Replanting Decisions Following Glyphosate Application to Peanut

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Bridget R. Lassiter, Research Assistant, David L. Jordan, Professor, P. Dewayne Johnson, Research Technician III, and Gail G. Wilkerson, Professor, Department of Crop Science, Box 7620, North Carolina State University, Raleigh, NC 27695-7620

Abstract

Growers in the United States often rotate cotton (Gossypium hirsutum L.), corn (Zea mays L.), or soybean [Glycine max (L.) Merr.] with peanut (Arachis hypogaea L.), and peanut injury from accidental application of glyphosate is a real possibility. Six experiments were conducted to determine peanut response to glyphosate at 0.11 or 0.38 lb ae/acre and to determine if replanting is economically justifiable once injury was observed. Yield loss following glyphosate applied at 0.11 lb/acre ranged from 300 to 1,000 lb/acre, while yield loss following glyphosate at 0.38 lb/acre ranged from 790 to 4390 lb/acre. Yield of replanted peanut did not differ from non-treated peanut planted early but exceeded peanut planted early when glyphosate was applied at 0.11 lb/acre in two experiments and in all experiments when glyphosate was applied at 0.38 lb/acre. Net return of non-treated peanut planted early was similar to peanut treated with glyphosate at 0.11 lb/acre in 5 of 6 experiments when peanut following both treatments was allowed to reach optimum maturity. When peanut was treated with glyphosate at 0.38 lb/acre, replanting was more economical in 3 of the 6 experiments. Net return of both severely damaged peanut and replanted peanut was lower than net return of non-treated peanut planted early in 5 and 4 of the 6 experiments, respectively.

Introduction

Growers on occasion will accidentally include in the spray tank an herbicide that can cause significant crop injury. A range of responses can be observed, from negligible injury to crop death. Many growers in the Virginia-Carolina production region rotate corn, cotton, or soybean with peanut. These growers have adopted Roundup Ready Technology on most of these acres, and accidentally including glyphosate a spray tank designated for peanut is a real possibility. Cost of replanting Virginia market type peanut is approximately $124/acre when including seed, planting operation, and in-furrow insecticide (5). Determining when replanting is economically justifiable is important when making this decision.

The planting window for peanut is relatively narrow and occurs from early May through early June in North Carolina (7). Therefore, growers need to make a decision about replanting as quickly as possible, often before the full effect of glyphosate injury is known. Previous research has documented peanut response to glyphosate rate in simulated drift studies (17). However, in these trials peanut was dug on the same day regardless of glyphosate rate, injury level, or crop maturity. Peanut yield was reduced only when the glyphosate rate exceeded 0.125 lb/acre. Although yield was reduced by glyphosate, it is plausible that peanut could recover from early season injury and yield would not be compromised if peanut were allowed sufficient growth time to reach full pod maturity. A range of biotic and abiotic stresses can reduce peanut yield. However, in some cases peanut can recover from stress, in part because of its indeterminate growth habit. In these cases yield is not compromised if additional time for maturation occurs (11).

Considerable research has been conducted to document the effects of glyphosate drift on susceptible crops (1,2,3,6,8,9,10,14,15,16,19). Research has not been conducted to determine if delayed maturity occurs after glyphosate injures peanut or if replanting is justified when appreciable injury from glyphosate occurs. Therefore, research was conducted to: (i) compare visual injury following application of glyphosate at rates simulating typical nonionic surfactant rate or a rate similar to many herbicides applied routinely to peanut; (ii) determine if early season injury from glyphosate delayed row closure of peanut and reduced yield; and (iii) to determine if replanting peanut shortly after glyphosate injury appeared was economically justifiable.

Determining Peanut Response to Glyphosate and the Benefits of Replanting Following Accidental Glyphosate Application

Six experiments were conducted in North Carolina in 2005 through 2006 at the Peanut Belt Research Station located near Lewiston-Woodville on a Norfolk sandy loam soil (fine-loamy, siliceous, thermic, Typic Paleudults). The experiment was conducted in three fields during both years. Peanut was seeded in conventionally prepared raised seedbeds at a rate to achieve a final in-row population of 4 plants/ft. Aldicarb was applied in the seed furrow prior to seed drop. The cultivars Gregory (2 experiments), NC-V 11 (3 experiments), and VA 98R (1 experiment) were evaluated. Plot size was 2 rows wide (36-inch spacing) by 30 ft long. Two non-treated rows separated each plot.

