Although cotton acreage has declined in recent years, nearly 200,000 acres of some of the world’s finest quality cotton will be harvested this year in California. A large majority of the California crop is Pima cotton, a type that especially commands a higher market price when compared to more conventionally grown Upland cotton found throughout much of the US cotton belt.

This year has been particularly difficult for cotton growers because many of them work highly productive farmland that has seen severe curtailment of surface irrigation water resulting from ongoing environmental concerns in the Sacramento/San Joaquin Delta. And while this has been a year of acreage and water cutbacks, we are in a time of the season that is especially critical to the performance of the crop.

Most of the state’s crop is currently in peak bloom, a plant development period that is critical for setting a good crop. Blooms, should they succeed the temperature and physiologic stresses for a 4 to 6 day period, have a very good chance to mature into harvestable bolls at the end of the season.

Too much water or physiological stress at this time of year can result in premature plant cutout or the abortion of young fruit that often takes place within a day or two of bloom. The key therefore is to minimize severe stress and instead follow established irrigation guidelines. This is particularly true this year because peak bloom closely coincides with a near record number of July days over 100 degrees, thereby increasing plant water stress levels.

Many growers have elected to delay one or more of their irrigations earlier in the season, and we now see many fields moving toward a premature cutout at a time when there is still plenty of season to set and retain a large portion of the crop. This is particularly true for Pima cotton because it has a bloom set period of about 45 days when compared to Upland cotton’s approximate 30-day period. This tells us that there is still a large percentage of the crop to protect and that poor irrigation management decisions are amplified during this time of year.

There are many farms this year that simply don’t have the water available to fully meet ET for the 2009 season. This is surely understandable as growers choose to direct their limited water supplies in an effort to save their permanent plantings or other high valued crops that may have been forward contracted. Under these circumstances, deficit irrigation approaches in cotton may be an especially attractive option in the effort to stay profitable.

A successful deficit irrigation strategy should not be conducted in a “boiler plate” manner, but rather has several key components to consider. One factor that can get overlooked is the field’s soil water holding capacity. In clay loam and clay soils common to cotton acreage, there is often a deep reservoir of water that can be tapped at the end of the season and mitigate severe stresses that might otherwise result from an early irrigation termination. This sometimes underutilized resource should be tempered with the fact that some fields are variable in nature and growth responses will be accentuated as more severe water restrictions are applied. While many of these fields could eventually end up with some very respectable yields, expect to see greater variability if imposing more severe restrictions.

Another issue that helps in determining the sensitivity to late season water deficits is the depth of the late season root system. Plants that have a good tap root developed early in the season on soils with good tilth in the upper profile are often well situated to take advantage of deep, and increasingly available soil moisture as the root system continues to grow into lower soils zones. Good, well-drained clay loam soils on the West Side of the San Joaquin Valley might tap significant quantities of water at or below 6 feet, and clay soils that are not too tight and compacted can often exploit water to a depth of 4 feet in the late season. Here again, keep in mind that variability can again become more of an issue and situations that include soil compaction or significant soil salinity should be evaluated separately.

Many University of California studies conducted in the San Joaquin Valley have demonstrated real success in using deficit irrigation as a method to minimize crop losses when full cotton ET cannot be met. Successful approaches have a few elements in common, first of which have already been discussed. Using knowledge of your soil water reservoir to evaluate the current water extraction and forecast future water extraction can be used to assist in determining to the duration of the stress at critical growth periods. Delaying an early season decision by a few days can sometimes make all the difference in determining whether an extra irrigation late in the season will be necessary.

Assuming that the field irrigation schedule to date imposed a moderate water stress accrual prior to each earlier irrigation event, furthering this strategy can continue to work provided the stress is not too severe and too early. Studies showing a move of the pre-bloom irrigation events from a minus 15 bar leaf water potential (LWP) trigger to a minus 18 bar LWP trigger have been successful as well as a move toward higher stress levels beyond the recommended minus 18 to minus 19 bars post bloom. Studies have shown that pre-September water stress levels up to minus 22 to minus 23 bars in Pima cotton have had minimal impact on crop yield and quality on deficit irrigation treatments.

If water availability mandates a more severe water stress regime, the timing of the irrigation event can be nearly as critical as the amount applied. It is best to avoid very high water stress levels as late as possible in the season. Avoiding severe water stress until later in the season will result in the least impact on yield and quality in most any situation.

Because the fiber length is largely determined in the first 20 days following bloom, anything that delays water stress until after the fiber elongation phase will moderate any fiber quality problems that result from water stress. It is therefore recommended that extreme water stress be put off as late as possible until after the plant moves into physiological cutout. While this situation will likely result in a significant yield reduction, it is less likely that grades will suffer as much as they would if the stress event occurred earlier in the season.

Some might suggest that in this relatively poor market environment that water is not as important to the crop because values are lower than they have been in the past. This position might be a mistake, particularly if there is some flexibility in water scheduling or ability to make use of supplemental water. As the optimum amount of applied water declines, by definition there is a reduction in yield. At the first signs of yield reduction in a given field, the yield reduction is relatively low. However the amount of yield decline later increases as water quantities continue to decline.

This simple fact is rather fortuitous because it tells us that we can impose minor water deficits without causing a very large impact on yield. But if we impose a more substantial and sustained deficit, the chances for a significant and costly reduction in crop value are very likely.

While water’s value for Pima and Upland cotton types is going to be significantly different as a result of the different market pricing for each crop, water production differences are not that large. San Joaquin Valley studies at UC research centers and growers fields have shown that if water is applied efficiently and pest pressures are well managed, the yield from deficit irrigated cotton generally increases 50 to 80 pounds for every inch of water applied.

Used properly, these water production efficiency values for irrigated cotton together with information on market conditions, are very useful in evaluating the decision of whether to deficit irrigate a field and to what extent. Evaluating each field’s ability to perform in terms of yield and quality responses to water deficit conditions, can be a broad-based tool in outlining current and future farm water management strategies.