Arizona agriculture utilizes ~ 70% of the water in this state and generates a strong and productive industry. Arizona agriculture generates more than $23B in sales as well as directly and indirectly supporting more than 138,000 Arizona jobs and employing more than 162,000 unique workers. Arizona ranks among leading states in the production of lettuce, spinach, broccoli, cauliflower, cantaloupe, honeydew melons, durum wheat, and other commodities. Arizona is an important area for the seed production of many crops that are used across the U.S. and worldwide. Many Arizona counties rank in the top 1% of all U.S. counties in terms of crop and livestock production (Murphree, 2018).
In response to the Colorado River (CR) water shortage and the current reductions in CR allocations to Arizona via the Central Arizona Project (CAP), which is primarily impacting agricultural irrigation districts in central Arizona, there is an increasing level of scrutiny on agricultural uses of Arizona water. This of course is accentuated with the recognition that agriculture utilizes ~ 70% of the Arizona water supply.
In the irrigation districts along the mainstem of the CR, there is a common adage of “First in use, first in right.” This is a fundamental aspect of the “law of the river”, which is an amalgam of the various laws, agreements, and rulings on the governance of CR water. Therefore, it is important for us to consider and prepare the positive case that can be made for the good stewardship of water resources provided by Arizona agriculture.
One common area of criticism that is directed towards Arizona crop production systems, is the use of surface and flood irrigation systems. The alternative irrigation methods that are commonly advocated for use instead of flood irrigation are methods such as drip irrigation, micro-irrigation systems, sprinklers, etc. Each of these are good irrigation methods and advantageous under the appropriate conditions. However, a good case can be made for the very efficient use of flood irrigation systems, particularly with high-flow turnouts and dead level (or very nearly so) basins for irrigation. When properly managed, these types of flood irrigation systems can be very efficient.
When we know the area to be irrigated, the flow rate of water in the irrigation delivery ditch, and the amount of water needed; then we can determine the proper time or duration for an irrigation event. If we can get fast and uniform coverage of the field to be irrigated, apply the proper volume of water to replenish the plant-available water supply to the soil, then cut off the flow of irrigation water into the field; we can do a very good job of delivery for high water-use efficiency.
To facilitate the process of managing individual irrigations for optimum efficiency, the Irrigator’s Equation can be used to estimate the depth of water applied or time (duration) of an irrigation event.
Q x t = d x A
Where: Q = the flow rate, in cubic feet per second (cfs);
t = the set time or total time of irrigation (hours);
d = the depth of water applied (inches) and
A = the area irrigated (acres).
With an understanding of the dominant soil type in the field being irrigated and the level of soil-water depletion at the time of irrigation, we can estimate the amount or depth of water needed to replenish the soil profile of plant-available water to support the crop and prevent water stress.
In managing crop fields and irrigations, we recognize that soil textures vary in terms of water holding capacities and it is important to understand the dominant soil textures in the field, not only on the surface but also through the depths of the soil profile through the effective rooting depth of the crop, Tables 1 & 2.
Collectively, we can manage surface or flood irrigation systems efficiently. In the crop production arena, it is important to communicate these points effectively.
Table 1. Soil texture and water holding capacity.
2. Depths to which the roots of mature crops will deplete the available water supply when grown in a deep permeable, well-drained soil under average conditions.
Source: Chapter 11, "Sprinkler Irrigation," Section 15, Natural Resources Conservation Service National Engineering Handbook
References:
Murphree, J. 2018. Arizona Agriculture is 23 Billion Dollars Beautiful, Arizona Farm Bureau.
https://www.azfb.org/Article/Arizona-Agriculture-is-23-Billion-Dollars-Beautiful
This study was conducted at the Yuma Valley Agricultural Center. The soil was a silty clay loam (7-56-37 sand-silt-clay, pH 7.2, O.M. 0.7%). Spinach ‘Meerkat’ was seeded, then sprinkler-irrigated to germinate seed Jan 13, 2025 on beds with 84 in. between bed centers and containing 30 lines of seed per bed. All irrigation water was supplied by sprinkler irrigation. Treatments were replicated four times in a randomized complete block design. Replicate plots consisted of 15 ft lengths of bed separated by 3 ft lengths of nontreated bed. Treatments were applied with a CO2 backpack sprayer that delivered 50 gal/acre at 40 psi to flat-fan nozzles.
Downy mildew (caused by Peronospora farinosa f. sp. spinaciae)was first observed in plots on Mar 5 and final reading was taken on March 6 and March 7, 2025. Spray date for each treatments are listed in excel file with the results.
Disease severity was recorded by determining the percentage of infected leaves present within three 1-ft2areas within each of the four replicate plots per treatment. The number of spinach leaves in a 1-ft2area of bed was approximately 144. The percentage were then changed to 1-10scale, with 1 being 10% infection and 10 being 100% infection.
