Introduction

Subsurface drip irrigation (SDI) of peanut in the humid southeast has been effective in increasing pod yield and market grade, specifically kernel size distribution of peanut (Sorensen et al., 2000), when compared with non-irrigated peanut production (Lamb et al., 1997). Various researchers have shown that SDI can increase crop yield and quality on tomato, Lycopersicon esculentum, (Bogle et al., 1989; Camp et al., 1989), cotton, Gossypium hirsutum, (Bucks et al., 1988; Henggeler, 1988), and corn, Zea mays (Powell and Wright, 1993). Bucks and Davis (1986) listed a number of potential advantages of drip irrigation which include the conservation of water, enhanced plant growth and yield, improved application of fertilizer, reduced weed growth, and decreased energy requirements. Phene et al. (1992) suggested that drip irrigation can contribute to maximizing water use efficiency with negligible soil evaporation, percolation, and runoff.

Surface drip irrigation, due to its simplicity of design, has been used to irrigate vegetables and high value crops for many years (Bucks et al., 1974; Hanson et al., 1997). It can precisely deliver water, nutrients, and chemicals to the crop root zone. One of the greatest advantages of using surface drip irrigation is that the system can be installed easily with low initial investment and provide flexible irrigation schedules without using large pumps and wells. Field tests have been conducted using drip irrigation in various crops to improve water usage. Previous research has shown that surface and subsurface drip irrigation are effective on many crops, locations, and environmental conditions with various techniques (Goldberg and Shmueli, 1970; Bucks et al., 1974; Sammis, 1980; Hodgson et al., 1990; Camp et al., 1993; Hanson et al., 1997; and Camp, 1998). Due to the increased use of irrigation in peanut production, research efforts have addressed irrigation management in peanut with respect to optimal timing and irrigation methods to suit regional production demands (Pahalan and Trapathi, 1984; Wright and Adamsen, 1993; Lamb et al., 1997; Davidson et al., 1998; Lanier et al., 2004; and Zhu et al., 2004). Bosch et al. (1992), O'Brien et al., (1998), and Sharmasarkar et al. (2001) using yield data and economic simulations reported that SDI would be more profitable for small areas (<30 ha) because of its lower per area investment and lower pumping costs compared to fixed or towable center-pivot systems. Subsurface drip irrigation would also be preferred in irregularly shaped fields where a full circle irrigation pivot cannot be installed. These areas often require drip irrigation to provide sufficient water to different irrigation zones according to the zone sizes and crop types. Little research has been done on the use of surface drip irrigation in peanut production and the effects on peanut yield and quality (Zhu et al., 2004). In addition, there is a lack of information concerning the economic impact of surface drip technology on peanut production.

The objectives of this research were to: 1) show pod yield response to surface drip irrigation with two lateral spacings, 2) document farmer stock grade and gross revenue, and 3) provide a partial economic analysis of surface drip irrigation versus non-irrigation.

Materials and Methods

This research was conducted at the USDA-ARS Multi-Crop Irrigation Research Farm in Shellman, GA during the 2002 (Zhu et al., 2004), 2003, and 2004 growing seasons on two soil series and topographies. Site 1 was a Faceville fine sandy loam (Fine, kaolinitic, thermic Typic Kandiudults) with up to 3% slope. The topography was undulating with a general slope towards the east with a north aspect. The soil series at Site 2 was a Greenville fine sandy loam (fine, kaolinitic, thermic Rhodic Kandiudults) with less than 1% slope.

In Site 1, three areas were selected for crop rotations of cotton, corn, and peanut. Each crop rotation was 60 m wide and 91.2 m long. Each area was split into an irrigated and non-irrigated block. There were a total of six treatments replicated three times in a randomized block design. Irrigated treatments included surface drip tube laterals spaced at 0.91 m (next to a crop row) and 1.83 m (alternate row middles). Peanut was planted either in twin or single row spacing and either irrigated or non-irrigated (planting and irrigation treatments will be described later). Individual plots were 1.83 m wide and 91.2 m long separated into six individual subplots or positions (15.2 m long) used to help identify yield versus slope interaction. Non-planted beds were used as travel rows so that once the drip tubing was installed no wheeled traffic was allowed in the plots until harvest (Zhu et al., 2004).

