Abstract Photoperiod‐sensitive (PS) sorghum [Sorghum bicolor (L.) Moench] has been developed as a bioenergy crop. However, little is known about PS sorghum production in semiarid environments. The objective of this study was to investigate water use, biomass yield, and water‐use efficiency (WUE) in recently developed PS sorghum genotypes. Field experiments were conducted in 2 years and two locations in the US Southern Great Plains. Six genotypes (TAM08001, TAM17500, TAM17600, TAM17650, TAM17800, and TAM18000) were grown in three water regimes (dryland, irrigation at 50% evapotranspiration (ET) demand, and irrigation at 100% ET demand). For both locations, soil water extraction (SWE) occurred at a 0–2.4 m profile in 2018 but at a 0–1.2 m profile in 2019. At Bushland, TX, biomass yield ranged from 4 to 31 Mg ha−1 and seasonal ET ranged from 251 to 743 mm. In contrast, variations of biomass yield (10–19 Mg ha−1) and seasonal ET (345–483 mm) were smaller at Colby, KS. The WUE in PS sorghum (3.19–4.09 kg m−3) did not differ among water regimes except for the dryland treatment (1.52 kg m−3) at Bushland (2018). The genotypic differences in SWE, biomass yield, ET, and WUE were more pronounced under dryland conditions. TAM08001, TAM17800, and TAM17600 had greater biomass yield and WUE under drought conditions. Overall, biomass yield levels of 10–17 Mg ha−1 can be achieved in dryland in western KS, but under irrigation at 50% ET demand in the TX High Plains. Further studies are needed to better understand shoot and root traits related to drought tolerance in PS sorghum.

Corn (Zea mays L.) is a major irrigated crop in the Texas High Plains (THP). Inadequate precipitation, declining groundwater resources, and inevitable seasonal drought in the area have led to a greater number of studies on crop hybrids, water regimes, and other yield-enhancing techniques. A four-year field study was conducted to investigate the effect of foliar fungicide timing on corn grain yield, grain moisture, and grain test weight under different irrigation regimes. A fungicide containing QoI + DMI was applied to four to six corn hybrids each year at various growth stages (V5, R1, and V5 + R1). For all years, corn plants were grown under three irrigation regimes to meet the seasonal evapotranspiration (ET) requirements of 50% (I50), 75% (I75), and 100% (I100). Hybrid response to the timing of fungicide application and irrigation regime was not consistent over the years. However, in 2 of the 4 years, timing of fungicide application resulted in yield differences among the hybrids, especially at I75 and L100. For those years, the fungicide applied at R1 resulted in a higher grain yield, grain moisture, and grain test weight followed by the fungicide applied at V5 + R1. Results showed that hybrids reacted differently to the foliar fungicide based on the availability of water under disease-free conditions. This study further revealed that irrigation water could be reduced from I100 to I75 with a limited (5.6% in 2011, 11.3% in 2012, 1.3% in 2013, and 0.3% in 2014) yield penalty in the THP region.

Deficit irrigation (DI) is an effective way to save irrigation water while maintaining sustainable yield in irrigated crops. However, limited information is available related to canopy structure and solar radiation use under DI condition. In this study, our objective was to assess maize hybrids for leaf development, photosynthetically active radiation (PAR) interception and water use under DI condition. Field experiments were conducted in 2016 and 2017 in four maize hybrids at well-watered (I-100, referring to 100% evapotranspiration [ET] requirement) and DI (I-75, referring to 75% ET requirement) water regimes. Compared to I-100, I-75 did not reduce maize biomass and grain yield. Although DI reduced the leaf appearance rates (1.5% in 2016 and 7.6% in 2017) and resulted in greater variations in leaf area index (LAI) among hybrids, the amount of PAR interception was not affected during the growing season. DI significantly reduced the seasonal ET in both years (19.8% in 2016 and 26.6% in 2017). All the hybrids extracted more soil water (29 mm in 2016 and 27 mm in 2017) at I-75 than at I-100. Maize plants at I-75 had greater water use efficiency (WUE) (1.68 kg m(-3)) than those at I-100 (1.41 kg m(-3)). However, DI did not affect radiation use efficiency (RUE). In conclusion, DI at I-75 maintained grain yield through improved soil water extraction and WUE but stable canopy radiation interception and RUE.

