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Managing Dry Fallow on Idle Irrigated Cropland

Posted by John Spring, Oregon State University | October 21, 2021

As drought persists across the west, irrigation water shortages and restrictions continue to expand. Parts of Central Oregon and the Klamath Basin were particularly heavily impacted in 2021, but by no means the only production regions to suffer. Water shortages and restrictions left many irrigated producers with the often unfamiliar challenge of managing fallow cropland on these idled fields, and near-term outlooks predict that dry fallow will continue on a meaningful number of acres into 2022 and beyond. With that in mind, I would like to share some observations on what I think have been successful management strategies, and invite any growers or consultants who have been forced to maintain idle irrigated ground to share particular solutions (or challenges) they have found.

While summer fallow typical of dryland wheat production systems can offer many lessons (or at least useful comparisons) for those managing dry fallow on idled irrigated ground, there are critical differences between the two types of fallow as well.

The major objectives of dryland summer fallow are:

  1. to store as much fallow-phase precipitation as possible in the soil profile for use by the following winter wheat crop
  2. hold the moisture line high enough in the soil profile that wheat can be successfully established solely from profile moisture when it is drilled in August or early September

Of course, preventing weeds from using soil moisture and making seed, preventing soil erosion, and minimizing input costs are all important considerations as well, but dryland summer fallow is really about storing soil moisture and keeping it in the seed zone.

Assuming that dry fallow irrigated fields will eventually return to irrigated production, soil moisture storage is not nearly as important. In most irrigated production regions in the PNW, annual precipitation is very low, and the amount of water that can be stored in a soil profile is minimal compared to typical irrigated crop needs, even in the best precipitation years. This is particularly true in areas that do not have the deep, fine-textured soils that make dryland wheat-summer fallow production possible. For example, most irrigated fields in Central Oregon have about 2 feet of soil to parent material (i.e. rock), which simply doesn’t provide enough moisture-holding capacity to store a large amount of water. A loam soil of the sort found in Central Oregon usually has available moisture-holding capacity of 1.5 to 1.75” water per foot of profile, totaling 3.5” of available water in a totally full 2-foot profile. This is not much more than the amount of water typically applied by a wheel line over a single 24-hour set and is unlikely to make a meaningful difference for crops requiring 2-3’ of water to supply full demand over a growing season. (Contrast that with a reasonably typical silt loam soil in a wheat-fallow region, often with a 6 + foot profile depth and holding capacity of 2.5” per foot, allowing it to hold 15” of plant available water at full capacity.) Additionally, I will assume that most irrigated crops are watered up so that the depth of soil moisture at the end of fallow – critically important in a dryland wheat system – is irrelevant to dry fallow returning to irrigated production.

So, if soil moisture storage is relatively unimportant to dry fallow, and the position of soil moisture in the profile is essentially irrelevant, what are the goals of dry fallow in idled irrigated cropland?

I would suggest 3 major objectives:

  1. prevent weeds from producing seed
  2. prevent soil erosion
  3. create seedbed conditions that optimize crop establishment and irrigation efficiency when the field eventually returns to irrigated production.

Minimizing input costs is also a major consideration.

A few more specific thoughts on ways that these objectives can be achieved follow.

Weed Control Frequency

Weed control action (either spraying or tillage) will be necessary 3 to 5 times over the course of the fallow season to maintain a clean field. Less often and weeds will get too large between control measures, dramatically increasing the risk of seed production and control failure. Frequent scouting is, of course, necessary to support timely action.

Weed Control Timing

The first control measure is usually needed to kill winter annual weeds like cheatgrass and tumble mustard before they get too large. This may be necessary in the fall in years with a late growing season, or following an early-harvested crop. If no fall application is made, one will be needed in early spring, around the time plants re-initiate active growth. From then on control measures are typically needed at 4 to 6-week intervals for the rest of the season as weed size and pressure dictates. A late fall application may be needed in late falls or following an early harvested crop.

