Determining Morphological Traits for Selecting Wheat (Triticum Aestivum L.) with Improved Early- Season Forage Production

Winter wheat (Triticum aestivum L.) is the major annual crop in the Southern Great Plains of the USA grown as dual-purpose (forage and grain) crop. Wheat breeding has focused on maximizing grain yield and tolerance to abiotic and biotic stresses. Because of a lack of clearly defined selection criteria for breeding forage-type wheat, breeders usually rely on very laborious means to measure forage quantity and quality or they use imprecise visual estimates to quantify forage production. In a series of experiments conducted at Vernon, TX during 20032005, we determined correlations between selected morphological traits and the early-season forage DM yield in a range of wheat breeding lines and commercial cultivars evaluated by the Wheat Breeding Program of Texas A&M AgriLife Research. Early-season forage DM yield was highly correlated with tiller number, leaf length and width, and inversely correlated with specific leaf weight. Environmental variables modified the responses. A number of wheat breeding lines and cultivars had combined three out of the four evaluated morphological traits, including Abilene Ag Exp., Cutter, Fannin, HG-9, Duster, TAM 110, TX01M5009, TX01V6016, TX03M1179, TX04M410009, and Weather master 135. These cultivars/breeding lines have been recommended for dualpurpose use; thus, the morphological traits evaluated in our studies were desirable for selection of wheat with improved forage productivity.


Introduction
Winter wheat (Triticum aestivum L.) is the predominant commodity in the Southern Great Plains of USA, sown on 10 M ha each year. It is often used as dual-purpose (forage and grain) crop for grazing stocker cattle (Bos spp.) and grain production (Pinchak et al., 1996;Hossain et al., 2004). Wheat forage has a high nutritional value capable of producing weight gains greater than 1.4 kg d -1 (Mack own et al., 2008). Grazing cattle on wheat forage is practiced from early winter (late November) until development of the first hollow stem, which usually occurs in early March in dual-purpose systems or through May in graze-out systems.
Historically, wheat breeding research has focused mainly on maximizing grain yield and tolerance to abiotic (drought, heat) and biotic (insects, pathogens) stresses (Lantican et al., 2005). Development of dual-purpose or forage-type wheat cultivars was not addressed until late 1980's (Rajaram and Hettel, 1995). The early research indicated that selection requirements for improved forage production might be highly specific to the target environment and management and should involve traits like early-season forage production, grain recovery potential, and reduced awns (Pfeiffer, 1992). Although grain yield potential of modern cultivars is higher than older cultivars, breeding progress for forage production, forage quality, and tolerance to grazing has been very limited (Kim et al., 2016). Texas A&M University System has released only three awn less cultivars bred primarily for grazing: Lockett, TAM 401, and TAM 204 (Rudd et al., 2012). In addition, two dual-purpose wheat cultivars have been released: TAM 202 (Worrall et al., 1995) and TAM 112 (Rudd et al., 2014). 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 (Krenzer et al., 1992). Such an approach may not be the most appropriate because high grain yield and high forage yield traits are not always correlated (Atkins et al., 1969;Ud-Din et al., 1993).
The objective of this study was to determine morphological traits as tools for selection of wheat cultivars with improved early-season forage productivity based on commercial cultivars and breeding lines evaluated by the Wheat Breeding Program of Texas A&M AgriLife Research (TAM Wheat Breeding Program) for high grain production and tolerance to insects, diseases, and drought.  (Raushel, 2011). Experiment I consisted of 40 wheat breeding lines (TXE collection) and 40 wheat cultivars (UVT collection) and was a part of the TAM Wheat Breeding Program state-wide evaluation test. Experiment II consisted of 25-28, depending on year, wheat entries selected from the TXE and UVT collections based on contrasting characteristics, i.e., tiller number, early-season forage dry matter (DM) production, leaf morphology, and grain yield. These forage characteristics were determined on the TXE and UVT collections during a preliminary study conducted at Chillicothe, TX (34 o 11´N, 99 o 31´W, elevation 442 m) in 2002 (data not presented). Pre-planting fertilization of 34 kg ha -1 N, 56 kg ha -1 P2O5, and 29 kg ha -1 K2O was applied each season. Wheat entries were planted in a tilled seedbed with a precision planter (Wintersteiger, Salt Lake City, Utah) at a seeding rate of 250 seeds m -2 . Plot size was 1.5 by 4.5 m. Experiments at this location were not irrigated. In October 2003 only, the experiment with TXE and UVT collections was planted at Lockett on Miles fine sandy loam [fine-loamy, mixed, Thermic Udic Paleustalfs (Raushel, 2011)]. The reasons for choosing a different location with irrigation capability were severe drought conditions during the whole 2003 and predicted drought extending into 2004. Due to severe precipitation deficit, the experiment was irrigated with a sprinkle-type irrigation system at the equivalent of 2 inch precipitation once a month during October-December.