In a semi-arid climate, heat and drought stress can significantly reduce photosynthesis during grain filling and hence, the remobilization of stored carbon reserves from stems becomes important to wheat (Triticum aestivum L.) yield. A 2-year field study was conducted to better understand the contributions of different internodes to grain yield. We measured remobilization of stored carbon reserves in eight wheat genotypes under irrigated and dryland conditions. For each genotype, plant samples consisting of ten culms were collected between anthesis and physiological maturity. There was no difference in single stem weight loss between two water levels, but the stem loss contribution to yield was higher in dryland wheat (54.4%) than that in irrigated wheat (38.5%). Among the genotypes, the stem loss contribution to yield ranged 38.9%-58.1% in the relatively wet season of 2017 and 36.5%-62.5% in the dry season of 2018. The amount of stem loss showed a strong relationship with grain yield, especially under the dryland condition. At each sampling date, length as well as the dry weight of peduncle (first internode from the top) was greater than the other internodes; however, more stem loss (in weight and percentage weight) occurred in the third internode, followed by second and fourth internodes. Results suggest that the stem loss contribution to yield is greater under dryland than irrigated condition and occurs mainly in the middle portion of the culm.

Grain sorghum (Sorghum bicolor L.) is a major dryland crop in the Texas High Plains. Currently, drought and infestation by the sugarcane aphid (SCA, Melanaphis sacchari) are the two major challenges to grain sorghum production in the area. A 2-year field study was conducted to investigate the effect of planting date (PD) and hybrid selection on yield, evapotranspiration (ET), water use efficiency (WUE), and SCA infestation. Five sorghum hybrids (86P20, SP-31A15, AG1201, AG1203, and DKS37-07) were grown on two planting dates (PD1—early May; PD2—late June) under dryland conditions. Insecticides were not used. There were significant differences in grain yield, WUE, evapotranspiration (ET), and SCA population between two PDs and among hybrids. For PD1, SCA infestation occurred after sorghum reached physiological maturity in 2017. Although SCA infestation was observed during late grain filling in 2018, SCA populations were low and did not affect yield. For PD2, SCA was present before anthesis in both years and significantly affected grain yield. Even with heavy SCA infestation in PD2, the grain yield was higher in PD2 than in PD1 due to timely precipitation. Among hybrids, AG1203, 86P20 and DK37-07 performed better with higher yield and less SCA infestation in PD2. Grain yield was more related to seeds per plant than to kernel weight and harvest index.

Using imbalanced historical yield data to predict performance and select new lines is an arduous breeding task. Genome-wide association studies (GWAS) and high throughput genotyping based on sequencing techniques can increase prediction accuracy. An association mapping panel of 227 Texas elite (TXE) wheat breeding lines was used for GWAS and a training population to develop prediction models for grain yield selection. An imbalanced set of yield data collected from 102 environments (year-by-location) over 10 years, through testing yield in 40–66 lines each year at 6–14 locations with 38–41 lines repeated in the test in any two consecutive years, was used. Based on correlations among data from different environments within two adjacent years and heritability estimated in each environment, yield data from 87 environments were selected and assigned to two correlation-based groups. The yield best linear unbiased estimation (BLUE) from each group, along with reaction to greenbug and Hessian fly in each line, was used for GWAS to reveal genomic regions associated with yield and insect resistance. A total of 74 genomic regions were associated with grain yield and two of them were commonly detected in both correlation-based groups. Greenbug resistance in TXE lines was mainly controlled by Gb3 on chromosome 7DL in addition to two novel regions on 3DL and 6DS, and Hessian fly resistance was conferred by the region on 1AS. Genomic prediction models developed in two correlation-based groups were validated using a set of 105 new advanced breeding lines and the model from correlation-based group G2 was more reliable for prediction. This research not only identified genomic regions associated with yield and insect resistance but also established the method of using historical imbalanced breeding data to develop a genomic prediction model for crop improvement.

