ARTÍCULOS ORIGINALES

Water stress and forage production in Tetrachne dregei Nees, Panicum coloratum L. and Eragrostis curvula (Schrad) Nees. (With 1 Table & 5 Figures)

Estrés hídrico y producción forrajera de Tetrachne dregei Nees, Panicum coloratum L. y Eragrostis curvula (Schrad) Nees. (Con 1 Tabla y 5 Figuras)

Ruiz¹ MA, AD Golberg ¹,², O Martínez ²

1INTA EEA Anguil "Ing. Agr. Guillermo Covas", Ruta Nac. 5, km 580, CC11 (6326) Anguil, La Pampa, Argentina.
2Facultad de Agronomía, UNLPam, Ruta 35, km 335, CC300 (6300) Santa Rosa, La Pampa, Argentina.
Address Correspondence to: Lic. María de los Ángeles Ruiz, e-mail: mruiz@anguil.inta.gov.ar, fax (54) 2954-495057.

Recibido/Received 17.10.2007. Aceptado/Accepted 22.10.2007.

Abstract. Tetrachne dregei, Panicum coloratum and Eragrostis curvula are perennial forage C₄ grasses, introduced from South Africa to the pampean semiarid region. This work was carried out to compare water stress tolerance; forage production and quality of T. dregei, P. coloratum and E. curvula. Studies were conducted under greenhouse and field conditions. In the greenhouse, watering was stopped after eighty one days of plant emergency in the water stress treatment. Water potential (ψ), stomatal resistance (SR) and shoot and root weights were evaluated. Under water stress, ψ diminished earlier and SR increases were higher in P. coloratum than in T. dregei and E. curvula. Plant survival in T. dregei and E. curvula was higher than that of P. coloratum. Under field conditions (INTA, Agricultural Experimental Station Anguil, La Pampa, Argentina), biomass production of T. dregei was lower than that of the other species (p<0.05) during the first year, but forage production was higher (p<0.05) in E. curvula, followed by T. dregei, than in P. coloratum in the following years. In spring, P. coloratum showed a greater forage digestibility (p<0.05) than E. curvula and T. dregei; P. coloratum and T. dregei had more protein percent than E. curvula. In summer, protein percent of T. dregei was higher (p<0.05) than that in the other species; however, there were no significant differences in forage digestibility among species. This study provided strong evidence that T. dregei could be a suitable species for semiarid environments. Panicum coloratum showed very good forage quality characteristics, although its high biomass production was limited to the first year.

Key words: Tetrachne dregei; Panicum coloratum; Eragrostis curvula; Water stress; Forage production; Forage quality.

Resumen. Tetrachne dregei, Panicum coloratum y Eragrostis curvula son gramíneas forrajeras perennes de ciclo C₄, introducidas de Sud África a la región semiárida pampeana (RSP). El objetivo del presente trabajo fue comparar la tolerancia al estrés hídrico; y la producción y calidad de forraje de T. dregei, P. coloratum y E. curvula bajo condiciones de invernáculo o de campo. En el estudio de invernáculo, luego de ochenta y un días de la emergencia, se suspendió el riego en el tratamiento de estrés hídrico. Se determinaron el potencial agua (ψ), la resistencia estomática (RS) y los pesos de la parte aérea y radical. Bajo estrés hídrico, el ψ de P. coloratum disminuyó antes que en T. dregei y E. curvula. El incremento de RS bajo estrés hídrico fue mayor en P. coloratum que en las otras dos especies. La supervivencia de las plantas de T. dregei y E. curvula fue superior a las de P.coloratum. En el ensayo de campo (Estación Experimental Agropecuaria Anguil, La Pampa, Argentina), la producción de forraje de T. dregei durante la primer temporada, fue inferior a la de las otras especies (p<0,05). Sin embargo, la producción forrajera de E. curvula, seguido de T. dregei, superaron (p<0,05) aquella de P. coloratum en los siguientes años. En primavera, P. coloratum presentó mayor digestibilidad que E. curvula y T. dregei (p<0,05) pero en verano, no mostraron diferencias. Panicum coloratum y T. dregei mostraron mayor porcentaje (p<0,05) de proteína que E. curvula en primavera, mientras que en el verano, el porcentaje de proteína de T. dregei fue superior al de las otras especies (p<0,05). Este estudio demostró que T. dregei parece ser una especie promisoria para ambientes semiáridos. Panicum coloratum presentó una muy buena calidad forrajera, aunque su alta producción de biomasa estuvo limitada al primer año.