Site 2 was part of a long term crop rotation and irrigation system project where part of a non-irrigated area was irrigated using surface drip tubing (Lamb et al., 2003). The overall project was a randomized block design with six crop rotations, three irrigation systems (subsurface drip, surface drip, and overhead sprinkler) and a non-irrigated control. Each non-irrigated subplot was 18 rows wide (0.91 m row spacing) and 61 meters long. This research will only report the surface drip irrigation and non-irrigated portion of the project.

Land preparation was the same for both sites, all rotations, and for each year. The land was disk harrowed, deep ripped, and then turned with a bottom plow in late spring or early fall depending on weather conditions. Lime was applied in early spring at rates determined by soil test. In the spring a field cultivator was used for weed control, soil amendment incorporation, and soil preparation for herbicide application. Preplant herbicide, Pendimethalin (N-(1-ethylpropyl)-3, 4-dimethyl-2, 6 dinitrobenzenamine), was applied at 2.8 L/ha and incorporated with a 1.83 m wide power rototiller. Boron was applied to foliage twice each season for a total of 0.56 kg B/ha. Fungicides, insecticides, and herbicides were applied at recommended rates and timing as determined by field scouting during the growing season for disease, insect, and weed control.

Peanut cultivars Georgia Green (GG) (Site 1 and 2) and ViruGard (VG) (Site 1) were planted all years with planters centered on the 1.83-m beds. The single (Site 1) and twin-row patterns (Site 1 and 2) were planted with a commercial vacuum type planter (Monosem planter, ATI Inc, Lenexa, KS). The seeding density was 20 seed/m as recommended to reduce the risk of Tomato Spotted Wilt Virus (TSWV) (Brown et al., 2002a, b). The single row planter placed seeds at 0.91 m row spacing for two rows on a bed. Twin-row pattern was planted at 1.17 m between the outside rows and 0.7 m between the inside rows with 0.22 m between the twin-rows with four rows on a bed. Aldicarb (2-methyl-2-(methylthio)-, O-((methylamino)carbonyl)oxime) was applied in each crop row at recommended rates for single and twin-row patterns.

Irrigation water was supplied through a series of 5 cm diameter flexible hoses with drip tubing connected using adapters containing shutoff valves (Agricultural Products, Inc., Ontario, CA, model 400-BV-06-LS). The drip tubing had 0.2 mm wall thickness with emitters spaced at 30 cm (Roberts Irrigation Products, Inc., San Marcos, Ca. www.robertsirrigation.com). The 0.91 m spaced lateral was about 5 cm away from the single row peanuts or placed in the center of the twin-row planted peanuts (see Zhu et al., 2004). The 1.83 m spaced laterals were spaced in alternate row middles. Emitter flow rate was 0.91 L/h (2002) and 0.56 L/h (2003 and 2004). Operating pressure was 70 kPa at the head of the field (100 kPa at the pump) and water flow was measured with mechanical water meters. Drip tubing cost $240/ha for 1.83 m lateral spacing and twice this amount ($480/ha) for the 0.91 m lateral spacing.

Research with drip irrigation has shown a water savings of about 25% (Sorensen et al., 2005) compared with recommendations by Irrigator Pro (Davidson et al., 2000) and Stansell et al. (1976). Irrigation events were determined by Irrigator Pro, but irrigation depths were reduced by an average of 82% less than recommended by Irrigator Pro. Precipitation was collected with an electronic weather station and verified with an onsite manual gauge.

Peanut maturity was determined by the hull scrape method (Williams and Drexler, 1981). Yield rows were dug with a 2-row inverter, allowed to field dry, and harvested with a two row field combine. Pod yield, farmer stock grade and kernel size distribution were determined after being mechanically dried, weighed, and adjusted to 7% moisture (wet basis) and using screens specified in USDA grading procedures (USDA, 1993). Gross revenue was determined using the USDA price table for 2005 ($/mT =  $5.323 * TSMK + OK * $1.543: where TSMK  =  total sound mature kernels and OK  =  other kernels).

Data from each site is described independently. Pod yield, farmer stock (market) grade and gross revenue were analyzed using a general analysis of variance procedure (Statistix8, 2003) with respect to year, lateral spacing, and variety (Site 1). Least significant difference (LSD) method range test was used to show differences among means (P ≤ 0.05) when ANOVA F-test showed significance.

Results

Precipitation received, water applied, and plant and harvest dates are shown in Table 1. Precipitation was lowest in 2002 resulting in increased irrigation compared with 2003 and 2004. Surface drip at Site 1 applied less water compared with recommendations proposed by Irrigator Pro for the overhead sprinkler associated with Site 2 by 88, 95 and 65% in 2002, 2003, and 2004, respectively (sprinkler data not shown).