Quantitative trait loci (QTL) analysis genetically dissects complex traits, discerns their genetic control and genotype-by-environment interactions, and ultimately helps marker development for assisted breeding selection. A mapping population of 124 F-5:7 recombinant inbred lines (RILs) derived from the cross of 'TAM 112' x 'TAM 111' was grown under seven diverse environments and evaluated for end-use quality traits including kernel, flour, and dough-mixing characteristics. The objective of this study was to detect QTL associated with end-use quality traits in these two cultivars. Through 5,948 single nucleotide polymorphisms (SNPs), 39 QTL regions were consistently identified in two or more environments or QTL analyses. Thirteen QTL regions associated to two or more traits were also identified. Among them, 11 QTL regions were in common on chromosomes 1A, 1B, 1D, 2D, and 4D associated to kernel hardness (HARD), kernel diameter (DIAM), single kernel weight (SKW), flour yield (FYLD), midline peak time (MLPT) with logarithm of odds (LOD) up to 42.0, and explained up to 49.3% of the phenotypic variation. Notably, Glu-D1 loci at 414.3 Mb on chromosome 1D strongly influenced dough rheology and explained up to 54.6% of the trait variation for MLPT, with the favorable allele derived from TAM 112. Novel QTL for HARD, FYLD, and ASH were also identified on the chromosome 1A and 2A, 6B, and 7D, respectively. This study confirmed previously identified loci at major genes, some newly identified QTL on chromosomes, and the favorable alleles carried forward for improved end-use quality.

Quantitative trait loci (QTL) analysis could help to identify suitable molecular markers for marker-assisted breeding (MAB). A mapping population of 124 F5:7recombinant inbred lines derived from the cross `TAM 112'/`TAM 111' was grown under 28 diverse environments and evaluated for grain yield, test weight, heading date, and plant height. The objective of this study was to detect QTL conferring grain yield and agronomic traits from multiple mega-environments. Through a linkage map with 5,948 single nucleotide polymorphisms (SNPs), 51 QTL were consistently identified in two or more environments or analyses. Ten QTL linked to two or more traits were also identified on chromosomes 1A, 1D, 4B, 4D, 6A, 7B, and 7D. Those QTL explained up to 13.3% of additive phenotypic variations with the additive logarithm of odds (LOD(A)) scores up to 11.2. The additive effect increased yield up to 8.16 and 6.57 g m(-2) and increased test weight by 2.14 and 3.47 kg m(-3) with favorable alleles from TAM 111 and TAM 112, respectively. Seven major QTL for yield and six for TW with one in common were of our interest on MAB as they explained 5% or more phenotypic variations through additive effects. This study confirmed previously identified loci and identified new QTL and the favorable alleles for improving grain yield and agronomic traits.

Abstract Thermal imaging has been used to determine canopy temperature and study plant water relationships. The objective of this study was to investigate the potential use of infrared thermal imaging to determine crop canopy temperature (Tc) and evaluate wheat (Triticum aestivum L.) genotypes under drought conditions. Thermal images were acquired at anthesis and grain‐filling stages from 17 genotypes grown under dryland conditions in 2015 and 2016 winter wheat growing season at Bushland, TX. A handheld thermal camera was used to acquire thermal images and the images were processed using customized image processing software. The customized software filters out the background soil from the thermal images and calculates the mean Tc. A significant difference (p < .05) in Tc among genotypes was found during grain filling in 2015 and at anthesis in 2016. Genotypes TAM 111, TAM 114, PlainsGold Byrd, and Jagalene had cooler canopies, and Billings, TAM 304, and TAM 105 had warmer canopies in both years. There was a significant negative correlation between grain yield and Tc measured at anthesis (r =–.48, p < .05) and grain‐filling (r = –.33, p < .05). Infrared thermal imaging showed a promising method to obtain Tc, which can be used to evaluate genotypes for drought tolerance.