Treatment factors, experimental design, and data collected. Treatments consisted of glyphosate (Roundup Weather MAX, Monsanto Co., St. Louis, MO) at 0.11 lb/acre or 0.38 lb/acre applied approximately 3 weeks after planting when peanut diameter was 4 inches. Peanut in this stage had not started flowering or pegging and had 4 to 6 leaves containing 4 leaflets each. These respective rates simulate glyphosate in a spray tank at a nonionic surfactant rate of 0.25% (v/v) in 15-gal aqueous solution/acre or herbicide application at 1 pint/acre. A non-treated control was included. One additional treatment included killing peanut with glufosinate and replanting peanut in early June within 2 weeks after glyphosate was applied. The first planting date was during the first two weeks of May in all experiments.

The experimental design was a randomized complete block with four replications. Visual estimates of percent peanut injury were determined 2 weeks after glyphosate application using a scale of 0 to 100% where 0 = no injury and 100 = peanut death. Foliar chlorosis, necrosis, and plant stunting were considered when making the visual estimates. The number of days between peanut emergence and row closure was also determined. Additionally, the number of days required for row closure following replanting was recorded. Peanut was dug at optimum maturity based on pod mesocarp color (13) for each treatment using a composite sample collected from all four replications. This approach to digging allowed the possibility for peanut to compensate for the early season injury. Peanut can recover from early season stress if allowed to continue the maturation process later in the season. However, late season digging runs the risk of problems associated with weather conditions and poor soil conditions for digging and vine inversion.

Net return was based on budgets for Virginia market type peanut and was calculated as the product of peanut yield (lb/acre) and crop value ($0.21/lb) less the cost of $727/acre (no replanting) or $851/acre (replanting) based on North Carolina Cooperative Extension budgets (5). All other production and pest management practices were applied uniformly across the experiment (4,11,12,18). Data for visual estimates of peanut injury, days from peanut emergence to row closure, pod yield, and net return were subjected to analysis of variance. Means were separated using Fisher’s Protected LSD at P ≤ 0.05 (SAS Institute Inc., Cary, NC).

Visual Peanut Injury Following Application of Glyphosate

Visual injury 2 weeks after application ranged from 15 to 43% when glyphosate was applied at 0.11 lb/acre, while injury from glyphosate at 0.38 lb/acre ranged from 48 to 81% (Table 1). Injury ranged from 15 to 19% in two experiments, 28 to 35% in three experiments, and 43% in another experiment when glyphosate was applied at 0.11 lb/acre (Table 1). At the higher rate of glyphosate (0.38 lb/acre), injury ranged from 48 to 58% in two experiments and 71 to 81% in the remaining four experiments (Table 1). Lassiter et al. (17) reported less than 10% injury when glyphosate was applied at 0.09 lb/acre or lower, 15 to 45% when applied at 0.11 lb/acre, and 78 to 90% when applied at 0.38 lb/acre when evaluated 7 days after application.

Table 1. Influence of glyphosate rate on peanut injury 2 weeks after application at Lewiston-Woodville during 2005 and 2006.

Glyphosate (lb/acre)	Cultivar
	Gregory		NC-V 11			VA 98R
	2005	2006	2005	Field 1, 2006	Field 2, 2006	2005
	Percent visual injury (%)
0.11	34	35	15	28	43	19
0.38	71*	58*	81*	48*	76*	78*

* Significance at P ≤ 0.05.

Days from Peanut Emergence to Row Closure

When compared with non-treated peanut planted early, the number of days from emergence to row closure was greater when glyphosate was applied at 0.11 lb/acre or 0.38 lb/acre in 3 and 5 experiments, respectively (Table 2). As expected, the number of days from emergence to row closure of replanted peanut was greater than the number of days for non-treated peanut planted early in all experiments (Table 2). However, the number of days for replanted peanut to close rows compared with non-treated peanut planted early was 1 to 25 days more rapid. Growing conditions in June are often more conducive to rapid vegetative growth than are conditions in May, and this partially explains the more rapid canopy closure by replanting peanut.

