Peanut (Arachis hypogaea L.), a leguminous crop, is capable of biological nitrogen fixation (BNF) by Bradyrhizobium (Schiffman and Alper, 1968; Shimshi et al., 1967; Walker et al., 1976). Inoculants can be applied with the seed or as granular or liquid products in the seed furrow at planting to ensure adequate amounts of Bradyrhizobium are present for root infection (Baughmann et al., 2007; Chapin et al., 2007; Godsey et al., 2007; Jordan, 2008a). Research indicates that in-furrow applications are often more effective than application to the seed when peanut is planted in fields that have never been seeded to peanut (Baughmann et al., 2007; Lanier et al., 2005). Cost of Bradyrhizobium inoculant is relatively low compared with other variable costs (Bullen and Jordan, 2008). Approximately 77% of growers in North Carolina apply inoculant to peanut (Rhodes et al., 2008).

Crop rotation is a critical component of pest management in peanut (Lamb et al., 1993; Rodriguez-Kabana et al., 1987; Rodriguez-Kabana and Touchton, 1984). Increasing the number of non-peanut crops between plantings of peanut often decreases incidence of disease and nematodes and can result in higher peanut yield (Bailey and Matyac, 1988; Pataky et al., 1983; Rodriquez-Kabana et al., 1987; Rodriguez-Kabana, 1984). Peanut also offer advantages for non-N fixing crops such as corn (Zea mays L.) and cotton (Gossypium hirsutum L.) by fixing N₂ into approximately 124 kg/ha soil N when planted the year prior to these crops (Elkan, 1995).

Practitioners often pose the question of whether fumigation for Cylindrocladium black rot (caused by Cylindrocladium parasiticum) is needed if a certain number of non-peanut crops separate peanut plantings or if soybean [Glycine max (l.) Merr.], a host for Cylindrocladium black rot, is included in the rotation. Similarly, determining if the number of non-peanut crops between peanut plantings that result in a positive response to inoculation can be established would benefit practitioners. Although benefits of BNF to other non-legume crops in rotation with peanut are relatively well defined, interactions of crop rotation and inoculation are not well defined for peanut with respect to the role of rotation and rotation length on survival of Bradyrhizobium. Cooperative Extension Services in several states in the United States recommend inoculation of peanut regardless of rotation (Baughmann et al., 2007; Chapin et al., 2007; Godsey et al., 2007; Jordan, 2008a). A definite length of rotation when peanut will respond to inoculation has not been established for Virginia market type peanut. However, there is some debate among peanut growers and their advisors as to whether this approach is viable given the diversity factors that can influence nodulation of peanut by Bradyrhizobium. Therefore, research was conducted in North Carolina to determine the effect of crop rotation on peanut response to Bradyrhizobium.

Experiments were conducted in North Carolina from 1999 through 2006 at the Border Belt Research Station near Whiteville, the Peanut Belt Research Station located near Lewiston-Woodville, and the Upper Coastal Plain Research Station near Rocky Mount on Norfolk sandy loam soil (fine-loamy, siliceous, thermic Aquic Paleudalts). Soil pH ranged from 5.7 to 6.1 and soil organic matter content ranged from 1.7 to 2.2%. The cultivars NC-V 11 (Wynne et al., 1991) (Lewiston-Woodville and Rocky Mount) and Perry (Isleib et al., 2003) (Whiteville) were planted in conventionally prepared seedbeds in early May to achieve a final in-row plant population of 13 plants/m. Plot size was 12 rows (Lewiston-Woodville and Whiteville) or 8 rows (Rocky Mount) by 15 m. Row spacing at all locations was 91 cm.

Cropping systems for the duration of the experiment varied considerably depending upon location (Table 1). At Lewiston-Woodville in one experiment, crop rotations from 2001–2006 included continuous peanut, peanut separated by five years of corn, three cycles of corn-peanut or cotton-peanut, and two cycles of corn-corn-peanut, cotton-cotton-peanut, cotton-corn-peanut, soybean-corn-peanut, or soybean-cotton-peanut. Peanut was planted in the entire test area during 2000. In a second experiment from 1999–2006 at this location, rotations included two cycles of cotton-cotton-cotton-peanut or cotton-cotton-corn-peanut and four cycles of corn-peanut or cotton-peanut. Peanut was planted in the entire test area during 1998. At Rocky Mount, rotations from 2001–2006 included five years of cotton followed by peanut, three cycles of cotton-peanut, two cycles of cotton-cotton-peanut, cotton-cotton-soybean-cotton-cotton-peanut, and continuous peanut. Rotations from 2001–2006 at Whiteville included five years of corn followed by peanut; two cycles of corn-corn-peanut, tobacco (Nicotiana tobacum L.)-corn-peanut, or corn-tobacco-peanut; and the rotation of corn-corn-tobacco-corn-corn-peanut.

In 2006 at all locations, peanut was not inoculated or was inoculated with Bradyrhizobium (Optimize Lift®, EMD Crop Bioscience, Brookfield, WI) in the seed furrow at planting. Optimize Lift® delivers 2.2 × 10¹² viable cells of bacteria/ha when applied at 1.2 L/ha in 47 L/ha aqueous solution immediately after seed drop but prior to furrow closure. Spray solution directly contacted seed. Peanut was not inoculated during any year of the experiment except in 2006. Peanut was planted in all fields the year prior to initiating the experiments. No attempt was made to quantify N concentration in soil.

Aldicarb [2-methyl-2-(methylthio)propionaldehyde O-methylcarbamoloxime] at 1.1 kg ai/ha was applied in the seed furrow. All other production and pest management practices were held constant over the entire test area and were based on Cooperative Extension Service recommendations appropriate for the region (Brandenburg, 2007; Jordan, 2008a 2008b; Shew, 2007).

During the second week of August in 2006, color of the entire peanut canopy was estimated visually on a scale of 0 to 5 where 0  =  the entire peanut canopy expressing a pale yellow color and 5  =  the entire peanut canopy expressing a dark green color. Determining canopy color at this stage of peanut development minimized possible confounding of canopy color due to development of Cylindrocladium black rot and other diseases resulting from crop rotation. While determining N concentration in peanut leaves or recording chlorophyll content would have been more informative than assessing the canopy visually, practitioners most likely will use a visual assessment for this comparison. Peanut was dug and vines inverted in late September or early October based on pod mesocarp color determination (Williams and Drexler, 1981). Digging was initiated when approximately 65% of pods were in the brown and black color pod mesocarp color category (Jordan et al., 2005). Pods were harvested 4 to 7 d after digging and vine inversion.

The experimental design in all experiments was a split plot with sub-plots replicated four times. Cropping system served as the whole plot unit and inoculation treatment served as the split plot unit. Data for canopy color and pod yield were subjected to analysis of variance appropriate for the treatment structure (SAS, 2006). Means of significant main effects and interactions were separated using Fisher's Protected LSD test at p ≤ 0.05.

The interaction of rotation by inoculation was not significant for peanut canopy color or peanut pod yield in any of the experiments or locations (Table 2). Crop rotation did not affect peanut canopy color but did affect pod yield in three of the four experiments (Table 2). Inoculation affected canopy color in one experiment at Lewiston-Woodville and affected pod yield in two of four experiments (Table 2). Data for peanut canopy color and pod yield are presented for all experiments regardless of significance of the main affect at a location or within an experiment at a particular location.