Forage DM Yield and Plant Morphological Measurements
To determine early-season (early December) forage production, a sample of wheat forage was harvested to the ground level from an area of 0.5 m 2 located in the middle of each plot on December 10 (±3 days) each year. Forage samples were oven dried at 60 o C until no changes in weight were detected. Samples were weighed to determine DM. Leaf length was measured with a ruler on 5 randomly selected leaves from each plot. Leaf area was measured on the same leaves with a LI-COR leaf area meter (LI-COR Biosciences, Lincoln, Nebraska, USA). Leaf width was calculated from leaf area and length measurements. After measurements, the 5 leaves were oven dried at 60 o C F until no changes in weight were detected and weighed to determine DM. Specific leaf weight (SLW) was calculated as a ratio between leaf DM (g) and leaf area (cm 2 ). Tiller number was determined at the time of forage sampling from plants grown in two neighboring rows on 0.3 m distance in each row and recalculated on 0.5 m 2 basis.

Statistical Analysis
The experiments were set up as completely randomized designs and repeated during three growing seasons (October-March) in 2003-2006. In each experiment, treatments were wheat entries replicated three times. Statistical analyses were performed separately for each growing season of Experiment I and II because evaluated wheat entries varied each season in accordance with the objectives of the TAM Wheat Breeding Program. Data for early forage DM production, tiller number, leaf length, leaf width, and SLW were analyzed using Procedure Mixed (SAS Institute, 1999) ( Table 1). Wheat entries were considered fixed effects, whereas replications were considered random effects in the analysis of variance (ANOVA). Significance of means was declared at P=0.05. Correlation and stepwise regression analyses of early forage DM with wheat morphological traits (tiller number, leaf length, leaf width, and SLW) were performed using the CORR Procedure and REG Procedure of the SAS software (SAS Institute, 1999). All variables left in the model of the stepwise REG procedure were significant at the P=0.15.

Weather Patterns
Severe soil moisture deficits during the early growing season (September-December) occurred in 2003 and 2005 ( Fig. 1). The long-term average annual precipitation for the experimental location is 711 mm. During the first 4 months of wheat growth (September-December), precipitation reached only 21% and 49% of the long-term average amount (225 mm) in 2003 and 2005, respectively. In contrast, precipitation amount during September-December 2004 was 37% higher than the long-term average precipitation, but most of the rainfall occurred during November. The severe soil moisture deficit conditions during wheat establishment and early growth were coupled with higher than normal temperatures. For the months September-December, average temperature was higher by 1.

Differences in early-Season Forage Production among Experimental Wheat Breeding Lines and Cultivars
Early-season forage production in winter wheat is determined by genetic potential, management, and environmental factors (Krenzer et al., 1992). Results of our studies showed a broad variability among wheat breeding lines and commercial cultivars for early-season forage production and correlated morphological traits.  Precipitation 1306 kg ha -1 , respectively) than lines TX02V7937, TX02D5813, TX03M1016, and TX02D5797 (695-822 kg ha -1 ), with the other lines being intermediate (Fig. 2).

