Investment Returns from Three Sub-surface Microirrigation Tubing Spacings

Sub-surface microirrigation (SMI) tubing costs make up 33 to 60% of total purchase and installation costs of the system, depending on tubing spacing. Purchase and installation costs of SMI systems can be lowered by $290/acre by increasing tubing spacing from 3 to 9 ft. Yields may be reduced due to less uniform placement of water in the crop root zone. The net present values (NPVs) of three SMI tubing spacings were estimated for systems irrigating corn (Zea mays L.) and peanut (Arachis hypogaea L.). The analysis included investment costs for purchasing and installing the irrigation system but did not include water source development costs. Irrigated yield responses and irrigation applications came from field experiments conducted from 1986 to 1995 at Suffolk, VA, on an Uchee loamy sand (loamy, siliceous, thermic Arenic Hapludult) with inclusions of Emporia loamy sand (fine-loamy, siliceous, thermic Typic Hapludult). The 6-ft tubing spacing had the highest NPV of $14/acre. The low NPV indicates it is unlikely to be profitable when water source development costs are included. NPVs of 3-ft and 9-ft spacings were −$28 and −$104/acre, respectively. Six-foot spacing had lower yields and lower investment costs than 3-ft spacing. Increasing the proportion of corn in the rotation lowered the advantage of 6-ft spacing. Higher peanut and corn prices or reduced tubing prices narrowed the difference in NPVs between 3-ft and 6-ft spacings. Six-foot tubing spacing results in highest returns when SMI is used for corn-peanut rotations on sandy soils in Virginia/North Carolina. Research QuestionSub-surface microirrigation (SMI) uses below-ground tubing to deliver water, nutrients, and chemicals to the crop root zone. Tubing costs of SMI comprise 33 to 60% of total purchase and installation costs of the system depending on tubing spacing. Investment costs of SMI systems can be lowered by $290/acre by increasing tubing spacing from 3 to 9 ft. Wider spacings may also reduce yields due to less uniform placement of water, chemicals, and nutrients in the crop root zone. The tradeoff between reduced investment cost and possibly lower yield income must be considered in order to achieve optimal returns on irrigation investments. This study investigates the effects of variations in lateral spacing of SMI tubing on investment returns from irrigating corn and peanuts. Effects of variations in crop rotation, crop price, and tubing cost on the optimal tubing spacing are analyzed. Literature SummaryPowell and Wright evaluated corn grain yields under 3-, 6-, and 9-ft spacings and found that corn grain yields were highest under the 3-ft spacing. Applying more irrigation water reduced the yield advantage of narrow spacings over wide spacings. Lamm et al. (1992, 1995) reported that lateral spacing for SMI systems of 5 ft (1.5 m) or less (between alternate rows) produced higher yields and net returns than wider spacings on corn in western Kansas. Study DescriptionNet present values (NPVs) were estimated for SMI systems with alternative tubing spacings. The NPV measures the present worth of net returns to an irrigation system after deducting irrigation costs. If all costs are included, a positive NPV indicates the irrigation investment is a more profitable alternative than nonirrigated production. The analysis was conducted for a corn-peanut rotation over a 10-yr period, the assumed lifetime of SMI tubing. Investment costs (excluding water source development costs), tax depreciation benefits, and salvage values of alternative tubing spacings were estimated. Peanut and corn yield responses to irrigation, additional input costs for irrigated production, and irrigation pumping costs were considered. Yield responses to imgation from alternative spacings were obtained from experiments conducted at Tidewater Agricultural Research and Extension Center, Suffolk, VA, from 1986 to 1991 on peanut and from 1986 to 1995 on corn. The experiments were located on Uchee loamy sand with 0 to 2% slopes. Applied QuestionsWhich tubing spacing gives the highest net return to the irrigation investment over the lifetime of the tubing? The 6-foot tubing spacing has the highest NPV of $14/acre, compared with — $28/acre for 3-ft, and −$104/acre for 9-ft spacing. However, water source development costs, which are not included in the NPV estimate, are likely to be greater than $14/acre indicating that the 6-ft spacing is unlikely to be profitable. While 6-ft spacing has somewhat lower yields yhan the 3-ft spacing, its lower investment cost causes its NPV to be higher. The 9-ft spacing has much lower yield response and lower NPV than the 3- or 6-ft spacings. How is optimal tubing spacing affected by crop rotation? Increasing the number of years of corn in the corn-peanut rotation reduces the NPV advantage of 6-ft spacing. The 6-ft spacing has the highest NPV for all rotations except continuous corn, for which 3-ft spacing has the highest NPV. The NPV of the 6-ft spacing exceeds that of 3-ft spacing by $42/acre with a corn-peanut rotation while the 3-ft spacing has a $2/acre higher NPV with continuous corn. How is optimal tubing spacing affected by crop prices? The spread between the highest NPV (6-ft spacing) and second highest (3-ft spacing) is reduced by higher corn and peanut prices. Higher prices benefit the 3-ft spacing relatively more because of its higher yields. However, corn prices would have to increase by 46% to $4/bu before 3-ft spacing would have the same NPV as 6-ft spacing. How is optimal tubing spacing affected by tubing cost? Decreasing the cost of tubing increased NPVs of all tubing spacings but benefited the 3-ft spacing relatively more because it uses more tubing per acre. If tubing costs fell by about 25% to $0.022/ft, the 3-ft and 6-ft spacings would have equal NPVs ($56/acre). RecommendationWhen SMI is used to produce corn and peanuts in rotation on sandy soils in Virginia/North Carolina, we recommend a 6-ft tubing spacing. If tubing costs are reduced to below $0.022/ft or if corn prices exceed $4/bu., a 3-ft spacing would produce higher returns. Some producers who have to maintain high yields in order to fulfill peanut quotas or produce adequate corn for feed on fixed land area may wish to consider the 3-ft spacing, if this is a cheaper alternative than purchasing grain.