The data (found in the accompanying Excel file) illustrate the degree of disease reduction obtained by applications of the various tested fungicides. Products that provided most effective control against the disease include Orondis ultra, Zampro, Stargus, Cevya, Eject .Please see table for other treatments with significant disease suppression/control. No phytotoxicity was observed in any of the treatments in this trial.
For those interested in autonomous ag field robots, Future Farming (Misset Publisher, BV, Doetinchem, Netherlands) put together an informative video that features 16 commercially available units. A wide range of robots are highlighted – tool carriers, planters that geo-reference seed placement, several weeding machines and even a rock picker. The short, 3 minute 20 second video is interesting and a quick way to get up to date on these technologies.
It will be interesting to see what the future holds and whether these types of machines will become adopted in Arizona vegetable production. When discussing the high cost of automated weeding and other “smart” machines with a grower, he stated that the tractor driver was the cheapest part of the system. It’s a good point and one that’s hard to argue with.
Click here or on the image to see Commercial Autonomous Ag Robot video.
It is much easier to kill weeds when there is no crop in the field and now is a good time to reduce the weed seed bank of weeds in fallow fields. It is an especially valuable time to reduce the population of perennial weeds, like nutsedge, that will continue to get worse every year until the field is almost unfarmable for many crops. Weed seeds are buried at variable depths in the soil, some have hard seed coats and there are other variables that cause them to germinate over a long period of time. If they all came up at the same time they would be much easier to control. It takes time, therefore, to repeatedly irrigate, germinate and kill weeds with either tillage or herbicides. We have conducted trials that indicate that in most years summer annual weeds begin to germinate in February, reach a peak in June but continue to germinate into October. Perennial weeds often have to be actively growing to be adequately controlled and now is a good time to get started.
Tillage Repeated germination and tillage of small annual weeds can be very effective at reducing the weed seed bank. It can also spread weeds in some situations. Proper timing of tillage to kill weeds can be important with some species. Succulent weeds like purslane can survive for several days after cultivation or hoeing. They can reroot at the nodes and continue to grow if they are allowed to get too big before they are uprooted. Growers sometimes allow early emerging weeds to get fairly big in an effort to germinate as many seeds as possible. Incorporating big weeds can temporarily build up large amounts of organic matter into the soil and have a negative effect on some preemergent herbicides used in vegetables. Many of the root and shoot inhibitor herbicides like Trifluaralin, Pendimethalin, Benefin, DCPA and others can bind to organic matter and be less available to kill weeds.
Tillage has the opposite effect on perennial weeds such as nutsedge and bermudagrass than it has on annual weeds. These weeds are spread vegetatively and repeatedly irrigating and tilling them will spread rather than kill them.
Herbicides Both contact and systemic herbicides are used during fallow periods to control weeds. The contact herbicides include Paraquat (Gramoxone, Firestorm), Carfentrazone (Aim, Shark), Pyraflufen (ET), Pelegonic Acid (Scythe) and others. Some of the advantages of these are that they are quick and have no soil residual allowing crops to be planted soon after application. Disadvantages are that they are effective primarily only on small weeds. The highest labelled rates of Paraquat is effective, however, on some big weeds.
The most commonly used systemic herbicide for fallow ground is Glyphosate. Timing is especially important when using glyphosate to control perennial weeds. Glyphosate moves both up and down from the foliage through plants. More of it moves down into the reproductive parts of perennial weeds late in the season. Late season applications will work better on perennial weeds. It is broad spectrum and has no soil residual except on very course soils. Many of the systemic herbicides registered for fallow use, such as Oxyfluorfen (Goal, Galigen) or EPTC (Eptam) require at least 90 days before planting many vegetable crops. If done correctly, Eptam can be very effective in controlling nutsedge during summer fallow. It is sensitive to use for fallow weed control and application procedure is important. Eptam is extremely volatile and can be leached or evaporated when it contacts water. It works best when incorporated into dry soil to create a chemical tarp in the top 2 to 6 inches. The best technique is to irrigate and cultivate or spray the weeds with glyphosate when dry. One half gallon of Eptam should the be incorporated into the dry surface and left smooth. Avoid getting the field wet. The longer it can be left the better. Preferably at least 30 days. Longer if possible. Some people have chemigated the Eptam on and had fair control. I am surprised that this works but higher rates than 0.5 gallon have been used which my offset much of what is lost in the water. A couple good irrigations will be necessary to remove the residual Eptam when it get warmer and before planting vegetables
Only the fumigants kill weed seeds. These include Chloropicrin, Methyl Bromide, Metam Sodium, Dazomet, Telone and others. Most preemergent herbicides only work after the seed has germinated. Preemergent herbicides are often used for fallow weed control only when at least 30 -45 days or longer are available. Fumigants are expensive, can be difficult to use and are often used for disease or nematode control with the added benefit of controlling weeds.
Soil solarization and flooding have become increasingly popular in recent years as techniques to control pests during summer fallow. Few regions are as well suited for these techniques as the low desert. They are used primarily to control diseases but have the benefit of controlling some summer annual weeds as well.
