Introduction

Peanut is a valuable commodity in the United States with approximately 634,800 ha harvested with an estimated value of $1.19 million USD (USDA 2016a; USDA 2016b). Approximately 7% of national peanut ha is in North Carolina (USDA 2016a) with the majority Virginia market type cultivars (Brown, 2016). Weeds compete with peanut for sunlight, moisture, and nutrients throughout the growing season (Wilcut et al., 1994). Peanut yield, quality, and economic value can be negatively affected by weeds (Everman et al., 2008; Walker et al., 1989). Season-long interference of combination of broadleaf and grass weeds can reduce peanut yield by 60 to 80% and reduced harvest efficiency in some instances (Everman et al., 2008; Wilcut et al., 1994). Common ragweed (Ambrosia artemisiifolia L.) (Clewis et al. 2001) and Palmer amaranth (Amaranthus palmeri L.) (Burke et al. 2007) at a density of one plant/m of row reduced peanut yield by 40 and 28%, respectively. The prostrate growth habit of peanut causes peanut to be vulnerable to interference from weeds throughout the season and requires effective season-long management strategies to protect yield and promote efficient digging and vine inversion (Walker et al., 1989; Wilcut et al., 1994).

Weed control programs often include preplant incorporated, preemergence, and postemergence (POST) herbicides to minimize interference of weeds with peanut and challenges with harvest (Clewis et al. 2007; Grichar and Dotray 2012; Henning et al. 1982; Jordan, 2016; Wilcut et al., 1994). However, in spite of these herbicide intensive programs weed escapes still occur. Herbicide options available for late-season weed control are limited because of injury potential and labeled pre-harvest intervals (Cahoon and Jordan, 2016; Jordan, 2016). While Palmer amaranth is the most troublesome and difficult to control weed in peanut in North Carolina, primarily due to resistance to acetolactate synthase (ALS)-inhibiting herbicides (Webster, 2013; Poirier et al., 2014), annual grasses and morningglory (Ipomoea spp.) are often found in fields late in the season (Jordan, 2016). Growers can apply diclosulam through the ground cracking stage in North Carolina (Anonymous, 2010) and up to 30 d after planting in other states to control Benghal dayflower (Commelina benghalensis L.) (Anonymous, 2004). Imazapic, lactofen, and 2,4-DB can be applied no closer to harvest than 90, 45, and 45 d, respectively (Anonymous, 2015; Anonymous, 2016; Anonymous, 2014a). Imazapic and lactofen are marginally effective on Palmer amaranth and morningglory that are large (Chahal et al., 2011; Lancaster et al., 2005 2007) or express resistance to ALS-inhibiting herbicides which is widespread in North Carolina (Poirier et al., 2014). Diclosulam suppresses several broadleaf weeds including annual morningglory, common ragweed, and Benghal dayflower when applied POST (Prostko, 2004; Lancaster et al., 2007). Of these herbicides 2,4-DB is applied late in the season more often to suppress Palmer amaranth and morningglory, although 2,4-DB is ineffective in controlling pitted morningglory (Ipomoea lacunosa L.) (Jordan, 2016). Determining efficacy of carfentrazone-ethyl and pyraflufen-ethyl herbicides against this weed and how peanut respond to these herbicides could lead to improved harvesting of peanut.

Carfentrazone-ethyl and pyraflufen-ethyl inhibit protoporphyrinogen oxidase (PPO) in sensitive plants including morningglory (Anonymous, 2011, Anonymous, 2014b; Dayan et al., 1997; Reed et al., 2004). However, carfentrazone-ethyl and pyraflufen-ethyl applied 28 to 51 d after planting injured peanut 62 to 48% when evaluated 14 d after treatment (Dotray et al., 2010). Grichar et al. (2010) reported 7 to 52% injury and 4 to 26% stunting of peanut when carfentrazone-ethyl and pyraflufen-ethyl were applied 35 to 56 d after planting. Peanut injury from these herbicides at these timings of application reduced pod yield of runner market types, but did not impact peanut market grade characteristics (Dotray et al., 2010; Grichar et al., 2010). Grichar et al. (2010) reported that peanut tolerance to carfentrazone-ethyl and pyraflufen-ethyl was cultivar dependent. Response of large-seeded Virginia market-type peanut to carfentrazone-ethyl and pyraflufen-ethyl is limited in the peer-reviewed literature. Therefore, research was conducted to determine peanut tolerance to these two herbicides when POST applied at different rates within 2 weeks prior to digging peanut pods and inverting vines and how peanut respond to these herbicide compared with other herbicides that have potential for late-season use.

Materials and Methods

Results and Discussion

Peanut response to carfentrazone-ethyl and pyraflufen-ethyl rate and timing

Peanut injury from carfentrazone-ethyl 1 WAT was influenced by the interaction of experiment by rate (P ≤; 0.0371) and experiment by application timing (P ≤ 0.0001); therefore data are presented by experiments. The interaction of carfentrazone-ethyl rate by application timing was significant for peanut injury 1 WAT for 2014 at Lewiston-Woodville (P =; 0.0010) but was not observed at Lewiston-Woodville and Rocky Mount during 2012 and 2013 (Table 1). However, at Lewiston-Woodville, the effect of carfentrazone-ethyl rate was significant for 2012 (P =; 0.0044), with injury increasing as the rate carfentrazone-ethyl increased (Table 1). The effect of application timing was significant for 2012 at Rocky Mount (P =; 0.0055) and 2013 Lewiston-Woodville (P =; 0.0035) and treatments applied 1 WBD caused higher injury (25 and 28%, respectively) when compared to treatments applied 2 WBD (16 and 13%) (Table 1). In 2014 at Lewiston-Woodville, carfentrazone-ethyl applied at 17.5 g/ha caused injury ≤ 10% regardless of application timing. However, increasing the carfentrazone-ethyl rate to 35 g/ha caused higher injury when applied 2 WBD than 1 WBD (25% versus 7%, data not shown in tables). The interaction of experiment by herbicide rate was not significant for peanut injury at 2 WAT (P =; 0.0570); therefore, data are combined over experiments. Carfentrazone-ethyl at 35 g/ha caused more peanut injury than when applied at 17.5 g/ha (17% versus 9%) (Table 2).