To meet the demands of different wheat-based food products, traits related to end-use quality become indispensable components in wheat improvement. Thus, markers associated with these traits are valuable for the timely evaluation of protein content, kernel physical characteristics, and rheological properties. Hereunder, we report the mapping results of quantitative trait loci (QTLs) linked to end-use quality traits. We used a dense genetic map with 5199 SNPs from a 90K array based on a recombinant inbred line (RIL) population derived from ‘CO960293-2’/‘TAM 111’. The population was evaluated for flour protein concentration, kernel characteristics, dough rheological properties, and grain mineral concentrations. An inclusive composite interval mapping model for individual and across-environment QTL analyses revealed 22 consistent QTLs identified in two or more environments. Chromosomes 1A, 1B, and 1D had clustered QTLs associated with rheological parameters. Glu-D1 loci from CO960293-2 and either low-molecular-weight glutenin subunits or gliadin loci on 1A, 1B, and 1D influenced dough mixing properties substantially, with up to 34.2% of the total phenotypic variation explained (PVE). A total of five QTLs associated with grain Cd, Co, and Mo concentrations were identified on 3B, 5A, and 7B, explaining up to 11.6% of PVE. The results provide important genetic resources towards understanding the genetic bases of end-use quality traits. Information about the novel and consistent QTLs provided solid foundations for further characterization and marker designing to assist selections for end-use quality improvements.

MAIN CONCLUSION:The expression of stay-green (SG) characteristic in sorghum under water stress was related to N supply. SG genotype performed better than a non-stay-green (NSG) genotype at medium and high N levels. The differences in physiological parameters between SG and NSG genotypes were not significant at low N level and severe water stress. Grain sorghum [Sorghum bicolor (L.) Moench] with stay-green (SG) trait has the potential to produce more biomass and use soil water and nitrogen (N) more efficiently under post-flowering water stress. Previous studies were mostly conducted without N deficiency and more information is needed for interactions among soil N availability, SG genotype, and post-flowering water stress. In this study, the differences in leaf growth and senescence, shoot and root biomass, evapotranspiration (ET), water use efficiency (WUE), leaf photosynthetic responses, and nitrogen use efficiency (NUE) between a SG genotype (BTx642) and a non-stay-green (NSG) genotype (Tx7000) were examined. The two genotypes were grown at three N levels (Low, LN; Medium, MN; High, HN) and under three post-flowering water regimes (No water deficit, ND; Moderate water deficit, MD; Severe water deficit, SD). The genotypic difference was generally significant while it frequently interacted with N levels and water regimes. At medium and high N levels, SG genotype consistently had greater green leaf area, slower senescence rate, more shoot biomass and root biomass, and greater WUE and NUE than the NSG genotype under post-flowering drought. However, differences in several variables (e.g., leaf senescence, ET, WUE and NUE) between genotypes were not significant under SD at LN. At HN and MN, photosynthetic function of SG genotype was better maintained under drought. At LN, SG genotype maintained greater green leaf area but had lower photosynthetic activity than the NSG genotype. Nonetheless, adequate N supply is important for SG genotype under drought and greater root biomass may contribute to greater NUE in SG genotype.

Two drought-tolerant wheat cultivars, 'TAM 111' and 'TAM 112', have been widely grown in the Southern Great Plains of the U.S. and used as parents in many wheat breeding programs worldwide. This study aimed to reveal genetic control of yield and yield components in the two cultivars under both dryland and irrigated conditions. A mapping population containing 124 F5:7 recombinant inbred lines (RILs) was developed from the cross of TAM 112/TAM 111. A set of 5,948 SNPs from the wheat 90K iSelect array and double digest restriction-site associated DNA sequencing was used to construct high-density genetic maps. Data for yield and yield components were obtained from 11 environments. QTL analyses were performed based on 11 individual environments, across all environments, within and across mega-environments. Thirty-six unique consistent QTL regions were distributed on 13 chromosomes including 1A, 1B, 1D, 2A, 2D, 3D, 4B, 4D, 6A, 6B, 6D, 7B, and 7D. Ten unique QTL with pleiotropic effects were identified on four chromosomes and eight were in common with the consistent QTL. These QTL increased dry biomass grain yield by 16.3 g m-2, plot yield by 28.1 g m-2, kernels spike-1 by 0.7, spikes m-2 by 14.8, thousand kernel weight by 0.9 g with favorable alleles from either parent. TAM 112 alleles mainly increased spikes m-2 and thousand kernel weight while TMA 111 alleles increased kernels spike-1, harvest index and grain yield. The saturated genetic map and markers linked to significant QTL from this study will be very useful in developing high throughput genotyping markers for tracking the desirable haplotypes of these important yield-related traits in popular parental cultivars.

