both growth stages, but the MPOST treatments averaged 40 to 65% less DM accumulation at the V8 stage and 40% less at the R1 stage than the PRE treatment in both years. Glyphosate had to be applied by the V3 (2003) to V4 (2002) stage, regardless of hybrid maturity, to maximize DM accumulation at the R1 stage, which is closely correlated with DM yields at harvest (Muchow and Davis, 1988).
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Table 3. Dry matter accumulation of two corn hybrids at the eighth leaf stage (V8) and silking (R1) stage of corn growth under different weed control treatments at Aurora, NY in 2002 and 2003.
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Weed control treatments did not affect DM content at harvest in 2002 (Table 4) because dry July conditions delayed silking in all treatments until 1 to 2 d after an 11-mm precipitation event on 27 July. Weed control treatments did affect DM content at harvest in 2003, and there was no hybrid x weed control treatment interaction. The untreated control averaged about 290 compared with about 345 g kg–1 DM content at harvest in the PRE weed-free and EPOST treatments, in part because weed interference delayed silking by 4 d. The MPOST and LPOST treatments averaged about 305 g kg–1 DM content at harvest in part because early season weed interference delayed corn silking by 3 d. The DM content of corn forage at harvest must be more than 300 g kg–1 for safe storage in horizontal silos and more than 350 g kg–1 for safe storage in tower silos (Bastiman and Altman, 1985). When weed densities are high, as in 2003, a delay in glyphosate application until the V5 stage may delay corn forage harvest by several days or more because early season weed interference delays corn phenological development.
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Table 4. Dry matter (DM) content at harvest and DM yield of two corn hybrids under different weed control treatments at Aurora, NY, in 2002 and 2003.
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Weed control treatments affected DM yields in both years, and there were no hybrid x weed control interactions (Table 4). The untreated control vs. the PRE weed-free treatment averaged 70 to 75% lower DM yields, which far exceeded the previously reported 15 to 20% DM yield reductions (Tollenaar et al., 1994; Evans et al., 2003) because of highly competitive growing conditions in this study. The EPOST and PRE weed-free treatments yielded similarly in both years, but the MPOST treatment yielded 20 to 25% less. The MPOST treatment had significant DM yield reductions because corn is most susceptible to early season weed interference under competitive growing conditions (Dalley et al., 2004). The LPOST treatment yielded 50% less compared with 75% less in the untreated control, despite similar DM accumulation at the R1 stage, so a glyphosate application at the V7–V8 stage prevented further DM yield losses by maintaining weed-free conditions during the grain-filling period. Nevertheless, glyphosate had to be applied by the V3–V4 stage, regardless of hybrid maturity, to avoid significant DM yield losses.
Weed control treatments affected milk per megagram, a forage quality index, in 2002 but not in 2003, and there were no hybrid x weed control interactions in either year of the study (Table 5). The untreated control averaged the least milk per megagram in 2002 because moderate weed densities with dry conditions around silking resulted in limited grain development and very low starch and very high NDF concentrations (Table 6). In 2003, however, high weed densities with wet conditions around silking did not affect grain development or most forage quality characteristics. Season-long weed interference apparently affects corn forage quality only in years with dry conditions before and after silking by reducing grain development.
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Table 5. Milk per megagram and calculated milk yield of two corn hybrids under different weed control treatments at Aurora, NY, in 2002 and 2003.
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Table 6. Starch, neutral detergent fiber (NDF), NDF digestibility (NDFd), crude protein (CP), and in vitro true digestibility (IVTD) of two corn hybrids under different weed control treatments at Aurora, NY, in 2002 and 2003.
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The MPOST and LPOST treatments averaged the greatest milk per megagram in 2002, despite low CP concentrations, because of lower NDF and greater starch and NDFd concentrations (Table 6). When conditions are dry for 4 wk before and 4 wk after silking, delayed glyphosate application until the V6–V8 stage may improve corn forage quality because early season weed interference can reduce stem and leaf growth, thereby increasing stover NDFd and conserving stored soil water for increased grain development. Dry conditions in July and August cannot be accurately predicted in June so a delay in glyphosate application until the V6–V8 stage to improve forage quality cannot be a recommended weed management practice in the rain-fed conditions of the northeastern USA. The PRE weed-free and EPOST treatments had similar milk per megagram values in both years, which indicates that an initial glyphosate application at the V3–V4 stage for weed control, regardless of hybrid maturity, results in similar corn forage quality as that of weed-free corn.
Hybrid affected milk per megagram in 2003. The 92-d RM hybrid, DKC42-70RR, had greater milk per megagram vs. DKC53-33RR, the 103-d RM hybrid, because of greater starch and CP concentrations and lower NDF concentrations. The greater forage quality and digestibility (IVTD) of DKC42-70RR in 2003 was probably associated with its earliness, which reduced the impact of the dry August conditions on grain development, rather than timing of weed removal in a shorter vs. longer-season hybrid.
Weed control treatments affected calculated milk yields in both years, and there were no hybrid x weed control interactions (Table 5). When compared with the PRE weed-free treatment, the untreated control averaged 70 to 80% less calculated milk yield because of low DM yield and forage quality in 2002 and low DM yield in 2003. The MPOST treatment had similar calculated milk yield when compared with the PRE weed-free treatment in 2002 because greater forage quality offset its 20% lower DM yield. A delay in glyphosate application beyond the time for optimum DM yields, however, cannot be a recommended weed management practice in rain-fed environments, as indicated by similar forage quality and 30% lower calculated milk yield in the MPOST vs. the PRE weed-free treatment in 2003. The PRE weed-free and the EPOST treatment averaged the same calculated milk yields in both years, which indicate that an initial glyphosate application at the V3–V4 stage, regardless of hybrid maturity, should result in similar milk production on dairy farms as that of weed-free corn forage.
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CONCLUSIONS
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Dairy producers in the northeastern USA should apply glyphosate by the V3–V4 stage to maximize DM yields and forage quality in competitive growing conditions. The V3–V4 stage can occur after mid-June for early May planted corn, such as in 2002, which would allow dairy producers to complete the first harvest of perennial forages and then make a timely glyphosate application to corn. The V3–V4 stage, however, can occur in early June, such as in 2003. In those years, dairy producers will be unable to apply glyphosate by the V3–V4 stage because of overlap with perennial forage harvest so they should hire custom applicators to ensure a timely glyphosate application. Hybrids, which differed in RM by 11 d, did not interact with weed control treatments for DM yield, milk per megagram, and calculated milk yields so the use of longer-season hybrids would not delay the optimum timing of glyphosate on corn forage.
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REFERENCES
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ABSTRACT
INTRODUCTION
