Introduction

Excessive vine growth of peanut (Arachis hypogaea L.) reduces digging and inverting efficiency often attributed to poor row visibility. Additionally, pods often shed from plants in the digging process and can lead to substantial yield loss, especially when soil conditions are not conducive for separation of pods from soil. Prohexadione calcium (Apogee®, calcium salt of 3,5-dioxo-4 propionylcyclohexanecarboxylic acid, BASF Corp., 26 Davis Dr., Research Triangle Park, NC 27709) is registered for management of vegetative growth of peanut and other crops (Anonymous, 2003; Byers and Yoder, 1999; Grossman et al., 1994; Lee et al., 1998; Nakayama et al., 1992; Yamaji et al., 1991). Culpepper et al. (1997) and Mitchem et al. (1996) reported that prohexadione calcium improved row visibility of peanut and increased pod yield. Beam et al. (2002b) reported that increased pod yield of peanut by prohexadione calcium was attributed in part to increased pod retention and less pod loss during digging and inversion of vines.

Although the benefits of prohexadione calcium have been established in the literature, the current price of prohexadione calcium is cost prohibitive in many circumstances, especially in light of changes in 2002 Federal farm legislation that reduced value of peanut at the farm level (Bullen and Jordan, 2006). Developing alternative application methods of prohexadione calcium might allow growers to control vine growth more economically. One alternative to broadcast applications is banding prohexadione calcium. Savings might also be realized if the actual rate of prohexadione calcium was reduced in certain parts of the canopy by using spray nozzles delivering different spray volumes and subsequently lower rates. Both of these approaches could allow efficient digging and vine inversion with lower expense for broadcast applications. However, these approaches to applying prohexadioine calcium have not been evaluated.

Field conditions can prevent timely applications of crop protection chemicals, and this is also the case with prohexadione calcium. Growers often pose the question of how effective is prohexadione calcium when applied after the label recommendation of 50% row closure (Anonymous, 2003). Determining if prohexadione calcium applications made later than recommended improve row visibility or increase yield would help growers make this decision. Benefits of applying prohexadione calcium closer to digging and inversion timing have not been extensively evaluated.

Defining interactions of agrichemicals is important when developing production and pest management strategies. In peanut, timing of application of numerous agrichemicals often coincide, and growers would prefer to apply products simultaneously to increase efficacy and/or efficiency and reduce application costs. However, some combinations of agrichemicals are not compatible (Lancaster et al., 2005). Although prohexadione calcium is compatible with the majority of agrichemicals applied to peanut (Beam et al., 2002a), efficacy of sequential applications of prohexadione calcium alone or with other products has not been evaluated.

Growers often apply sequential applications of 2,4-DB to control sicklepod (Jordan, 2006). Sequential applications of 2,4-DB within a 2 to 3 wk period are possible (Jordan, 2006). Pyraclostrobin is a relatively new fungicide that controls several important pathogens in peanut (Shew, 2006). Although compatibility of prohexadione calcium with several fungicides applied to peanut has been documented (Beam et al., 2002a), efficacy of prohexadione calcium with pyraclostrobin has not been documented. Determining if pyraclostrobin and/or 2,4-DB are compatible with prohexadione calcium is important in developing strategies for late season weed and disease control and managing peanut vine growth.

Beam et al. (2002b) reported when prohexadione calcium was applied based on the manufacturer's suggested use rate and timing, increased yield following application of prohexadione calcium was partially attributed to reduced pod shed. Benefits of applying prohexadione calcium later in the season at rates exceeding those recommended by the manufacturer have not been determined.

A considerable amount of literature exists documenting peanut response to prohexadione calcium when applied according to the manufacturer's recommendation (Beam et al., 2002b; Culpepper et al., 1997; Mitchem et al., 1996). However, efficacy of prohexadione calcium applied in unique ways to reduce cost has not been reported. Additionally, efficacy of prohexadione applied later than recommended by the manufacturer, or in combination with pyraclostrobin or 2,4-DB has not been published. Therefore, research was conducted from 2000–2007 to determine peanut response to prohexadione calcium either banded or applied at different rates over the peanut canopy, applied at various rates and timings later in crop development than currently recommended by the manufacturer, and applied alone or with pyraclostrobin or 2,4-DB.

