Plant populations of American pima cotton (Gossypium barbadenseL.) and upland cotton (G. hirsutumL.) giving maximum lint yields vary widely in reports in the literature. This study attempts to explain the wide variations in plant populations giving maximum lint yield for pima cotton by considering the plant population X plant height interactions with lint yield. Field tests were conducted for 5 yr at four locations on Anthropic Torrifluvents, Torrifluventic Haplostolls, and Typic Torrifluvents soils. Fifteen pima cotton tests with varied plant populations, and with 1.02‐m row spacing, had plant population, plant height, and lint yield measured on each plot. The relationships among yield, population, and height were analyzed by multiple linear regression in each test. In three of the 15 individual test analyses all regression components were significant. A combined analysis of the 15 tests showed a significant negative (P=0.001) plant population × plant height interaction for lint yield. Most of the plant population and plant height treatment means in these tests were within the range of 20 000 to 200 000 plants ha −1and 60 to 190 cm high. Plant population giving maximum yield decreased about 11 000 plants ha −1for each 10‐cm increase in plant height. Cultivars and irrigation treatments did not greatly alter the interaction. Year and geographical location effects were more varied. The variation in plant population producing maximum lint yield for different plant heights was great enough to explain the varied plant populations for maximum lint yield reported in the literature.

Conservation tillage techniques have gained widespread, but not universal, use in irrigated corn (Zea mays L.) production. This study was conducted to evaluate yield impacts associated with conservation tillage methods when compared with conventional techniques in a replicated field experiment for 17 consecutive years in Nebraska (1976–1992). Six tillage treatments were imposed on continuous corn production and replicated three times. Tillage treatments ranged from traditional (disk and surface plant) to no‐till (slot plant on existing ridge). Post‐plant operations were uniformly applied to accommodate furrow irrigation. Over the life of the project the conservation tillage treatments (ridge till, rotary till, and slot plant) had a 4 bu/acre increase in grain yield over the conventional treatments (those having chisel, disk, or lister operations). The advantage of the conservation tillage systems was amplified by corresponding lower production costs. The rotary, ridge, and slot tillage systems had $7.72, $14.22, and $16.22/acre lower annual tillage and herbicide costs (1990 dollars), respectively, than the conventional tillage system with a disking operation. The conservation tillage treatments significantly lowered stalk rot and residual spring soil nitrate N concentrations. Other comparisons showing significant differences were list vs. chisel and disk for harvest population and soil potassium concentrations, and ridge vs. rotary for harvest population. None of the other comparisons tested had significant differences for grain yield or other soil parameters. Overall, this study indicates that conservation tillage on irrigated continuous‐corn is advantageous over conventional methods. Research Question Are furrow‐irrigated corn yields increased with conservation tillage? Furrow irrigation and conservation tillage techniques are both common to irrigated agriculture in the U.S. Corn Belt. This study evaluated corn yield for six different tillage systems over 17 yr in a furrow irrigated field trial in south central Nebraska. Literature Summary Long term field studies of the relationship between tillage activities and corn yield show mixed results. Some studies show that corn yield increased when less intensive tillage practices were used. Other studies showed no effect or even a decrease in corn yield with conservation tillage. In these other studies there are many factors involved—soil temperatures, weed control, disease problems, or seed placement, for example. Much of the information available concerning alternative tillage and corn yield is from nonirrigated studies where year‐to‐year climate plays a larger role. Study Description Corn yield was collected in a field study for 17 yr (1976–1992) at Clay Center, NE. The field was furrow irrigated. The tillage treatments evaluated were: conventional tillage (disk, surface plant, and ridge) conventional tillage with a chisel operation after shredding conventional tillage with a lister attachment on the planter ridge tillage rotary tillage slot planting Herbicide ...