Table 1.

Peanut injury 1 week after treatment and pod yield as influenced by experiment and carfentrazone-ethyl rate and timing of application.

Table 2.

Peanut injury 2 week after POST application of carfentrazone-ethyl.^a

In the experiment with pyraflufen-ethyl, the interaction of experiment × application timing and experiment × herbicide rate were significant for peanut injury at 1 WAT (P ≤ 0.0001) and 2WAT (P = 0.0197). Therefore, data are presented by each year. Peanut injury at 1 WAT was not affected by the interaction of pyraflufen-ethyl rate × application timing for experiments during 2012 and 2013 (P > 0.05); this interaction was significant in 2014 (P 0.0001) (Table 3). Main effect of rate (P 0.0001) was significant for 2012. Peanut injury was higher when pyraflufen-ethyl was applied at 3.6 g/ha compared with to lower rate of 1.8 g/ha (Table 3). In 2014 at Lewiston-Woodville, pyraflufen-ethyl applied at 1.8 g/ha caused no more than 11% injury at both application timings. As was noted when comparing injury at the two timings, pyraflufen-ethyl at 3.6 g/ha injured peanut 33% at 2 WBD compared with only 8% at 1 WBD (data not shown in tables). At 2 WAT, the main effect of pyraflufen-ethyl rate was significant in 2012 (P ≤ 0.0367) and 2014 (P =; 0.0002), with the higher application rate of 3.6 g/ha causing more peanut injury compared with the rate of 1.8 g/ha (Table 4).

Table 3.

Peanut injury 1 week after treatment and pod yield as influenced by experiment and pyraflufen-ethyl rate and timing of application.

Table 4.

Peanut injury 2 week after POST application of pyraflufen-ethyl.^a

Peanut yield following carfentrazone-ethyl and pyraflufen-ethyl was not influenced by the main effect of herbicide rate or application timing and their interaction alone or with experiment. Peanut yield was similar to non-treated peanut regardless of herbicide rate or timing of application (Tables 1 and 3). Dotray et al. (2010) reported that carfentrazone-ethyl and pyraflufen-ethyl applied late POST 93 to 121 d after planting reduced peanut yield reduction at only at 1 of 6 locations. Peanut in our experiments was in the R8 stage of growth with 60-70% of pods in orange, brown, and black pod mesocarp classifications (Boote, 1982). It is postulated that herbicide phytotoxicity to foliage at this stage of growth and development did not affect foliage adequately to cause pod shed or the pod and seed maturation process.

Peanut response to carfentrazone-ethyl and pyraflufen-ethyl compared with diclosulam, lactofen, and 2,4-DB

The main effect of herbicide and the interaction of experiment × herbicide were significant for visible injury (P ≤ 0.0001). Although the main effect of herbicide was significant for pod yield, the interaction of experiment × herbicide was not significant (P = 0.3805). Therefore, data are combined over experiments for this interaction. The interaction of experiment × herbicide for visible injury was caused by lower peanut injury at Whiteville following carfentrazone-ethyl compared with pyraflufen-ethyl, and due to lack of differences in injury caused by lactofen and 2,4-DB compared with different results at Lewiston-Woodville (Table 5). At Lewiston-Woodville in both fields, no difference in injury was observed when comparing carfentrazone-ethyl and pyraflufen-ethyl. Visible injury at this location caused by lactofen exceeded that of diclosulam and 2,4-DB. Pod yield was 9% to 10% lower when carfentrazone-ethyl and pyraflufen-ethyl were applied compared with non-treated peanut (Table 5). Diclosulam, lactofen, and 2,4-DB did not negatively affect pod yield when compared with non-treated peanut. Other research (Lancaster et al., 2007) demonstrated no detrimental effect of diclosulam applied POST to peanut 7 to 35 d after planting prior to the R1 stage of growth (Boote, 1982). Diclosulam can be applied up to 30 d after planting with minimal concern for negative impact on peanut (Anonymous, 2004; Prostko, 2004). Although Baughman et al. (2002) reported no adverse impact of 2,4-DB on peanut yield when applied 30 to 120 d after planting which corresponds to pre-flowering to pod maturity, other research has shown that lactofen can negatively affect yield in some but not all instances (Ferrell et al., 2013; Grichar, 1997; Jordan et al., 1993). Results from these experiments indicate that the injury caused by carfentrazone-ethyl and pyraflufen-ethyl observed 4 WBD is more deleterious to peanut growth and development and subsequent yield than the same level of injury within 2 WBD.

Table 5.

Peanut injury 2 weeks after treatment and pod yield as influenced by carfentrazone-ethyl, pyraflufen-ethyl, diclosulam, lactofen, and 2,4-DB applied 4 weeks before digging peanut.^a

Conclusions

Overall, these results demonstrate that variation in peanut injury can occur depending on rate of carfentrazone-ethyl and pyraflufen-ethyl and application timing. However, when applied within 2 weeks of digging, these herbicides will not reduce peanut yield and may have potential to minimize weed interference with digging and vine inversion.

The North Carolina Peanut Growers Association and FMC Corporation proved financial support for this research. Appreciation is extended to P. D. Johnson, A. T. Hare, T. Buck, and staff at the Border Belt Research Station, the Peanut Belt Research Station, and Upper Coastal Plain Research Station for technical assistance.

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