Physiological and Morphological Bases for Breeding Dual-purpose Wheat Cultivars with Improved Forage Production

Rationale

    Wheat historically has been bred for increased grain yield and for tolerance to abiotic (drought, soil mineral imbalance) and biotic (insects, pathogens) stresses. Although grain yield potential of modern cultivars is higher than older cultivars, breeding progress for forage production, forage quality, and grazing has been very limited. Texas A&M University has released only one variety (Lockett) bred exclusively for grazing and one dual-purpose (TAM-202) wheat variety. The lack of adequate selection criteria has hampered breeding efforts to develop improved forage-type and dual-purpose wheat varieties.

    Because of a lack of clearly defined selection criteria for breeding forage-type wheat, breeders usually rely on forage quantity and quality during the fall-spring growing season as selection tools. Such an approach may not be the most appropriate to develop disease and insect resistant, productive cultivars with a maximal potential to withstand various grazing pressures and climate fluctuations.

    There are several criteria determining the potential of forage production in wheat. Leaf appearance rate (phyllochron) affects the number of leaves on a tiller and the tillering rate. The phyllochron of wheat is strongly related to air temperature, soil water availability, and nitrogen supply. Tillering rate and final tiller number depends on cultivar characteristics, especially hormonal regulation (auxins), and seeding rate. Leaf morphology determines leaf area, which is often correlated with plant biomass. The rate of leaf area development is closely associated with embryo size, so selection for large embryo size should improve early growth rates. Selection for leaf width, however, may not be beneficial under drought; thus, traits like glaucousness or rapid leaf rolling in response to water deficit should be considered. Residual leaf area following defoliation has been considered more important for regrowth of grasses than the pool of nonstructural carbohydrates; therefore, our candidate lines should have at least the first leaves on tillers placed horizontally (to avoid being grazed). Following defoliation, residual leaves should express a high degree of compensatory photosynthesis in order to maximize regrowth rate after grazing. 

    A forage-type, dual-use wheat has to be resistant to pests and foliar diseases. Such resistance is often related to production of secondary metabolites such as phenolic compounds. Recent results suggest phenolic compounds may be one group of metabolites in wheat forage controlling frothy bloat, a serious digestive disorder of cattle grazing wheat. In an independent series of studies (Malinowski, Min, Pinchak 2003-2005), we evidenced a relationship between rapid changes in solar radiation and temperature (e.g., during passing cold fronts) and phenolic concentration in wheat forage. Frothy bloat incidences usually amplify during conditions of rapid weather changes in the late winter-early spring season. Previous research evidenced the importance of foam stability in the rumen for the potential of frothy bloat (Gregory and Auricht, 2003). We showed that wheat entries with low phenolic concentrations exhibited an increase in foam strength measured in vitro.

Objectives

    The objective of this study was to determine morphological and physiological traits for selection of dual-use wheat with improved forage productivity. In this presentation we discuss correlations between forage production in the early grazing season (November-December) and wheat plant morphological parameters, and phenolic concentrations in wheat cultivars and breeding lines. 

Materials and Methods

    During the 2003-2005 winter growing seasons we evaluated forage and grain yield, grazing tolerance, morphological and physiological traits, and resistance to pests and diseases of a range of breeding lines and cultivars selected from the Texas Elite (TXE) and Uniform Variety Trial (UVT) wheat collections. Each wheat entry was strip-planted on 0.04-ac plots (18 x 100 ft) in blocks repeated 3 times. Seeding rate was 23 seeds/sq ft, which corresponds to 75 lbs/ac of Lockett wheat (check variety). The experimental site was a part of a 35-ac wheat pasture grazed from December through February each year at 0.75 head/ac stocking rate. Forage yield was measured at 28-d intervals from grazed and enclosed, non-grazed areas by harvesting 5.4 sq ft area of each plot. Tillers were counted from 1-ft row. Wheat samples for phenolic compounds assessments were collected in January, February, and March 2005 during periods of sudden weather changes.     

    The experimental design was a completely randomized block replicated three times. All data were analyzed using the Mixed Procedure of the Statistical Analysis System (SAS, 1994). Replications were considered random and wheat entries were considered a fixed factor. Mean separation was performed using the protected least square means (LSMEANS) procedure. Significance was declared at P < 0.05.     

Results

    The 2003 winter growing season was extremely dry until January 2004. Precipitation during October-December 2003 was only 1.10 inches (long-term average is 4.80 inches). Early forage yield was positively correlated (R²=0.66) with tiller number in the dryland wheat, but not significantly correlated with tiller number in the irrigated UVT (R²=0.22) and TXE (R²=0.16) wheat collections (Fig. 1 A-C). Precipitation during October-December 2004 (11.63 inches) was above normal (4.80 inches). Under such wet conditions, early forage yield was not correlated with tiller number in the dryland study (R²=0.07) and weakly correlated with tiller number in the non-irrigated UVT (R²=0.41) and TXE wheat collections (R²=0.52) (Fig. 1 D-F). 

Figure 1. Correlations between tiller number and early (December) forage yield in dryland wheat, irrigated UVT and TXE collections in 2003 (A-C) and dryland wheat and non-irrigated UVT and TXE wheat collections in 2004 (D-F).

    Leaf length was not correlated with early forage yield (R²=0.08 to R²=0.25), except for dryland wheat in 2003 (R²=0.88; data not presented). Leaf width was also not correlated with early forage yield (R²=0.004 to R²=0.16; data not presented).

    Specific leaf weight (SLW) was negatively correlated with early forage production in dryland wheat in 2003, but there were weak correlations between these traits in irrigated UVT and TXE wheat collections (Fig. 2 A-C). In 2004, early yield was negatively correlated with SLW in dryland wheat and non-irrigated UVT and TXE collections (Fig. 2 D-F). Producing leaves with lower SLW enables the construction of more leaf area per unit of leaf mass, which is a typical strategy of fast growing grass species.

Figure 2. Correlations between specific leaf weight and early (December) forage yield in dryland wheat and irrigated UVT and TXE wheat collections in 2003 (A-C) and in dryland wheat and non-irrigated UVT and TXE wheat collections in 2004 (D-F).

    Phenolic compounds varied among wheat cultivars and breeding lines during the 2004-2005 growing season. Cultivars TAM 100, TAM 111, TAM W-101, Deliver, and a breeding line TX98V9628 had the highest concentrations of phenolics, while cultivars TAM 400, OK 102, Jagger, and breeding lines TX01V5314, TX00V1117 and TX01U2598 had the lowest phenolic concentrations (Fig. 3).

Figure 3. Phenolic concentrations in wheat cultivars and breeding lines during 2004-2005 growing season, averaged for 3 sampling periods.

Conclusions

    Morphological traits such as tiller number or specific leaf weight are easy to measure and they are correlated with early wheat forage production. It is important to conduct the wheat selection process for increased forage productivity under conditions in which the cultivars will later grow. Leaf parameters such as length or width are not useful in selecting lines for high forage productivity.

    Concentrations of phenolic compounds (which may play a role in frothy bloat prevention) vary among wheat cultivars and breeding lines, suggesting a potential for selection of wheat with high and stable phenolic content. 

Acknowledgements:  The research project has been partially supported by grants from the Texas Wheat Producers Board during 2003-2005. 

This research program is being conducted with cooperation of Dr. Jackie Rudd (TAES Amarillo, TX), Dr. William E. Pinchak, and Dr. Byeng-Ryel Min (TAES Vernon, TX).

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