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Plant populations giving the highest lint yield of American pima cotton (Gossypium burbdense L.) grown in 1‐m spaced rows change as plant heights change. Therefore, plant height should be considered in future plant population studies with pima cotton. This study was conducted to determine the number and type of plant height measurements needed to give a satisfactory estimate of mean plant height for a plot. Plant height uniformity data were obtained from 12 plots (four locations with three plots per location) in Arizona. Soils were Torrifluvents Haplostolls, Anthropic Torrifluvents, and Thermic Torrifluvents. Measurement of four to eight plants per plot, half of the number on each end, appeared to be adequate to characterize the plant height of a plot. Plant heights were normally distributd. A systematic selection produced lower variation (about onehalf as many plants needed to be measured) but over‐estimated mean plant height by an average of 12.5%.

The crux of the Howitt et al. [ 1980] article is that the use of quadratic programing (QP) rather than linear programing (LP) in estimating the derived demand for irrigation water will yield more elastic estimates of demand. Their empirical example purports to demonstrate these results, which they first claimed to be conceptually correct on the basis of a mathematical proof. Their assertion was so lacking in intuitive appeal that we were led first to examine the empirical illustration for logical consistency. When the example did not prove credible, we then examined the basic mathematics. While the mathematics itself was correct, interpretation of the crucial equation was not. INTUITIVE CONCEPTSThe practical advantage of QP over LP in estimating the derived demand for water is that instead of assuming product prices constant as the price of water rises (the LP formulation), the QP formulation provides for rising product prices as less of the product is produced for sale. This QP formulation is in conformance with the generally accepted notion of However, intuition suggests that the QP formulation should normally develop a derived demand curve for water that is less elastic than a demand curve developed from a LP formulation, rather than more elastic as Howitt et at. claim. Because product prices are allowed to rise as less water is used in production and output is decreased, the products will be better able to pay for higher-cost water than is projected in the LP formulation, and the QP demand curve will rise more steeply than the LP curve from a common base (see our Figure 1). Beginning at any given point a, representing current reality, a rise in the price of irrigation water should have less of an effect on water use under the QP formulation. One interpretation would be to substitute Py = Py* (a constant) for their market clearing condition (6). However, if output is allowed to vary while the product price is held constant, their theoretical model provides indeterminate results. This difficulty arises from the fact that under a constant product price the homogeneous of degree 1 (constant returns to scale) production function they assume results in undefined maximum profits as well as undefined input and output levels.An alternative interpretation is that the product price is fixed in their market clearing equation. Then, output is also fixed, and

The economic analysis of experimental crop production data illustrates how some production strategies can be eliminated from consideration regardless of prices. Also, other production strategies can be isolated as applicable only under substantially different prices. Research on reducing water applied to furrow irrigated cotton (Gossypium hirsutum L.) through a shortened irrigation season and skip‐row planting is reported and evaluated using basic economic principles that can be applied by researchers in various disciplines. Upland cotton was grown at Marana, AZ, for 3 yr under eight alternative furrow irrigation strategies. Dropping the last of the seven scheduled post‐plant irrigations resulted in the highest yield and, therefore, required less land and water to produce the same amount of cotton than treatments with the same number (dropping the first irrigation) or more scheduled irrigations (all seven irrigations). Deletion of two irrigations required more land and water to produce the same amount of lint when compared with dropping the first irrigation on part of the acreage and deleting three Irrigations on the remainder. Yield was reduced significantly under 2 × 1 skip row (plant 2 and skip 1) and 2 × 2 skip row (plant 2 and skip 2) although using 2 × 1 skip rows could be an efficient use of land and water where water is relatively expensive and seed, fertilizer, and picking costs are increased. Irrigation treatments of economic interest were thereby narrowed to: (i) dropping the last irrigation, (ii) dropping the first two and the last irrigation, and (iii) the 2 × 1 skip row.

Crop simulation models are increasingly used to evaluate the effect of alternative agricultural practices on crop yields and the environment. Difficulty in obtaining concise field data often requires that simulation results reflect a single point assumed to represent average field condition. Results of this study indicate that the single point analysis can have significant effects upon estimates of nitrogen leaching and crop yield under various surface irrigation practices. Practices considered illustrate the effects of differences in infiltration along the row, between irrigations, and between irrigated sets. Attempts to add intermediate realism in describing the irrigation system may not improve yield and leaching estimates over the single point simulation when compared to simulation results where even more realism is introduced.

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