Table 2. Influence of glyphosate rate on the number of days between peanut emergence to row closure at Lewiston-Woodville during 2005 and 2006.

Glyphosate (lb/acre)		Cultivar
		Gregory		NC-V 11			VA 98R
		2005	2006	2005	Field 1, 2006	Field 2, 2006	2005
Days between peanut emergence and row closure	0	95 c	88 c	85 c	102 c	77 b	86 c
	0.11	101 bc	91 c	93 b	146 a	83 ab	94 b
	0.38	118 a	132 a	104 a	152 a	82 ab	103 a
	replanted	107 b	99 b	95 b	127 b	84 a	95 b
Days between replanting peanut and peanut row closure	replanted	70	66	61	101	62	61
Days between peanut emergence and peanut digging	0 or 0.11	141	145	141	143	141	141
Days between peanut emergence and peanut digging	0.38 or replanted	158	162	162	166	166	162

* Means within a cultivar, year, and field followed by the same letter followed by the same letter are not significantly different at P ≤ 0.05.

The difference in the number of days from emergence to digging ranged from 17 to 25 when comparing peanut treated with glyphosate at 0.38 lb/acre or replanting peanut without glyphosate. Digging in these experiments was based on pod mesocarp color and projected optimum maturity using a composite sample of pods from each plot (13).

Pod Yield and Economic Comparison of Replanting

Pod yield of non-treated peanut planted early or when peanut was planted early and treated with glyphosate at 0.11 lb/acre did not differ in 4 of 6 experiments (Table 3). In contrast, yield loss following glyphosate at 0.38 lb/acre was noted in all experiments when compared with non-treated peanut and in four experiments when compared with replanted peanut. Lassiter et al. (17) reported no yield loss when glyphosate was applied at 0.11 lb/acre. However, in that study, glyphosate applied at 0.38 lb/acre resulted in yield reductions ranging from 0 to 83% of non-treated yield in five experiments conducted during 2001 and 2002. Yield losses of 32 to 77% were noted in our study when glyphosate was applied at 0.38 lb/acre compared with non-treated peanut planted on time (Table 3). Yield reductions noted in the study by Lassiter et al. (17) occurred when peanut was not allowed to compensate for early season injury due to digging on the same day irrespective of pod maturation. In contrast, in this study peanut were allowed to recover as much as possible under conditions in North Carolina. Yield measurements reflected digging glyphosate-injured peanut 17 to 25 days later based on pod mesocarp color (13).

Table 3. Influence of glyphosate rate on pod yield at Lewiston-Woodville during 2005 and 2006.

Glyphosate (lb/acre)	Cultivar
	Gregory		NC-V 11			VA 98R
	2005	2006	2005	2006, Field 1	2006, Field 2	2005
	Pod yield (lb/acre)
0	2460 a*	3470 a	4910 a	3810 a	5680 a	4440 a
0.11	1920 ab	2830 ab	4610 a	2810 b	4750 a	3870 b
0.38	1670 b	2090 b	3040 b	1840 c	1290 b	2600 c
Replanted	2230 ab	2750 ab	4840 a	3210 a	5230 a	4080 a

Peanut for each treatment was dug at optimum pod maturity based on mesocarp color using a composite sample for each treatment (13).

* Means within a cultivar, year, and field followed by the same letter are not significantly different at P ≤ 0.05.

One component of the objectives in our study and of Lassiter et al. (17) was to define a level of visual injury that could be used to predict yield loss from either accidental application of glyphosate or from drift when glyphosate was applied to adjacent glyphosate-resistant crops. In our experiment, injury of 43% or lower did not reduce yield in 4 of 6 experiments. However, injury at 19% or 28% was sufficient to reduce pod yield in two of the experiments. Our experiment did demonstrate, however, that a yield reduction will most likely occur when visual estimates of peanut injury are 48% or higher when observed within several weeks after glyphosate application. It is also important to note, however, that comparisons of visual injury following application of glyphosate at different rates may be confounded partially because of physiological affects glyphosate may have that are not reflected in visual estimates of peanut injury. Additionally, a yield reduction with injury as low as 19% suggests that while determining if replanting is needed based on visual injury is perhaps the most reasonable indicator for practitioners, other factors including environmental conditions most likely will minimize the accuracy and consistency of using this approach.

