Linda Benedict, Millhollon, Eddie P. | 8/4/2009 10:38:28 PM
E. Merritt Holman and E.P. Millhollon
Cotton is a perennial that continues to grow vegetatively during reproductive development. A consequence of this characteristic can be excessive vegetative growth under conditions of above optimum soil moisture and fertility, thereby delaying maturity and increasing the incidence of lodging. The excessive foliage can create an environment favorable for boll rot and can decrease picker harvest efficiency. In an attempt to limit excessive vegetative growth in cotton, several growth retardants have been tested over the past 25 years. One that has been investigated extensively is 1,1-dimethylpiperidinium (mepiquat chloride).
In 1980, BASF Corporation began marketing a formulation of mepiquat chloride under the trade name Pix, the first widely marketed plant growth retardant for cotton. Currently, several formulations of mepiquat chloride are marketed and sold in Louisiana as Pix, Pix Plus, Mepex, MepPlus and Mepichlor. Although plant growth regulation using Pix has been under investigation since the mid 1970s in Louisiana and other cotton-producing states, results concerning the economic benefits have been contradictory. Determining where Pix belongs in a cotton production program in Louisiana necessitates a closer examination of the research and the underlying physiological effect of Pix on cotton growth and development.
Pix and plant growth
Once inside the plant, Pix reduces the synthesis of a class of plant hormones referred to as gibberellins. One of the major effects of this hormone class is to promote cell expansion in plant tissues. As some cell expansion is obviously needed, complete inhibition of gibberellin is not desirable; therefore, the rate of Pix applied to plants is critical to achieve the desired plant growth response. Pix concentration in the plant is a function of the rate applied and the plant size at application. Following application, the concentration of Pix in the plant is diluted by plant growth. Thus, Pix applied when plants are small can result in a significant plant response, whereas the same amount applied when plants are larger can result in little or no response.
Research conducted in Louisiana and other states has shown that Pix applications reduce internode elongation, resulting in a reduction in overall plant height and length of vegetative branches. Pix also causes smaller, thicker leaves, resulting in a characteristic dark-green color. Pix does not reduce existing plant tissue but limits further expansion following application. In theory, because Pix-treated cotton requires less energy and nutrients for leaf expansion and stem elongation, more energy should be available for boll production and, consequently, higher lint yields. In practice, however, the effect of Pix on yield has been highly inconsistent and in some cases negative.
Louisiana research results
Plant height. The effect of Pix on cotton plant height has been well documented under a multitude of environments, rates and timings by several researchers at the Northeast Research Station near St. Joseph since 1975. In the late 1970s, Steve Crawford demonstrated in small and large plot research that Pix consistently reduced height by 15 percent. Don Boquet, in the early 1990s, found that Pix reduced height by an average of 13 percent across two row spacings, five varieties and two soil types. More recently, Holman observed a 12 percent decrease in final plant height with Pix applied when the average of the upper five internodes was longer than 1.8 inches. Height reductions have been the most stable response to Pix measured in Louisiana research.
Crop maturity. Research in other states has suggested that Pix results in earlier crop maturity. Early maturity could help lower a farmer’s production costs by avoiding late populations of insect pests. In Louisiana, only two of the nine studies reviewed showed an increase in earliness associated with Pix. Most of the studies conducted in Louisiana have been non-irrigated, which could allow a dry period during the summer to negate any differences in crop maturity as a result of using Pix. On the other hand, in the situations where earlier maturity was observed with Pix, the difference in days to harvest was rather small: less than or equal to five days.
Lint yield. The most important economic response, lint yield, has also been the most inconsistent, ranging from a 24 percent increase to a 19 percent decrease. In Crawford’s studies in northeast Louisiana, yield increases or decreases with Pix were closely associated with differences in soil moisture. Thus, in dry conditions, Pix limited plant growth to the extent that yield was negatively affected (two of seven years), whereas yield increases with Pix occurred only under highrainfall conditions (two of seven years) where excessive plant growth and boll rot were noted.
Cotton boll rot. About 10 percent of the Louisiana cotton crop is lost each year to cotton boll rot. The reduction in plant growth with Pix could allow more sunlight penetration and air circulation around lower bolls and create an unfavorable climate for the fungi that cause boll rot. Snow and colleagues initiated a study at the Dean Lee Research Station near Alexandria in 1978 to determine the effect of Pix on boll rot and yield. The incidence of boll rot was reduced indirectly by Pix in all three years of the study with yield increases up to 24 percent occurring during years with high rainfall, vigorous plant growth and a high incidence of boll rot in untreated plants. Thus, some of the variability in yield response to Pix can be explained by conditions (high rainfall and excessive plant growth) conducive to boll-rotting fungi.
Different varieties. There is also some evidence that varieties respond differently to Pix. Jones and Dickson in Baton Rouge found a yield increase from Pix in one of two studies in 1987. The yield increase from Pix occurred with experimental strains (okra-leaf shape) when soil moisture was adequate to excessive, but there was a larger yield increase in the more vigorous strains. However, Millhollon and colleagues at the Red River Research Station in Bossier City found no yield differences in three normal-leaf varieties and one okra-leaf variety in response to Pix. Boquet and Coco also found no difference in yield response among five varieties of varying growth habit. Thus, the evidence supporting a Pix-by-variety interaction is not conclusive. However, it is reasonable to expect some degree of variety interaction with Pix because boll rot can be more severe in large, vigorous varieties.
Row spacing. Row spacing and loss of early squares are other factors that have also been investigated for interaction with Pix. Boquet and Coco found no interaction between Pix applications and row spacing. In addition, Cook and Kennedy in 1992 observed that Pix application following early square loss improved yield. However, they concluded that the interaction between Pix rate and timing and the amount of square loss would make it difficult to estimate the appropriate rate and timing for maximum benefit.
New product with bacillus cereus
In 1998, Micro Flo Company (Lakeland, Fla.) introduced a new plant growth regulator under the trade name Mep Plus, which contains mepiquat chloride and bacillus cereus. Researchers in several states are currently investigating the effect of bacillus cereus on plant growth. Testing by Holman in 1997-98 indicated that Mep Plus applications resulted in similar plant responses in height reduction and yield as the same rate of Pix.
What it means to Louisiana cotton producers
Producers must avoid yield decreases to break even with Pix priced at 70 cents to 80 cents per fluid ounce. To increase net return, producers must be conservative and apply Pix only when conditions warrant.
The shorter, more consistent plant height and dark green color associated with Pix-treated cotton are highly attractive to producers. However, Pix should be used only in fields with a prior history of excessive growth and adequate soil moisture. Producers should consider soil type, irrigation and previous rainfall. If it is determined a Pix application is justified, multiple applications at low rates will reduce the risk of a yield decrease if drought conditions ensue.
The recent interest in ultra-narrowrow cotton may establish a new niche for Pix in Louisiana. This cotton production system requires substantially shorter cotton plants with greatly reduced vegetative growth. Research on ultranarrow- row cotton in Louisiana is limited but will be an area of future research.
E. Merritt Holman, Assistant Professor, Northeast Research Station, St. Joseph, La., and E.P. Millhollon, Associate Professor, Red River Research Station, Bossier City, La.
(This article was published in the summer 1999 issue of Louisiana Agriculture.)