It is estimated that 20% of cotton producers in the Texas High Plains use the strip tillage practice of planting cotton (Gossypium hirsutum L.) in residues of chemically terminated winter wheat (Triticum aestivum L.). The effect of the residue on water use partitioning between soil (Es) and crop (Ec) evaporation has not been reported. This study tested the hypothesis that the cumulative evapotranspiration (Et), of crop ina strip tillage system and in conventional tillage are similar, and that the residue modifies the components of total Et by reducing Es and increasing Ec. Winter wheat was planted in the fall of 1990 and was chemically terminated in the spring of 1991 in an Olton soil (fine, mixed, thermic Aridic Paleustoll) at Lubbock, TX. The plot was divided into a wheat‐stubble and a conventional treatment. Residue on half of the plot was left as stubble and residue on the other half was shredded and incorporated. Both plots were planted with cotton on 20 May 1991. Measurements included soil water and temperature profiles, soil evaporation, weather, and crop phenology and lint yield. Input values obtained from these data were used in the energy and water balance model (ENWATBAL) to calculate the daily and seasonal water lost to Es and Ec for 100 d. Calculated daily Es varied <1 SD from measured values. Measured mean total Et were similar, i.e., 305 ± 20 mm for conventional and 304 ± 41 mm for cotton in the wheat stubble. Simulated Et was within 10% of the measured value. The ratio of total Ec to Et was 0.50 in the conventional and 0.69 in the wheat stubble cotton. The more efficient use of Et in the wheat stubble increased lint yield by 35%. The Ec water use efficiency was not modified by the wheat stubble and was 3.8 g lint kg−1 of transpired water. The modified version of ENWATBAL was adequate to calculate the daily and seasonal water use of a cotton crop in a terminated wheat stubble, and the results support our hypothesis. The Et water use efficiency was increased by the wheat residue without additional input of water.

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Because of recently enacted conservation compliance legislation, reduced and no‐till farming systems in which crop residues are left on the soil surface are becoming more important in areas where soils are highly erodible. On the southern high plains of Texas, many producers are planting cotton (Gossypium hirsutum L.) into wheat straw (Triticum aestivum L.). This study was conducted to determine the allelopathic potential of wheat residues on cotton germination, emergence, seedling growth, and lint yield. Laboratory bioassays revealed that cotton seedling development was inhibited by aqueous extracts of wheat straw. Cotton cultivars were screened for the ability to tolerate the inhibitive effects of wheat straw in laboratory bioassays and greenhouse pot studies. Tolerant ‘Paymaster 404’ and intolerant ‘Acala A246’ were identified and used in field experiments that were conducted in 1986 and 1987 to determine the influence of wheat stubble residues on their emergence and yield. Major reductions in emergence only occurred when above ground residues were present in the seedbed. Emergence was reduced by an average of 9% for Paymaster 404 and 21% for Acala A246 when wheat stubble residues were present in the seedbed. The allelopathic effect of wheat stubble indirectly influenced lint yield by affecting population densities. The negative effect of wheat stubble on cotton stand establishment can apparently be overcome by; limiting the amount of above ground residues that are incorporated into the seedbed during planting, increasing the seeding rates, and planting tolerant cultivars.

Measurement of leaf area index (LAI) is useful for understanding cotton (Gossypium hirsutum L.) growth, water use, and canopy light interception. Destructive measurement is time consuming and labor intensive. Our objective was to evaluate sampling procedures using the Li‐Cor (Lincoln, NE) LAI 2000 plant canopy analyzer (PCA) for nondestructive estimation of cotton LAI on the southern High Plains of Texas. We evaluated shading as a way to allow PCA measurements in direct sunlight and the influence of solar direction when using this procedure. We also evaluated a test of canopy homogeneity (information required for setting PCA field of view), determined the number of belowcanopy measurements required, examined the influence of leaf wilting on PCA LAI determinations, and tested an alternative method (masking the sensor's two outer rings) for calculating LAI from PCA measurements. The best agreement between PCA and destructively measured LAI values was obtained when PCA observations were made either during uniformly overcast conditions or around solar noon using the shading method. Heterogeneous canopies with large gaps between rows required both a restricted (45°) azimuthal field of view and averaging the LAI values for two transects, made with the field of view parallel and then perpendicular to the row direction. This method agreed well (r2 = 0.84) with destructively measured LAI in the range of 0.5 to 3.5 and did not deviate from a 1:1 relationship. The PCA underestimated LAI by ≥ 20% when measurements were made on canopies wilted due to water stress. Masking the PCA sensor's outer rings did not improve the relationship between estimated and measured LAI in the range of LAI sampled.

The hydraulic press (HP) is a highly portable instrument designed for rapid measurement of leaf water potential of plants. This study compared grain sorghum [Sorghum bicolor (L.) Moench] leaf water potentials measured with a pressure chamber (PC) and using two endpoints with a HP, under glasshouse conditions. The first endpoint is when water exudes from one vein and the second endpoint when water exudes from all veins on both sides of the leaf tissue. The results showed that the HP and PC measurements agreed well in the range of −0.5 to −3.5 MPa of leaf water potentials when the second endpoint was selected for the HP. The HP can be used to measure the leaf water potential of grain sorghum, provided it is calibrated using the second endpoint against a PC or another method.