Correlations Between Morphological Traits and Early-Season Forage Production
The correlation coefficients showed significant relationships between the studied morphological traits and the early-season forage DM yield, both under supplemental irrigation in Experiment I during 2003 growing season and under rainfed conditions (Experiment I and Experiment II) during 2003, 2004, and 2005 growing seasons ( Table 2 and Table 3   ns, * and **: Not significant, significant at the 5% and 1% levels of probability, respectively. Results of the stepwise regression variance analysis indicated that early-season forage DM yield was mainly determined by tiller number, both in Experiment I (Table 4) and Experiment II (Table 5). Depending on the growing season and experiment, tiller number explained 10-66%, leaf length 5-26%, leaf width 2-21%, and SLW 2-26% variation in early-season forage DM yield.
Our results indicate that high tiller number, long and wide leaves, and low SLW are correlated with early-season forage DM yield. In grasses, selection for high yielding forage cultivars aims to increase both the number of tillers per plant and the biomass per tiller (Hanson and Carnahan, 1956). Tillering potential (and resulting earlyseason forage productivity) in wheat is determined by genetic makeup (Li et al., 2010), environmental factors such as light, rainfall and temperature, and certain management factors, i.e., mineral nutrition (Assuero and Tognetti, 2010). Among TXE wheat breeding lines evaluated in Experiment I and Experiment II, the highest tiller  numbers were noted for TX01M5009, TX01V6008, TX01U2527, TX01A7326, TX99V2822-A2, TX02V8033,  TX01V5425RC, TX04M410009, and TX97V5304 (  (Nelson et al., 1977), and smooth bromegrass (Bromus inermis Leyss.) (Tan et al., 1979). Leaf length and leaf width were also closely correlated with early-season forage productivity in our studies. The longest leaves were produced by TXE breeding lines TX03M1179, TX01A7340, TX01V6016, TX04M410009,  TX04V072075, TX02V7937, TX99A0556, TX01D3215, TX00V1117, TX02U2557, TX02D5797, TX03V76009, and TX99A0153-1 (Table 7). Among wheat cultivars in the UVT collection, the longest leaves were produced by HG-9, Duster, Weather master 135, Endurance, Fannin, Abilene Ag Exp, Wintex, Longhorn, and Stanton. Among TXE wheat breeding lines, the widest leaves were produced by TX04M410067 , TX02A0115, TX01A7340, TX03A0382,  TX03M1179, TX03V73029, TX01D3232 (TAM 304), TX03M1016, TX02U2557, TX02D6112, TX01M5009,  TX01A5936, TX01V5719, and TX01V6334 (Table 8). Cultivars with the widest leaves in the UVT collection were Dumas, Cutter, Weather master 135, Guymon, 2145, HG-9, Overly, TAM 110, Longhorn, Abilene Ag Exp, Doans, and Jagelene. Several studies have indicated a negative correlation between leaf elongation rate and tillering in tall fescue (Nelson et al., 1977;Zarrough et al., 1984) and perennial ryegrass (Gautier et al., 1999). Thus, it may be difficult to combine high tillering potential and leaf elongation rate in one wheat cultivar. In our studies, we generally did not observe significant correlations between leaf length or leaf width and tiller number in wheat. Two exceptions were found in Experiment II during 2003 growing season (leaf width) and 2005 growing season (leaf length). TAM-W101 (13.5) †Numbers in parentheses indicate average leaf length (cm). The lowest SLW was noted for TX02V7426, TX98A0190, TX03M1179, TX02U2510, TX03M1008, TX01V5425WC, and TX00D1390 in the TXE collection (Table 9). Among the UVT wheat cultivars, HG-9, 2174, Lockett, TAM 400, Cisco, Abilene Ag Exp, Weathermaster 135, AP502CL, Wintex, Stanton, Intrada, Jagger, and TAM 110 had the lowest SLW. Specific leaf weight (an inverse of specific leaf area) has been associated with relative growth rate (RGR) in numerous non-woody C3 species (Poorter and Remkes, 1990). Fast growing species with a high RGR express lower SLW (or higher specific leaf area, SLA) than slow growing species with high SLW. Results of our studies suggest that a relationship between SLW and growth rate occurs also in wheat, i.e., low SLW is associated with high tiller number and forage production in the early growing season. Specific leaf weight may affect forage DM accumulation in wheat because of a positive association between SLW and net CO2 exchange rate per unit leaf area (Khan and Tsunoda, 1970;Delaney and Dobrenz, 1976;Dornhoff and Shible, 1976) and a negative association between SLW and the rate of leaf area expansion (Rawson et al., 1987;Hay, 1990;Solhaug, 1991). The negative correlation between SLW and early-season forage production, however, varied among seasons, suggesting that environmental factors may have played more significant role in affecting SLW (and early-season forage DM yield) than wheat genotype effects. Similar results were reported by López-Castañeda et al., (1995) and Rebetzke et al., (2004) who found that environment had the largest influence on SLW both directly and through interactions with wheat genotypes.

CONCLUSIONS
Among the wheat breeding lines and cultivars evaluated in our studies, there was no single one with all four morphological traits (high tiller number, long and wide leaves, and low SLW) combined to maximize early-season forage DM yield. Tiller number and leaf area are considered important traits for the improvement of forage DM productivity in wheat (Kim et al., 2016). Several wheat entries expressed high tillering potential and long or wide leaves, including Fannin, Duster (Edwards et al., 2012), TX01M5009, and TX04M410009. Other wheat entries expressed long and/or wide leaves and low SLW, including Abilene Ag Exp, Cutter, HG-9, TAM 110, TX01V6016, TX03M1179, and Weathermaster 135. Most of these cultivars/breeding lines have been recommended for dualpurpose use; thus, the morphological traits evaluated in our studies were desirable for selection of wheat with improved forage productivity. It is also interesting to note that TX01M5009 and TAM 112 (Rudd et al., 2014) were parents of the recently released forage and dual-purpose wheat cultivar, TAM 204 (Ledbetter, 2014), indicating that the morphological traits correlated with high early-season forage DM yield may be heritable.