Cost and Benefits of Mechanical Straw Mulch Application to Irrigation Furrows
The primary method of water application for Treasure Valley crops is furrow irrigation. Furrow irrigation is a method that is fairly easy to use, has been used for many years, and has some large advantages associated with it when applied to certain crops. In the past hundred years, large investments have been made in the effort to improve furrow irrigation. The use of field leveling, control structures, and water conveyance techniques, are just a few examples of the progress that has been made and is being made. There are, however, some disadvantages of using furrow irrigation. These include soil erosion (soil movement within the field), a large water requirement, loss of sediments and nutrients from the field in runoff, and the excessive deep infiltration of water, which can carry nitrate below the root zone requiring more fertilizer to be placed on the crop. One method of reducing soil movement within the field and loss of sediment and nutrients off the field is to use mechanical straw mulching techniques.
In the early 1980's Vernon Nakada and Joe Hobson were applying straw mulch by hand to reduce irrigation-induced erosion. One method of reducing soil movement within the field and loss of sediment and nutrients off the field is to use mechanical straw mulching techniques (1, 2, 3, 4, 5). The process of using straw mulch on fields is not a new concept. In fact, the hand mulching of onions and other various crops has been used for many years. Spreading the mulch by hand can be extremely expensive, so there was a need for another cost effective way to spread mulch.
Joe Hobson, a neighbor of the Malheur Experiment Station, realized the great time and effort necessary to hand-apply straw mulch. Joe Hobson started to build the mechanized mulcher in Ontario in the mid 1980’s to help reduce mulching costs. His mechanical mulcher made the spreading of mulch economically feasible for farmers. Several variations of his original idea are used in the Treasure Valley. Early mechanical mulching trials starting in 1985 demonstrated its effectiveness to reduce erosion (2) and improve sugar beet yields (6).
There are many different factors to take into consideration when mechanically spreading mulch. None of these factors had been evaluated. The size, type, and rate of mulch application determines the costs and benefits of spreading. Another financial consideration is the initial start-up cost of purchasing a mechanical mulcher versus renting one, and the cost of labor to run the mechanical mulcher. These factors play a major role for the grower's cost-benefit analysis (7).
The cost of straw can vary greatly based upon whether or not you choose to bale your own straw or buy it from a neighbor. The average cost for wheat straw grown by the grower is around $0.50 per bale. Average bale weight is around 50 lb. per bale, and the optimized amount of straw application for most fields is 800-1000 lb. per acre. It is also very important to monitor the length of the straw in the bales. Having straw that is too long can cause water to seep over the rows, while having straw that is too short can cause the straw to be washed down the furrow. Experience developed by Joe Hobson in the Treasure Valley indicated that the best straw length to use is around 8-10”. Applicators should try to use a straw that is chaff-free as possible, from a weed-free field, and without any grain heads baled along with the straw to minimize any unfavorable results. Anecdotal reports by growers, who do straw mulching, state that by careful planning they can create the correct type of straw for mulching. It starts at wheat harvest by removing (disabling the straw chopper (or spreader) on the rear of the combine so that the threshed straw is left in a windrow. They then remove chaff and seed heads by incorporating two rows into one with a side delivery rake before baling. Labor costs for using the mechanical mulcher can also widely vary depending on the size of the machine and the proper placement of the bales at the end of the furrows. An operator can usually mulch about one acre per chamber per hour. Using a three-chamber machine you can mulch three acres per hour, using a five-chamber machine, five acres an hour, etc.
According to the Parma Company (Parma, ID), a local mechanical mulcher manufacturer, chamber cost (one chamber per furrow) for a mechanical mulcher starts out at about $5,000 each and decreases per chamber as the number of chambers on the mulcher increases. Purchasing a new mulcher for use in Oregon has benefits. Until recently, a 50% income tax credit was available for buyers offered by the Oregon State government for non point source pollution control equipment spread over 10 years. After January 1, 2002 the Oregon legislature reduced the tax credit from 50% to 35% of the total cost. The average useful life of a machine is 20 years.