Palabras clave: Tetrachne dregei; Panicum coloratum; Eragrostis curvula; Estrés hídrico; Producción de Forraje; Calidad de forraje.

INTRODUCTION

Rainfall in the Pampean semiarid region decreases from NE to SW with a high inter-annual variation. Since soils have a low water retention capacity (Casagrande & Vergara, 1996), it is important to include water stress resistant grasses as a rancher management strategy. Tetrachne dregei Nees (Green grass, Td), Panicum coloratum L. (Kleingrass, Pc) and Eragrostis curvula (Schrad) Nees (Weeping lovegrass, Ec) are C₄ perennial forage grasses, introduced from South Africa and neighbouring countries. It is important to compare T. dregei performance under water stress conditions with that of E. curvula and P. coloratum. This is because of the well-known perfomance of the last two perennial grasses in the Pampean semiarid region (Cairnie, 1984; Covas, 1991; Petruzzi et al., 2003; Ruiz et al., 2004).
Eragrostis curvula shows a high performance under semiarid conditions (Voigt, 1991; Colom & Vazzana, 2001, 2003), but it has good forage quality characteristics for cattle grazing during a short time period (Stritzler et al. 1996). Panicum coloratum was introduced with the purpose of obtaining a better forage quality (Stritzler et al., 1996; Petruzzi et al., 2003).
Tetrachne dregei was introduced in Argentina from South Africa (www.fao.org/AG/aGp/agpc/doc/Gbase/Safricadata/tetradre.htm, 2006), where the cultivated surface is scarce. Since 1970, studies conducted in the Pampean semiarid region showed that T. dregei produces better forage quality than E. curvula and presents good adaptation to environmental conditions (Galván, 1971; Milano & Rodríguez Sáenz, 1971; Stritzler et al., 1996).
Rainfall limitation and soil with low water retention capacity cause water deficit in plants; decreases of water potential can affect seriously leaf growth, CO₂ exchange rate, tillering and root/shoot ratio in several plant species (Echenique & Curveto, 1986; Boyer, 1995; Ben Haj & Tardieu, 1997; Brevedan et al., 2004; Blum, 2005). Under water stress, plants can increase stomatal resistance and reduce transpiration in seconds (Pugnaire et al., 1994; Colom & Vazzana, 2003). Plants also present other mechanisms to control transpiration such as leaf abscission or increases of leaf specific density (Pugnaire et al., 1994).
Our working hypothesis was that T. dregei shows a water stress tolerance similar to that in E. curvula, which is well known because of its good performance in semiarid environments. Panicum coloratum was included in this study because of its good forage quality. The objectives of this work were to compare the water stress tolerance, forage production and quality of T. dregei, E. curvula and P. coloratum in a semiarid environment.

MATERIALS AND METHODS

Greenhouse trial. A greenhouse study was conducted at the Facultad de Agronomía, Universidad Nacional de La Pampa, Santa Rosa, La Pampa Province (36° 34' S, 64° 16' W; 210 m.a.s.l) from March 2002 to October 2002. Greenhouse environmental conditions were: average temperature 17 °C, humidity 69%, average photosynthetic photon flux density 525 mmol/m²/s. Seeds were sown in PVC pots, filled with 5 kg of regional soil (entic haplustol). After 30 days, only one plant was kept in each pot.
A Complete Randomized Design with a factorial treatment (3 species X 2 water conditions; 6 replicates) was used. Treatments were water stress and control. In the water stress treatment, plants were first watered near field capacity and thereafter watering was withheld during 81 days after plant emergence. At the same time, control plants were maintained near field capacity throughout the experimental period. At the time water was withheld, number of tillers was 1-3 in T. dregei, 1-4 in E. curvula and 2-4 in P. coloratum, and number of expanded leaves in the main stem was 5 in T. dregei and E. curvula, and 8 in P. coloratum.
Stomatal resistance (SR) was determined weekly after withholding water by using a Delta T Porometer, AP4-UM-2, 2.28 Version, 1991. At the same time, leaf water potential (ψ) was evaluated with a Scholander pressure chamber.
Leaf area was measured at the end of the experiment using a LICOR, LI 3000 leaf area meter. Shoot and root dry weights were determined. This allowed calculation of specific leaf area (leaf area/leaf weight) and root/shoot ratios. Data were statistically analysed by ANOVA and LSD test (p<0.05).
Field trial. This study was conducted in the Estación Experimental Agropecuaria Anguil, Instituto Nacional de Tecnología Agropecuaria (INTA) (36º 30' S, 63º 59' W, 165 m.a.s.l), La Pampa, Argentina. Soil was an entic haplustol (organic matter: 1.46%; Phosphorus: 30.25 ppm and pH 6.14). The experimental design was a complete randomized block with eight replicates. Sowing was performed in November 2001. Plots (2.1 x 3.0 m each) had row interspaces of 0.70 m. Rainfall during the study is shown in Fig. 1. For 2003 and 2005, annual rainfall was 449 or 543 mm, respectively; these values were lower than or similar to the historical long-term
mean, respectively (Meteorological Service, EEA Anguil).