Discussion

Surface drip laterals spaced in alternate row middles cost half per unit area compared with drip laterals spaced next to every crop row. Laterals spaced at 1.83 m cost about $270/ha for tubing ($0.05/m for 0.200 mm thick tubing). Average pod yield in non-irrigated peanut for these three years across both sites was 4179 kg/ha. Laterals spaced at 1.83 m had an average positive net revenue about $132/ha when subtracting the cost of the tubing compared with net revenue of the non-irrigated areas. To cover the cost of the drip tubing for the 1.83 m spacing ($270/ha) there would need to be a 675 kg/ha yield increase assuming 2005 prices. Site 1 had an irrigated to non-irrigated yield difference of 752 kg/ha while Site 2 had a yield difference of 916 kg/ha. The average yield difference between irrigated and non-irrigated sites equates to about $64/ha. This low yield difference and associated partial dollar return can be attributed to precipitation received in 2003 which increased the non-irrigated yield 1274 kg/ha compared with the average non-irrigated yield measured in 2002 and 2004. If we removed the 2003 non-irrigated yield from the non-irrigated average the difference between irrigated and non-irrigated yield be then be 1146 kg/ha or a net partial return of $188/ha or triple the partial return shown previously. Net returns for surface drip irrigation over non-irrigation are dependant on quantity and timing of precipitation and resultant yields and market grade values. However, for this three year average at both sites and across all years, surface drip irrigation for either lateral spacing may not be cost effective when accounting for installation costs, fuel, equipment, pumping energy, and system maintenance. This also assumes that drip tubing is installed every year and the old tubing is removed and destroyed.

Surface drip irrigation does not require the same infrastructure (pipe, appurtenances, filtering, etc) compared with subsurface drip irrigation (SDI) or even overhead sprinkler irrigation. In addition, this research used an irrigation percentage of about 80% of that recommended by Irrigator Pro for overhead sprinkler systems which implies water and energy savings. Previous researchers have shown that SDI is more cost effective on small fields compared with sprinkler systems (Bosch et al., 1992; O'Brien et al., 1998; and Sharmasarkar et al., 2001). Since surface drip is less expensive than SDI, surface drip would be less expensive than overhead sprinkler on small fields. More research and economic analysis is needed to document the estimated savings and economic value of surface drip versus SDI versus overhead sprinkler.

Irrigated peanut with twin-row configuration had one percentage point increase in market grade and lower percentage of OK compared with single row spacing. These yield and market grade values are similar to those described by Baldwin et al. (2000, 2001), Beasley et al. (2000), Lanier et al. (2004) and Sorensen et al. (2004) for twin-row configuration.

Gross revenue for irrigated peanut at Site 1 averaged $1844 while Site 2 averaged $2083. Site 2 gross revenue was very similar to values described by Zhu et al. (2004) while Site 1 was $200/ha lower. Yield differences and associated revenue are probably due to differences in slope and aspect not difference in soil series. Site 1 had higher slope variation compared to Site 2 (Zhu et al., 2004) such that during high intensity precipitation events water would tend to move more rapidly off Site 1 and not infiltrate compared with Site 2.

Sorensen et al. (2007) showed that surface drip tubing can be used in the field for longer than one season, provided the previous crops were not conventionally tilled, i.e., strip tillage or no-tillage, such that surface drip tubing was not removed. They showed that a light covering of soil or plant debris over the tubing reduced rodent damage and would allow the tubing to remain in the field for three years and possibly longer with cotton and corn rotations. Over a three year life span, tubing would cost the grower about $90/ha per year for the surface drip tubing (other infrastructure costs would be incurred). This longer life span and lower cost/ha would make surface drip feasible even during years with high precipitation.

A grower may increase revenue with surface drip irrigation on peanut using an alternate row lateral spacing (1.83 m) and twin row configuration when precipitation is limited. The increased revenue may not be high enough to pay for the cost of the tubing in one year compared with non-irrigation, especially during a wetter than normal growing season. However, if drip tubing were installed and used for previous crops (a typical rotation of cotton, corn, peanut), which implies the use of strip/no tillage, there would be greater net revenue from increased yield and market grade for irrigated compared with non-irrigated peanut. More research is needed to identify how long surface drip tubing can remain on the soil surface before replacement is required.

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