Weed Size

Weed size is absolutely critical to reliable performance of systemic herbicides. Weed height of 2-4” is ideal, and the bigger weeds get, the more likely herbicide failure becomes. For dry fallow 6-8” weed height is probably a good general rule for maximum acceptable size. Active growth is also required for systemic herbicides to perform well. Weeds should not be sprayed during severe drought stress, in very dusty conditions, or once temperatures exceed 85 F for the day, as these situations can all reduce performance of systemic herbicides. Once weeds have entered reproductive phases (bolting in broadleaves or heading in grasses) their herbicide susceptibility often decreases substantially, so weeds should be sprayed before reproductive stage. Performance of contact herbicides and tillage is less sensitive to weed size and physiological status, but weeds should still be controlled well prior to the formation of any seeds.

Herbicide Options (early season)

For herbicides, the fallow season can be divided into 2 parts: the early season, when conditions are relatively moist and mild; and the late season, when conditions are dry and hot. A glyphosate-based herbicide program is ideal in the early season. For herbicide resistance management, and to broaden the spectrum of control, glyphosate should never be used alone in fallow, or at low rates. Tank-mixing a second broadleaf herbicide with glyphosate should be considered mandatory in fallow. The synthetic auxin (Group 4) herbicide 2,4-D is probably the most common, but may not be the best choice early. Dicamba (e.g. Clarity, and many generics) at 16oz/ac is another cost-effective synthetic auxin herbicide that gives good post-emergent broadleaf activity, as well as offering 3 to 6 weeks of good soil residual broadleaf control when applied in early spring in all but the driest years. Plant-back restrictions with dicamba vary by following crop, but often preclude its use later in the season. At this time, 2,4-D or fluroxypyr) offer shorter rotational restrictions to many crops, but make sure to check labels to confirm. Another tank-mix option to consider for economical broad-spectrum broadleaf control are the ALS-inhibitors (Group 2 herbicides), such as Affinity, Harmony, and Express. Be careful to check the rotational interval on the product you want to use, as the length varies widely within this herbicide group. Resistance to ALS-inhibitors is widespread, so use prior experience in your fields to evaluate the potential for success with this mode of action. Pre-emergence herbicides have been used successfully in chemical fallow in other regions of the country, but performance under extremely dry conditions is often poor, and possible rotational damage to high-value irrigated crops means that risks likely outweigh potential gains in his setting.

Herbicide Options (later season)

As the season progresses and conditions turn hot and dry, achieving good results with systemic herbicides becomes more challenging. If weeds are still small and actively growing, a glyphosate-based tank-mix is still a good option. If weeds are larger or under drought stress, however, a switch to contact herbicides or tillage may be advisable. Although it requires extra care in handling and use, there is no herbicide equal to properly applied paraquat (e.g. Gramoxone) on large, drought-stressed broadleaf weeds in fallow. Paraquat also makes an excellent rescue treatment – and resistance management tool – for hardened-off weeds that were not killed by an earlier application of systemic herbicide. For those that are unwilling to handle paraquat but want a contact herbicide alternative to tillage, the new product Reviton (tiafenacil) has shown good burn-down performance on broadleaf weeds without the applicator toxicity concerns of paraquat. Sharpen (saflufenacil) is another reasonably efficient contact option, but may have longer rotation restrictions depending on crop rotation plans.

Tillage

Early in the season, if tillage is not required for ground preparation, it is probably best avoided. When soils are moist, the soil disturbance associated with tillage often immediately stimulates another flush of weed germination, requiring re-treatment much sooner than a no-till chemical application would. On the other hand, tillage is considerably less sensitive to weed size than systemic herbicides. Later in the season, as weeds become larger or drought-stressed, tillage is an excellent option. In dry, hot soil later in the season, it does not usually stimulate new weed flushes and is less prone to failure than any herbicide. Tillage is also an excellent rescue treatment for hardened-off weeds that survived an earlier herbicide application. From a soil protection standpoint, minimizing tillage and maximizing crop residue retention on the surface are good general guidelines. If no residue is available, practices that maximize surface roughness and clod size during dry, high-wind periods can help reduce erosion to some extent.