Sustainable corn ( Zea mays L.) production in the semiarid Texas High Plains (THP) relies on irrigation water from the Ogallala Aquifer, but it is well known that the aquifer is declining. A 3-yr (2010–2012) field study was conducted to find whether successful corn production is possible using only 60% (300 mm) of the current average irrigation for the region (500 mm). Three corn hybrids (Pioneer P31G96, Pioneer P33D49, and Monsanto DKC67-87) were planted at four densities (5, 6, 7, and 8 plants m −2 ) under a center pivot sprinkler system. Seasonal precipitation was highest in 2010 (224 mm), followed by 2012 (132 mm), and 2011 (85 mm). Seasonal evapotranspiration for corn ranged between 426 and 635 mm. Yield was significantly lower in 2011 (2.4 Mg ha −1 ) and 2012 (6.0 Mg ha −1 ) than 2010 (13.3 Mg ha -1 ) due to severe drought conditions combined with unusually high temperatures in those years. Increase in planting density increased biomass at silking but did not increase yield. Grain yield was related to biomass at maturity, harvest index, and biomass increase during grain filling. Results suggest that corn yields of about 13 Mg ha −1 are possible with only 300 mm of irrigation in the THP, but production risk is high due to frequent years with low seasonal precipitation.

Corn is the major irrigated crop in the Texas High Plains (THP) and uses 53% of the total agricultural regional water resource budget. Currently, the declining water level of the Ogallala Aquifer, coupled with irrigation pumping restrictions by regional groundwater conservation districts, is challenging sustainable, high level corn production. The objective of this article is to review production levels and evaluate corn management practices in the THP with reduced or limited levels of irrigation. Long-term field studies demonstrate that yield and water use efficiency (WUE) have increased significantly over the last forty years while seasonal corn evapotranspiration (ET) under full irrigation conditions has not increased. Among management practices, irrigation remains the single-most important factor in corn production. With recent advances in corn breeding and genetics, irrigation requirements can be reduced by up to 25% in some years and result in similar yields as compared to irrigation amounts at the 100% ET level. Also, WUE is generally maximized at irrigation levels meeting 75–80% ET demand. Newly developed drought tolerant corn hybrids have been shown to provide yield benefits of 10–15% under limited (reduced) irrigation water levels. At the higher irrigation levels (75–100% ET requirement), corn yield increased as seeding rate increased initially but did not increase further when the seeding rate exceeded 94,000 seeds ha−1. Also, a multi-year planting date study indicated that high corn yields can still be achieved with a long-season hybrid when planted in the middle of May and early June. When the planting date is delayed to late June and early July, mid- and short- season hybrids showed a yield advantage over the long season hybrids.