Materials and Methods

Results and Discussion

Comparison of broadcast and banded applications of prohexadione calcium

The interaction of site by treatment was not significant for row visibility, main stem height, or pod yield. Although the main effect of prohexadione calcium treatment was not significant for pod yield, it was significant for row visibility and main stem height. Row visibility was the highest when prohexadione calcium was broadcast over the entire peanut canopy or when prohexadione calcium was banded over lateral branches (Table 1). When banded over main stems, row visibility was higher than that of non-treated peanut but lower than broadcast application or banded application of prohexadione calcium over lateral branches. Peanut main stem height was higher for non-treated peanut or peanut treated with prohexadione calcium on a band over lateral branches than when prohexadione calcium was banded or broadcast over main stems (Table 1). These data indicate that prohexadione calcium does not translocate significantly from the contacted area to other portions of the peanut plant. Decreased internode elongation was noted only on lateral branches or on main stems when prohexadione calcium was applied to that region of the plant. These data are consistent with those of Beam (2004) reporting rapid metabolism and little translocation from treated tissue when prohexadione calcium was applied to annual bluegrass (Poa annua L), creeping bentgrass (Agrostis stolonifera), Kentucky bluegrass (Poa pratensis L.), and perennial ryegrass (Lolium perenna L.).

Table 1

Peanut response following banded and broadcast application of prohexadione calcium.^a

No difference in yield was noted when comparing prohexadione calcium-treated peanut with non-treated peanut (Table 1). Previous research (Beam et al., 2002b; Culpepper et al., 1997; Jordan et al., 2000) indicated that a consistent increase in row visibility does not always translate into increased yield.

Comparison of broadcast application of prohexadione calcium using variations in spray nozzle arrangement

Main effect of prohexadione calcium was significant for row visibility and main stem height but not for pod yield. Additionally, the interaction of year by prohexadione calcium treatment was not significant for these measurements. Row visibility was higher when prohexadione calcium was broadcast at a total rate of 280 g/ha or when the nozzle arrangement provided the highest rate of prohexadione calcium applied over lateral branches (Table 2). There was no difference in row visibility when the highest rate of prohexadione calcium was applied over main stems compared with non-treated peanut. Peanut main stems were shorter when prohexadione calcium was applied at the highest rate over main stems of peanut (Table 2). Main stem height was similar when prohexadione calcium was broadcast over the entire canopy, when the highest rate of prohexadione calcium was applied to lateral branches, and for non-treated peanut.

Table 2

Row visibility, main stem height, and peanut pod yield following broadcast application of prohexadione calcium using different spray nozzle arrangements.^a

Influence of application rate and timing on peanut response to prohexadione calcium

The site by prohexadione calcium treatment was not significant for row visibility or pod yield; however, the main effect of prohexadione calcium treatment was significant for row visibility and pod yield. Row visibility was greatest when prohexadione calcium was applied at the recommended rate at 50% row closure (Table 3). Delaying applications of prohexadione calcium past the optimum timing of 50% row closure resulted in lower row visibility irrespective of prohexadione rate (Table 3). These results were not surprising because prohexadione reduces internode elongation through inhibiting gibberellin biosynthesis by blocking the 3β-hydroxylation of GA₂₀ (Nakayama et al., 1992). Applications later in the season most likely will reduce internode elongation of fewer components of the peanut canopy than applications earlier in the season. These data also indicate that rates of prohexadione calcium higher than those recommended by the manufacturer will not compensate for applications made after the optimum timing. There was no difference in pod yield when comparing yield of non-treated peanut and prohexadione calcium at 140 g/ha applied at 50% row closure or when this rate was applied 1 wk after 50% row closure (Table 3). Pod yield following application of prohexadione calcium at 210 or 280 kg/ha one week after 50% row closure was lower than pod yield following application of prohexadione calcium at 140 g/ha applied at 50% row closure. The decrease in pod yield suggests that rates that exceed the manufacturer's suggested use rate applied later in the season may have a negative impact on peanut pod yield. However, lack of treatments including prohexadione calcium applied at higher rates at 50% row closure limits conclusions relative to this research.

