Peanut grown in the U. S. are increasingly being produced using conservation tillage practices (Sholar et al. 1995). The traditional advantages of conservation tillage peanut production are less soil erosion and improved water infiltration compared to conventional tillage systems (Tubbs and Beasley 2009). Of practical significance to peanut growers in the southeastern U. S., conservation tillage offers savings over conventional tillage by eliminating the need for multiple tillage operations in rapid succession which can be complicated by logistics, weather delays, shortages in skilled agricultural labor, and increasing fuel costs (Tubbs and Gallaher 2005). Furthermore, conservation tillage is a critical factor that lessens incidence and severity of spotted wilt (tomato spotted wilt tospovirus) (Brown et al. 1999; Johnson et al. 2001; Marois and Wright 2003) and early leaf spot (Cercospora arachidicola Hori.) (Cantonwine et al. 2007a, 2007b) in peanut.

There are many conservation tillage variants used in the southeastern U. S. peanut producing region, with strip-tillage being the most common. Cereal cover crops are seeded the previous autumn and killed with a non-selective postemergence herbicide in early spring. Seedbeds are prepared using an implement with in-row sub-soil shanks, multiple gangs of fluted coulters to cut cover-crop debris, and ground-driven crumblers that till a band approximately 30 cm wide. Crops are seeded with planter units either tandem mounted on the strip-tillage implement or as a separate operation.

Residual control of annual grasses and small-seeded broadleaf weeds in strip-tillage peanut typically included either pendimethalin or ethalfluralin combined with a second application of a non-selective herbicide applied immediately after seeding peanut. This system has been shown to be an effective component in an integrated system to manage annual grasses (Grichar 2005; Johnson et al. 2002, 2010) and pigweed (Amaranthus spp.) (Grey and Wehtje 2005). Dinitroaniline herbicides are potentially adsorbed by cover crop debris (Helling 1976; Weber 1990) and it was speculated that this phenomenon was the primary reason for reduced efficacy in strip-tillage peanut production (Johnson et al. 2002).

Current surveys show that annual grasses continue to be among the most common and troublesome weeds of peanut (Webster 2009), with Texas millet [Urochloa texana (Buckl.) R. Webster], several species of crabgrass (Digitaria spp.), and crowfootgrass [Dactyloctenium aegyptium (L.) Willd] commonly found throughout the southeastern U. S. Despite the regular incidence of annual grasses in peanut, well documented potential yield losses (Johnson and Mullinix 2005), and need for an aggressive control strategy (Johnson et al. 2002; Prostko et al. 2001), dinitroaniline herbicides are often applied long after planting in strip-tillage production systems. Johnson et al. (2010) determined that either imazapic or paraquat must be tank-mixed with pendimethalin if applications were delayed later than peanut emergence. While no visual injury was reported from delayed applications of pendimethalin, it was beyond the scope of those studies to closely investigate the effects of delayed applications of pendimethalin on peanut growth and yield. Therefore, trials were initiated to study the effects of delayed applications of the dinitroaniline herbicides ethalfluralin and pendimethalin on peanut growth, development, and yield.

Irrigated, weed-free field studies were conducted in 2004 and 2005 at the Coastal Plain Experiment Station Ponder Farm, near Ty Ty, GA. The soil was a Tifton loamy sand; 88% sand, 6% silt, and 6% clay with 0.5% organic matter. This soil type is representative of soils in the southeastern U. S. peanut production region.

The experimental design was a randomized complete block with a factorial arrangement of treatments replicated four times. Treatments were all possible combinations of two dinitroaniline herbicides and seven application timings. Dinitroaniline herbicides evaluated were pendimethalin4 (1.1 kg ai/ha) and ethalfluralin5 (0.8 kg ai/ha), each applied at rates registered for use on peanut. Application timings for each herbicide were preemergence (PRE) immediately after seeding peanut, vegetative emergence (VE) of peanut, 1wk after VE (VE+1wk), VE+2wk, VE+3wk, VE+4wk, and a nontreated control. Herbicides were applied with a tractor-mounted CO₂-pressurized plot sprayer calibrated to deliver 234 l/ha at 207 kPa using flat-flan nozzle tips. All plots were irrigated with sprinkler irrigation (1.0 to 1.8 cm) immediately after herbicide application to ensure maximum herbicide performance.

Plots were seeded with rye at 63 kg/ha using a grain drill (18 cm spacing) in the autumn after the preceding crop harvest. In early April, the rye cover was killed with glyphosate at 1.1 kg ai/ha. Seedbeds were formed with a two-row strip-tillage implement6 that prepared a tilled seedbed 30 cm wide and planted to peanut with vacuum planters7 in a separate operation. ‘C99R’ peanut (Gorbet and Shokes 2002) were seeded in early May each year at a rate of 112 kg/ha. After seeding peanut, the entire experimental area was treated with paraquat (0.6 kg ai/ha) alone to control emerged weeds. Plots were maintained weed-free throughout the season with one application of bentazon (1.1 kg ai/ha) plus 2,4-DB (0.3 kg ai/ha). An additional treatment of clethodim (0.14 kg ai/ha) was used to control escaped annual grasses. Other than herbicide treatments, crop and pest management practices were based on recommendations by the University of Georgia Cooperative Extension Service (Prostko 2009).

Parameters measured were visual estimates of crop stunting, vegetative biomass, pod biomass, and yield. Visual estimates of crop stunting were made 78 and 85 days after emergence (DAE) in 2004 and 2005, respectively. Vegetative and pod biomass samples were collected from 1 m of row, partitioned into pods and foliage, dried at 38°C for 72 hours, and weighed. Mid-season biomass samples were collected 68 and 71 DAE while late-season biomass samples were collected at 120 and 110 DAE in 2004 and 2005, respectively. Peanut yields were collected from the remainder of the plot by digging, inverting, and combining peanut using commercial two-row equipment. Yield samples were mechanically cleaned to remove foreign material. Yields are reported as cleaned, farmer-stock peanut.

Data were analyzed using a mixed-model analysis. Degrees of freedom were partitioned to test singularly and in combination the effects of dinitroaniline herbicides and time of application on peanut growth parameters and yield. Means were separated using Fisher's Protected LSD (P≤0.05).

Analysis of variance indicated a nonsignificant year by treatment interaction for all parameters. Therefore, data are combined across years. The same analysis also showed nonsignificant interactions between dinitroaniline herbicides and application timings for all parameters. Therefore, all data are presented as main effects at P≤0.05.