Planting date (PD) can significantly affect corn (Zea mays L.) yield. Our objective was to investigate the effects of hybrid and PD later than traditional time on irrigated corn performance in the Texas High Plains. A 2-yr field study was conducted using four (2013) to six (2014) hybrids and four PDs (mid-May [PD1], late May-early June [PD2], mid-June [PD3], and late June-early July [PD4]). In PD1 and PD2, long-season hybrids had greater yields (12-15 Mg ha(-1)) than short-season hybrids (9-13 Mg ha(-1)). In PD4, short-season hybrids (8-11 Mg ha(-1)) had yield advantage over long-season hybrids (4-10 Mg ha(-1)). Low yield at late planting was related to reduced biomass, harvest index, kernel weight, and kernel number. Seasonal evapotranspiration (ET) was only reported in 2013. The average ET over PDs was 767 mm in a long-season hybrid and 718 mm in a short-season hybrid. Late planting resulted in lower ET (600 mm in PD4 vs. 822 mm in PD1). Hybrid and PD did not affect water use efficiency (WUE) except for 33D53AM, which had low WUE in PD4 due to low yield with late planting of this long-season hybrid. This study demonstrated that high yield (similar to 15 Mg ha(-1)) and WUE (similar to 1.5 kg m(-3)) can be achieved with long-season hybrids when planted before mid-June, but short-season hybrids were a better choice after mid-June. With water limitations, further study is warranted on planting short-season hybrids after mid-June to reduce water use and irrigation.

Hybrid adoption, irrigation, and planting density are important factors for maize (Zea mays L.) production in semiarid regions. For this study, a 2-yr field experiment was conducted in the Texas High Plains to investigate maize yield determination, seasonal evapotranspiration (ETc), and water-use efficiency (WUE) under limited irrigation. Two hybrids (N74R, a conventional hybrid, and N75H, a drought-tolerant (DT) hybrid) were planted at three water regimes (I-100, I-75, and I-50, referring to 100%, 75%, and 50% of the evapotranspiration requirement) and three planting densities (PD 6, PD 8, and PD 10, referring to 6, 8, and 10 seeds m(-2)). At I-50, drought stress reduced grain yield by 4.78t/ha for the conventional hybrid but only 4.22t/ha for the DT hybrid, when compared to I-100. Although ETc decreased at I-75 and I-50, the highest WUE was found at I-75. The DT hybrid did not yield more than the conventional hybrid but had greater yield stability at lower water regimes and extracted less soil water. Drought decreased biomass, harvest index, and kernel weight but did not affect kernel number. Higher planting densities increased biomass and kernel number but decreased kernel weight. Kernel number and kernel weight of the conventional hybrid were more sensitive to planting density than the DT hybrid. These data demonstrated that limited irrigation at I-75 is an effective way to save water and maintain the maize yield in semiarid areas, and that DT hybrid shows a greater yield stability to plant density under water stress.

Adoption of drought-tolerant (DT) hybrids is a viable strategy for maize production in drought-prone environments. We conducted four-year field studies (2011–2014) to investigate yield, crop evapotranspiration (ETc), and water-use efficiency (WUE) in one conventional (N58L) and one DT hybrid (N59B-DT) under three water regimes (I100, I75, and I50, where the subscripts were the percentage of irrigation applied relative to meeting full ETc) and three plant densities. At I100 and I75, N59B-DT did not show advantage in yield and WUE relative to N58L, however, at I50 it showed an advantage of 8.5% and 10.5%, respectively. At I100 and I75, high plant density treatment had greater grain yield (9.1%) and WUE (9.4%) than low plant density. Comparing hybrids, N59B-DT had greater yield (5.9%) and WUE (7.3%) than N58L at high plant density. N59B-DT had large advantage over N58L in yield (18.0%) and WUE (26.2%) when the hybrids were grown under severe water deficit (I50) and high plant density (9.9 plants m−2). At I50, increasing plant density reduced yield (14.1%) for N58L but did not affect yield for N59B-DT. On average, plant density had no effect on seasonal ETc but N59B-DT had more seasonal ETc than N58L at I100 and I75. The results of this study indicate that DT hybrid was tolerant to high panting density. Planting a DT hybrid with a higher plant density may provide greater yield stability under water-limited conditions while also maintaining maximum yield potential when moisture is sufficient.