Table 3

Row visibility and peanut pod yield as influenced by application rate and timing of prohexadione calcium.^a

Peanut response to a single application of prohexadione calcium within 2 to 3 wks prior to digging

The interaction of site by prohexadione calcium treatment was significant for row visibility in North Carolina. Prohexadione calcium did not affect row visibility in the Georgia trial. Prohexadione calcium increased row visibility when applied within 2 to 3 wks prior to digging at 5 of 6 sites in North Carolina (Table 4). However, the magnitude of improvement in row visibility was much lower with these applications than with applications of prohexadione calcium at 50% row closure (Tables 1– 3). This is not surprising because prohexadione calcium prevents internode elongation, and by the time prohexadione calcium was applied in September, the peanut canopy was well developed, and therefore benefits of prohexadione calcium with respect to row visibility were minimized. In addition to improving row visibility, prohexadione calcium can increase harvestable yield by reducing pod shed (Beam et al., 2002b). In our experiment the interaction of site by prohexadione calcium as well as the main effect of prohexadione calcium was not significant for pod yield. These results indicate that applications made within 2 to 3 wks prior to digging do not improve yield (Table 4).

Table 4

Peanut row visibility and pod yield as influenced with prohexadione calcium applied within 2 to 3 wks of digging and vine inversion during 2005 in North Carolina and 2006 in Georgia.

Influence of pyraclostrobin and 2,4-DB on peanut response to prohexadione calcium

The interaction of site by treatment was not significant for row visibility and pod yield at sites in North Carolina in the experiments evaluating interactions of prohexadione calcium with pyraclostrobin or 2,4-DB. When pooled over sites, row visibility did not differ and ranged from 9.3 to 9.4 when prohexadione calcium was applied alone or with pyraclostrobin and was significantly better than the non-treated peanut row visibility of 5.2 (Table 5). A value of 6.9 to 8.3 was noted when prohexadione calcium was applied alone or with 2,4-DB compared with a value range of 2.9 to 4.5 for non-treated peanut (Table 6). However, the improvement in row visibility did not translate into increased yield regardless of prohexadione calcium or pesticide treatment (Table 5 and 6). These results suggest that pyraclostrobin or 2,4-DB are compatible when tank mixed with prohexadione calcium and do not reduce the effectiveness of prohexadione calcium on improving row visibility of peanut.

Table 5

Influence of prohexadione calcium on row visibility and pod yield when applied with pyraclostrobin.^a

Table 6

Influence of prohexadione calcium on row visibility and pod yield when applied with 2,4-DB.^a

Summary

Collectively, these data indicate that prohexadione calcium can be applied less expensively without compromising benefits of improved row visibility by banding applications or by broadcasting prohexadione calcium using spray nozzles delivering different spray volumes with different orifices. These respective approaches could save producers up to 50% of the expense associated with applying prohexadione calcium, crop oil concentrate, and nitrogen solution. However, these applications require greater precision, and most likely would require using a sprayer that matches sets of planted rows rather than using standard broadcast sprayers that cover multiple sets of planted rows. When comparing across experiments, row visibility was improved the greatest when prohexadione calcium was applied at the recommended rate at 50% row closure. These data also suggest that prohexadione calcium is compatible with two commonly applied crop protection chemicals in peanut. However, few differences in pod yield were noted when prohexadione was applied, even when applied based on the manufacturer's recommendation. These results are in contrast to other research demonstrating positive yield response of peanut to prohexadione calcium (Beam et al., 2002b; Culpepper et al., 1997; Jordan et al., 2000; Jordan et al., 2001; Mitchem et al., 1996). However, in other research, prohexadione calcium improved row visibility but did not increase yield when applied based on the manufacturer's recommendation (Faircloth et al., 2005). The full benefit of improved row visibility in terms of digging equipment staying in the proper position relative to peanut rows was not realized in our small-plot research, and additional efforts are needed to document benefits of row visibility provided by prohexadione calcium in large-scale peanut production.

Appreciation is expressed to personnel at the Peanut Belt Research Station, the Upper Coastal Plain Experiment Stations, the Border Belt Tobacco Research Station, and the USDA-ARS National Peanut Research Laboratory for assistance with these experiments. Carl Murphey, Brenda Penny, Josh Gaddy, Michael Williams, Sam Uzzell, Tommy Bennett, Bobby Hagler, Corey Collins, and Jesse Bolton provided technical assistance. Appreciation is also expressed to Jart Hudson, Ed and Lin Jordan, and Kent Fann for allowing these experiments to be conducted on their farms. BASF Corporation provided the prohexadione calcium, and the North Carolina Peanut Growers Association, Inc. provided partial funding for the research.

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