Replanted peanut that was allowed to reach optimum maturity by delayed digging yielded similar to peanut planted early but not treated with glyphosate. These data also indicate that peanut planted in early June can yield comparable to peanut planted in early May in North Carolina when conditions are favorable throughout the season for peanut growth and development. These results are supported by those of Carley et al. (7) showing yield of the cultivars NC-V 11 and VA 98R being similar when planted in early or mid May in two of two experiments and in two of three experiments, respectively.

In the economic assessment (Table 4), net return of non-treated peanut planted early was similar to peanut treated with glyphosate at 0.11 lb/acre in all but one experiment. When peanut was treated with glyphosate at 0.38 lb/acre severe injury was noted, and replanting was more economical in 3 of the 6 experiments. Net return of both severely damaged peanut and replanted peanut was lower than net return of peanut planted early and without glyphosate in 5 and 4 experiments, respectively.

Table 4. Net return as influenced by glyphosate injury and cost of replanting at Lewiston-Woodville from 2005 and 2006.

Glyphosate (lb/acre)	Cultivar
	Gregory		NC-V 11			VA 98R
	2005	2006	2005	2006, Field 1	2006, Field 2	2005
	Net return ($/acre)
0	-210 a	3 a	304 a	74 a	466 a	205 a
0.11	-323 ab	-133 ab	241 a	-137 ab	271 b	138 a
0.38	-375 ab	-287 b	-89 b	-340 c	-456 c	-181 c
Replanted	-383 b	- 264 b	166 a	-177 bc	247 b	6 b

Means within a cultivar, year, and field followed by the same letter followed by the same letter are not significantly different at P ≤ 0.05.

Results from these experiments suggest that when injury is relatively low, replanting is not justifiable. In contrast, replanting when higher levels of injury are observed (48% or higher) was often more economical even with the added expense than not replanting. However, this response was not observed in every experiment and in most cases economic return was lower than when peanut was planted on time and was not injured by glyphosate. The decision to replant peanut is challenging in North Carolina because of the limited window of production and the concern that peanut planted after the optimum planting date will have insufficient heat unit accumulation to allow optimum pod maturation. The additional cost of approximately $124/acre associated with seed, aldicarb, and planting constitutes 17% of the total budget associated with producing Virginia market type peanut in North Carolina (5).

Pod mesocarp color was used to estimate the optimum digging date in these experiments using a composite sample combined from all replications. Recording the percentage of mature pods at digging to allow statistical comparison of treatments would have been more informative but was not done in these experiments. Although peanut injured by glyphosate at 0.38 lb/acre and those replanted were later maturing than non-treated peanut or peanut treated with glyphosate at 0.11 lb/acre, peanut may not have progressed to the 65% mature pod threshold often used to initiate digging of Virginia market type peanut (13). Continued maturation and pod development in mid October is often minimal in North Carolina due to cooler temperatures. Therefore, in these experiments optimum pod maturation was likely never obtained with peanut having excessive glyphosate injury.

These data suggest that when glyphosate is accidentally added to the spray tank or applied to a peanut field at 0.11 lb/acre (0.25% v/v in 15 ga/acre) yield most likely will not be compromised. As was noted in studies by Lassiter et al. (17), peanut with injury ranging from 19 to 43% most likely will not experience yield loss. However, when a higher rate of glyphosate, on the order of 0.38 lb/acre is applied (a rate equivalent to 1 pint formulated product per acre), yield will be reduced, and replanting will most likely result in net return equaling or exceeding injured peanut remaining in the field under most conditions.

Acknowledgments

The North Carolina Peanut Growers Association supported this research financially. Appreciation is expressed to the staff at the Peanut Belt Research Station for assistance with these experiments.

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