The normalized difference vegetation index (NDVI) has been widely used to quantify vegetation by measuring the difference between near-infrared (NIR) and red light. Measuring NDVI throughout a growing season helps to evaluate the effect of continuous phenological and morphological changes on grain yield. A 2-year field study was conducted to characterize plant response to water stress in 20 winter wheat (Triticum aestium L.) genotypes during the season based on their NDVI values under the dryland and irrigated conditions. In addition, final biomass and yield were measured at maturity. The 2018 season was extremely dry with only 23 mm of precipitation, whereas 2016 was more favorable for wheat production with 315 mm seasonal precipitation. Except in a severe drought condition (2018, dryland), NDVI values increased from early spring to mid-season (anthesis) and decreased from mid-season to physiological maturity, indicating gradual leaf senescence. There was a significant (P = 0.05) positive correlation between NDVI and grain yield, especially for NDVI values after jointing. However, under the severe drought condition of 2018 (dryland), NDVI often did not show a strong relationship with grain yield. Even genotypes with higher NDVI at early growth stages ended up with lower yield because of the severe water stress at later growth stages. Hence, the use of NDVI is not suggested in screening genotypes for yield under extreme weather conditions.

Monitoring wheat (Triticum aestivum L.) performance throughout the growing season provides information on productivity and yield potential. Remote sensing tools have provided easy and quick measurements without destructive sampling. The objective of this study was to evaluate genetic variability in growth and performance of 20 wheat genotypes under two water regimes (rainfed and irrigated), using spectral vegetation indices (SVI) estimated from aerial imagery and percentage ground cover (%GC) estimated from digital photos. Field experiments were conducted at Bushland, Texas in two growing seasons (2014-2015 and 2015-2016). Digital photographs were taken using a digital camera in each plot, while a manned aircraft collected images of the entire field using a 12-band multiple camera array Tetracam system at three growth stages (tillering, jointing and heading). Results showed that a significant variation exists in SVI, %GC, aboveground biomass and yield among the wheat genotypes mostly at tillering and jointing. Significant relationships for %GC from digital photo at jointing was recorded with Normalized Difference Vegetation Index (NDVI) at tillering (coefficient of determination, R-2 = 0.84, p < 0.0001) and with %GC estimated from Perpendicular Vegetation Index (PVI) at tillering (R-2 = 0.83, p < 0.0001). Among the indices, Ratio Vegetation Index (RVI), Green-Red VI, Green Leaf Index (GLI), Generalized DVI (squared), DVI, Enhanced VI, Enhanced NDVI, and NDVI explained 37-99% of the variability in aboveground biomass and yield. Results indicate that these indices could be used as an indirect selection tool for screening a large number of early-generation and advanced wheat lines.

Wheat (Triticum aestivum) is grown in a wide range of water regimes and produces both grain and forage in the U.S. Southern Great Plains (SGP). Despite being relatively adapted to water stress, decreased water supply has become the main abiotic factor limiting wheat yield in the region. A 5-yr field study was conducted to investigate the yield determination in winter wheat under a center pivot irrigation system. Twenty elite wheat cultivars were grown at four water regimes, I-100, I-75, I-65, and I-5(0) to meet 100%, 75%, 65%, and 50% evapotranspiration (ET) requirement, respectively. The 2011 and 2013 seasons had all irrigation levels, while there were only two irrigation levels in 2012 (I-50 and I-65) and 2014 (I-75 and I-100), and three irrigation levels in 2015 (I-50, I-75, and I-10(0)). In most seasons, ET, biomass, and grain yield decreased consistently from higher to lower water regime, but water use efficiency (WUE) and harvest index (HI) were the highest at I-75. Averaged across all years and genotypes, grain yield was reduced by only 5% from I-100 to I-75 indicating that reducing irrigation to meet 75% of ET requirement can still achieve similar yield to I-1(00) and maximize HI and WUE. Newer cultivars including Duster, Endurance, Winterhawk, TAM 112, and TAM 113 had greater yield at I-50 compared to other cultivars. Grain yield showed a strong relationship with biomass, HI, and ET. At lower water regimes (I-50, I-65), all three yield components (spikes per square meter, seeds per spike, and kernel weight) were correlated to grain yield. At higher water regimes (I-75 and I-100) grain yield was consistently related to kernel weight. This study demonstrated that high yield can be achieved at an irrigation level of I-75. In addition, planting newer drought tolerant cultivars may provide yield benefits under water-limited conditions in the SGP.