An estimated total cost for mechanical mulch would be approximately $50.00 per acre. The rental cost of a automated mulcher (requiring only a tractor operator for spreading and another person to load the machine) can be estimated at $17.50/acre. Labor cost (including benefits) for a tractor operator would be about $14.00 an hour, and the person to load the straw into the mulcher would be about $10.00 an hour. Using a four-chamber machine that can mulch 4 acres an hour, labor cost per acre for the tractor operator would be $3.50 an acre, and the person loading $2.50 an acre. Tractor time per hour (160-60 hp tractor, fuel and lubrication, repair and maintenance., depreciation and interest, insurance) can be estimated at $26.00 an hour, (four acres mulched an hour) or $6.50 an acre. Straw cost (spread at a rate of 1,000 lb per acre, using purchased wheat straw) can be estimated at $20.00/acre.
Mechanical straw mulching improved onion yield and size in furrows that were compacted by tractor wheel traffic (3, 7). Five replicated trials were conducted between 1988 and 1995, in commercial fields and at the experiment station.
In the 1988 trial (commercial field) the mulch did not affect yield in rows without tractor wheel compaction. In rows with compaction, mulch improved yield through the increased production of jumbo onions by 30%. The yields of medium and colossal onions were not affected in this trial. The mulch was applied all at once at a rate of 650 lb per acre.
In 1990, (commercial field) straw mulch increased onion yield 24% in furrows without a wheel track traffic and 38% in furrows with a wheel track traffic. The yields of medium and jumbo onions were not affected by the straw mulch in this trial. The colossal yields were improved by the straw mulch. The mulch was applied all at once at a rate of 650 lb per acre.
In the 1991 trial, onion yield was unaffected in the commercial field. The mulch was applied at a rate of 650 lb per acre. The same year trial at the experiment station in a field with 3 percent slope showed that straw mulch increased onion yield by 64%. The increase was through both jumbo and colossal onions, and decreased the yield of mediums. In the 1995 experiment station trial in the same field with 3 percent slope, similar results were obtained with a 74% onion yield increase. Both of the trials conducted at the experiment station, wheel-compacted furrows were irrigated. The experiment station rate in 1991 was 800 lb per acre, and was 560 lb per acre in 1995, with split applications of straw.
During the trials at the experiment station, water runoff, infiltration, irrigation efficiency, and water use efficiency of the onion crop were measured in addition to onion yield and grade responses. The correlation for the increase in onion yields and straw mulch is attributed to reduced water runoff, increased lateral water movement, and improved soil moisture. The straw mulch placed in the furrows caused more water to move laterally into the beds as a result of slow water movement at the furrow bottom and the higher level of water in the furrows.
In 1992 trial, beet yield, quality, and recoverable sugar were measured for the sugar beets grown at the Malheur Experiment Station on a Nyssa silt loam with 3 percent slope, with and without mechanical mulching wheat straw in the irrigation furrows (4). Water inflow, water outflow, and sediment loss were measured over time on each of the 24 plots for all 13 irrigations. Infiltration was calculated, and irrigation durations were managed so that total water infiltration would be the same for strawed and non-strawed beets. The goal was to apply the amount of water necessary to meet the evapotranspiration demand for suagr beets at the locality. Runoff water and sediment from every plot was independently analyzed for nitrate, ammonium, total N, phosphate, and total P during one irrigation. Irrigation efficiency increased with furrow mulching. Strawed furrows required 36 percent less water than non-strawed furrows to meet evapotranspiration. Mechanically applied straw mulch increased beet yields by 2.5 tons per acre and recoverable sugar by 866 lb per acre. Furrow mulching decreased the loss of sediment from 78.8 tons to 6.7 tons per acre, decreased estimated P loss for 133 to 12 lb per acre for the season, and decreased estimated N loss from 334 to 75 lb per acre for the season.
The measurements in fields demonstrated that mechanical straw mulching had conservation benefits by reducing soil erosion and irrigation water runoff (1, 2, 3, 4, 5). Synthetic materials such as polyacrylamide (PAM) can control erosion and enhance water infiltration in irrigation furrows. A single application of straw mulch is apparently as effective as repeated applications of PAM to reduce erosion (8). Mechanically applied straw was more effective than PAM in increasing water infiltration and maintaining soil water potential, hence straw was more effective in increasing onion yields.
Straw mulch was also related to benefits in potato fields (9).