IMPROVING ONION YIELD AND MARKET GRADE BY MECHANICAL STRAW APPLICATION TO IRRIGATION FURROWS

C.C. Shock, L.B. Jensen, J.H. Hobson, M. Seddigh, B.M. Shock, L.D. Saunders,
and T.D. Stieber

Malheur Experiment Station, Oregon State University, 595 Onion Avenue, Ontario, Oregon, 97914.

Funds used for this study were provided in part by the Oregon State Univ. Experiment Station, Project 304. Contribution from Oregon Agric. Exp. Stn., Tech. Paper No. 10627. Contributions from SBIR are gratefully acknowledged.
 

Additional index words. soil erosion, furrow mulching, water runoff, water use efficiency, irrigation efficiency.
 
 

Summary

    Onion is primarily grown under furrow irrigation in western U.S. Wheel compaction of furrows increases water runoff and erosion, and can lead to poor lateral water movement and reduced yields. We studied the effects of 600 to 900 kg wheat straw ha-1 mechanically applied to the bottom of irrigation furrows on yield and market grade of sweet Spanish onions in commercial onion fields in 1988, 1990, and 1991, and at an experiment station in 1991 and 1995. Furrows in commercial fields were either compacted with tractor wheels or not. In the commercial fields, straw application increased onion yield in compacted furrows in 1988 and in all furrows in 1990. At the experiment station, straw mulch increased onion yield 64% in 1991, and 90% in 1995. Straw application primarily increased yields of jumbo and colossal onions, whereas there was no effect on medium onions and a slight decrease in small onions. We attributed yield improvements to decreased water runoff and increased lateral water movement.

    Onion growth is very sensitive to water stress (Goltz et al., 1971; Miller et al., 1971), and is commonly furrow irrigated in western U.S. (Ells et al., 1986). In a recent study in eastern Oregon, yield of onion cultivar Great Scott decreased when soil water potential at 20 cm depth decreased below -20 kPa (Shock et al., 1997). Onion requires intensive tractor-operated cultivation, which makes furrows subject to compaction by wheel track. This may increase water runoff and decrease lateral water movement. Onion growing on beds may not receive adequate water and thus not attain high yields when lateral water movement is restricted.

    Small amounts of grain straw uniformly placed by hand in sloping irrigation furrows can decrease water runoff and increase lateral water movement (Berg, 1984; Brown, 1985; Brown and Kemper, 1987). Manual application of straw is, however, onerous and time consuming. Commercial availability of equipments that mechanically apply straw mulch to the bottom of irrigation furrows have increased the interest in using the technique in onion fields. We (Shock et al., 1997) recently reported that mechanical straw mulching of irrigation furrows in onion plots substantially reduced soil erosion and surface water runoff. The objective of the present study was to assess effects of mechanical straw application in irrigation furrows on onion yield and market grade.
 
 

Materials and Methods

    Five replicated trials were conducted between 1988 and 1995. The first three experiments were on-farm studies conducted within 30 km of Ontario, Ore., on commercial onion fields in 1988, 1990, and 1991. The other two experiments were conducted at the Oregon State University Malheur Experiment Station 8 km south west of Ontario in 1991 and 1995. All fields had silt-loam soils. The slopes on commercial fields were 4% in 1988, 1% in 1990, and 1.5% in 1991, and at the experiment station were 3% in both years. Furrows were 1.12 m apart, and were 400 m long in commercial fields and 72 m long at the experiment station. Designated furrows in the on-farm experiments and all furrows at the experiment station were compacted by tractor wheels traffic. At the experiment station, sweet Spanish onion was seeded in April each year on rows 56 cm apart on furrow beds (each bed containing two onion rows) at about 2 kg ha-1. The crop was thinned to one plant for every 10 cm of row during the last week of June.

    Treatments for the on-farm experiments were two rates of straw applications (0 and 730 kg ha-1), and two compaction levels (wheel-traffic and non-wheel traffic furrows). At the experiment station, treatments were only two rates of straw applications (0 and 900 kg ha-1 in 1991, and 0 and 630 kg ha-1 in 1995) with all furrows compacted by wheel traffic. Plots in all experiments consisted of two furrow beds (400-m long for on-farm experiments and 76-m long for those at the experiment station), each containing two rows of onion. A randomized complete block was used in each year, with 4 replications in all experiments except for on-farm experiments in 1988 and 1991 when 10 replications were used.

    To prepare straw suitable for mechanical application, wheat from a weed free field was cut at 20 to 25 cm length during wheat harvest with a cylinder type combine, and was baled and stored dry. Straw was then applied to the bottom of furrows using a Hobson Mulching System mechanical straw applicator (Hobson Manufacturing Inc., Kaiser, Ore.). Half of the straw was applied prior to the first irrigation, and the reminder was applied after all cultivations in June. Splitting straw mulch application allowed cultivation for weed control.

    Uniform and effective weed control was obtained by a combination of herbicides, cultivation, and hand weeding. Chemical weed control included a combination of the foliar active herbicides bromoxynil at 175 g ha-1, oxyfluorfen at 56 g ha-1, and fluazifop at 210 g ha-1 in 108 L of water ha-1. Onion maggot was controlled by banding ethion at 3.36 kg ha-1 before onion emergence. When needed, onion thrips were controlled by aerial applications of cypermethrin at 8.8 g ha-1 in 42 L of water ha-1. Also, onions were fertilized in June with 200 kg N ha-1 as slow release fertilizer at the experiment station; different N rates were applied on growers field depending on soil test . All plots received 14 to 17 irrigations during each season. Irrigation duration in furrows with and without straw was identical and was adequate to allow the water to move laterally past the onion plants on the top of the beds in all rows.