Cover Crop

While cover crops indisputably use soil moisture and reduce wheat yields in dryland wheat-fallow systems, cover cropping has proven to be an excellent practice in dry fallow of irrigated ground. Here, a cereal cover crop contributes to weed suppression, erosion prevention, and can give superior seedbeds for following irrigated crops. Cereal rye is probably the best option for a cover crop, as it is most likely to establish and produce meaningful biomass using only natural precipitation. Winter wheat or triticale and spring wheat have also been used successfully. As nice as they sound in initial “theory”, multi-species mixes and legumes represent added cost with little to no realized benefit in actual experience, and are not recommended. If possible, conducting any seedbed preparation (tillage, ground leveling, bedding, etc.) early in the fall of the fallow year followed by a late-fall seeding of cereal rye seems to be the most successful strategy in Central Oregon. The cover crop can be allowed to grow until weeds necessitate it be terminated in the spring of the fallow year, or when it reaches an appropriate growth stage (flag leaf or early boot). The growing cover crop will provide weed suppression, as will the standing crop residue and lack of soil disturbance if managed with no-till. Leaving the undisturbed cover crop residue has been observed by some growers to maintain mellow soil conditions suitable for direct seeding of many irrigated crops without excessive or difficult residue, and, relative to a conventionally tilled seedbed, eliminates the need for pre-irrigation prior to seeding, reduces the risk of crusting and washing, improves infiltration capacity, and retains shallow seedbed soil moisture considerably longer after irrigations. All of this has led to notably better establishment and early growth of Kentucky bluegrass under cover crops than conventionally tilled seedbeds in Central Oregon and can be reasonably expected to give similar benefits to other shallow-planted small-seeded crops.


On the left, poorly managed tilled fallow has allowed large numbers of weeds to mature and produce seed. While the number of seeds produced per plant varies by weed species, it can be very high for the large plants pictured here. For example, Russian-thistle has been estimated to produce as many as 75,000 seeds per plant, kochia up to 30,000, hairy nightshade up to 45,000, and redroot pigweed over half a million seeds per plant under ideal conditions. While Russian-thistle and kochia seeds only last 1-2 years in the soil and many blow out of the field, both nightshade and pigweed seeds can survive for a decade or longer and leave a long-term legacy of increased weed problems in a field. An 8’x8’ uniform spacing gives 680 plants per acre, probably a reasonably conservative estimate here. Assuming an average seed production of 40,000 seeds per weed, this failure to control just late-season weeds returned roughly 27 million weed seeds per acre. For comparison, winter wheat seeded at 100lb/A drops roughly 1.6 million seeds per acre.

On the right, a visible legacy of weedy dry fallow the year preceding return to irrigated crop production. While issues with in-crop weed management undoubtedly contributed, heavy seed pressure from flixweed and redroot pigweed gave rise to considerable weed problems in this first year Kentucky bluegrass.
2. Effect of plant height on kochia control with Solstice (fluthiacet methyl + mesotrione, a premix of Cadet + Callisto registered in corn).


An excellent example of the importance of weed size on herbicide efficacy. Control of kochia with Soltice (fluthiacet-methyl + mesotrione) shows large declines when plants are treated at 20 cm (7.8”) height instead of the optimal 10 cm (4”) height. Figure 3. A. reproduced directly from Ganie et. al. 2010 (https://cdnsciencepub.com/doi/10.4141/cjps-2014-429). While not all weed/herbicide combinations result in such a large decline in effectiveness as plants get larger, it is not uncommon either, and the general point is widely applicable.

Kentucky bluegrass on left established following a season of dry fallow maintained with conventional heavy tillage. Bluegrass on right established following fallow under a non-irrigated, no-till spring wheat cover crop chemically terminated around flag leaf stage. Otherwise, both plots received identical treatment. Bluegrass seeded with no-till drill August 12th, photos taken September 28 from identical height. Bluegrass in no-till cover crop had 2-3 more tillers than that in the conventional seedbed at this time. Cover crop plots easily took 1” of water per application from LESA nozzles with no ponding or runoff evident, while conventional seedbeds showed notable ponding and runoff. Although not shown here, cover cropped plots also maintained surface soil moisture and morning surface recharge for 2-3 days longer than conventional till plots, a rather striking difference in August and early September. While plots were irrigated alike in the study, in a field setting the cover cropped plots likely could have received at least 1 less irrigation over the course of the fall growing season than conventionally tilled plots.

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