    Onions were lifted with a rod weeder in September and left on the soil surface until they were hand topped. Onions were then graded using a Kerian Speed Sizer (Kerian Machines Inc., Grafton, N. Dak.) into medium (<7.5 cm), jumbo (7.5-10 cm), and colossal (>10 cm) classes, and weighed. Data were analyzed as complete randomized blocks, and treatment means were compared using Fisher's protected LSD.
 
 

Results

    Straw mulch did not affect onion yield in 1988 when the irrigation furrow was not compacted by the wheel track (Table 1). When the furrow bottom was compacted by the wheel track, straw mulch increased onion yield by about 30%. The increase in total onion yield was through increase in production of jumbo onions, whereas straw did not affect the yields of medium and colossal onion.

    In 1990, straw mulch increased onion yield 24% in furrows with no wheel-track and 38% in wheel-track furrows (Table 1). The yield increase was through improvements in colossal onion yield, while straw mulch did not affect (or caused a non-significant decrease) in the yields of medium and jumbo onions.

    Straw mulch did not affect onion yield when grown on a commercial field in 1991 (Table 1). However, in the experiment conducted at the experiment station in that year, straw mulch increased onion yield 64% . The effect was through increases in both jumbo and colossal onions, whereas straw mulch decreased the yield of medium onions. Similar results were obtained in 1995 when straw much increased onion yield 74%.
 
 

Discussion

    The sensitivity of onion growth and yield to soil water stress has been well documented (Goltz et al., 1971; Miller et al., 1971; Hegde, 1986; Kruse et al., 1987; Ells et al., 1990; Shock et al., 1998). Results of the present study indicate that straw application to irrigation furrows can substantially improve onion yield. In addition to increasing yield, straw mulch increased the proportion of larger onions, which have higher market value. Other research (Drinkwater and Janes, 1955; Kruse et al., 1987; Ells et al., 1990) also indicate that irrigation increases onion yield through increases in jumbo (>7.5 cm) onions.

    Onion yield improvement by straw application to irrigation furrows is attributed to reduced water runoff and increased lateral water movement. To investigate this, water inflow and outflow were recorded during each irrigation at the experiment station in 1991. Straw mulch reduced water runoff substantially and increased infiltration by about three times (Table 2). Infiltration includes the water that moved laterally to planting beds as a result of slow water movement at the furrow bottom. Consequently, straw mulch increased both irrigation efficiency and water use efficiency (Table 2).

    Mechanical straw mulching of irrigation furrows of onion crop also has an important conservation value because it markedly reduces both soil erosion and irrigation water run-off (Shock et al., 1997; Shock and Shock, 1997). Synthetic materials such as polyacrylamide (PAM) control erosion and enhance water infiltration in irrigation furrows in onion fields, but a single application of straw mulch is apparently as effective as repeated application of PAM to reduce erosion and more effective than PAM to increase water infiltration and maintain soil water potential (Shock and Shock, 1997).

    Investigation of onion yield in 1991 as a function of seasonal water infiltration indicated that smaller amounts of straw than 900 kg ha-1 applied in that year could have been sufficient for maximum yield (data not shown). Effective onion rooting depth is generally shallower than 60 cm (Drinkwater and Janes, 1955; Kruse et al., 1987), suggesting that deep water percolation may not be beneficial, but increasing lateral water movement to maintain soil moisture at rooting profile can make more water available to onion. Effects of different rates of straw and irrigation durations on onion yield would depend on soil type and slope, and merit further investigation. Furrow spacing and plant position affects erosion from surface irrigation (Sojka et al., 1992). Effects of straw on onion yield and market grade under different planting conditions may be different and merit further investigation.
 
 

Conclusions

    Results indicated that mechanically applied straw mulch to the bottom of irrigation furrow can significantly improve potato yield and market value, especially in wheel rows where the furrow bottom is compacted. The technique also decreases water erosion and run-off, and improves water use efficiency. Under furrow irrigation, it is generally not practical to leave residue on the surface before constructing the furrows. Commercial equipments are now available that allow banding a layer of straw in irrigation furrows is now available that allows straw application in furrows after planting. Another advantage of this system is that fertilizers and pesticides may be applied directly on the soil surface before straw is applied.



 
 



 
 

Literature Cited

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Brown, M.J. 1985. Effect of grain straw and furrow irrigation stream size on soil erosion and infiltration. J. Soil and Water Cons. 40:389-391.

Brown, M.J. and W.D. Kemper. 1987. Using straw in steep furrows to reduce soil erosion and increase dry bean yields. J. Soil and Water Cons. 42:187-191.

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Shock, C.C., E.B.G. Feibert, and L. D. Saunders. 1998. Effect of soil water potential as irrigation threshold on onion yield. [Submitted for publication].

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Shock, C.C. and B.M. Shock. 1997. Comparative effectiveness of polyacrylamide and straw mulch to control erosion and enhance water infiltration. In Wallace, A. Handbook Of Soil Conditioners. Marcel Dekker, Inc. New York, NY. pp. 429-444.

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