Fig. 1. Monthly rainfall during 2002, 2003, 2004 and 2005, and longterm (1921-2002) monthly rainfall in the Agricultural Experimental Station of INTA Anguil, La Pampa, Argentina.
Fig. 1. Lluvias mensuales durante 2002, 2003, 2004 y 2005, y promedio de lluvias mensuales durante 1921-2002 en la Estación Experimental Agropecuaria INTAAnguil, La Pampa, Argentina.

Clipping of forage was made in April and November of 2002, 2003 and 2004, and in November of 2005. It was performed in the central row (two linear meters) in each plot, and fresh weight was determined. Afterwards, 200 grams were oven-dried at 60 ºC to constant weight. In November 2003 and April 2004, the following forage quality determinations were made: acid detergent fiber (ADF), dry matter digestibility (DMD), metabolised energy (Mcal/kgDM) and protein concentration (PC). Data were statistically analysed by ANOVA and LSD test (p<0.05).

RESULTS AND DISCUSSION

Greenhouse trial. The interaction water level x species was significant (p<0.05) for ψ and SR in all five dates. This was because E. curvula showed the least differences in the control and water stress treatments. In P. coloratum, ψ and SR of watered plants showed significant differences (p<0.05) compared to plants of the water stress treatment at 24 days after withholding water (DAWW). At 61 DAWW, ψ of water-stressed plants showed values lower than -1 MPa, and SR increased to 80 s/cm (Fig. 2 and 3). Leaf wilting was observed between 57 and 66 DAWW.

Fig.2. Effect of water stress on leaf water potential (MPa) of T. dregei, P. coloratum and E. curvula. DAWW: days after withholding water.
Fig.2. Efecto del estrés hídrico en el potencial (MPa) hídrico foliar de T. dregei, P. coloratum y E. curvula. DAWW: días desde la suspensión del riego.

Fig. 3. Effect of water stress on leaf stomatal resistance (s/cm) (SR) of T. dregei, P. coloratum and E. curvula during the water stress period. DAWW: days after withholding water.
Fig. 3. Efecto del estrés hídrico en la resistencia estomática (s/cm) (SR) de T. dregei, P. coloratum y E. curvula. DAWW: días desde la suspensión del riego.

Water potential and SR in control plants of T. dregei showed significant differences (p<0.05) in comparison to stressed plants after 24 and 35 DAWW, respectively. From 45 to 60 DAWW, however, stomatal resistance was not different between treatments. Plants under water stress started to wilt gradually by 104-157 DAWW; these plants showed a 40 day-delay in exhibiting this symptom in comparison to plants of P. coloratum. Water potential and SR of T. dregei showed the highest differences among individual plants at the wilting stage.
Water potential and SR showed significant differences (p<0.05) between treatments after 65 days of water withholding in E. curvula. This occurred 41 days later than in the other two species. Plants of E. curvula wilted by 93-106 DAWW, almost simultaneously with T. dregei. However, differences among individual plants were lower than those in T. dregei. Similar to results in our study, Colom & Vazzana (2001, 2003) found high stability of water potential, stomatal conductance, photosynthetic rates and leaf pigments content in cultivars of E. curvula exposed to water stress.
Leaf area of T. dregei and P. coloratum was significantly higher (p<0.05) in control than water-stressed plants. Eragrostis curvula, however, did not show significant differences between both treatments. While mortality of some mature leaves was observed, young leaves were kept alive in water-stressed plants of T. dregei. This is similar to results of Blum (2005) in Sorghum. Because of this, green leaf area of T. dregei plants was lower under water stress than in the control treatment. These plants, however, showed a higher level of stomatal conductance and ψ for a longer time period than P. coloratum.
Root / shoot ratios did not differ among treatments in none of the three species (Fig. 4a). This relation decreased under water stress because shoot growth was more affected than root growth (Blum, 2005).

Fig. 4. Effect of water stress on A) root / shoot ratios (R/S), and B) specific leaf area (SLA, cm²/g) in T. dregei, P. coloratum and E. curvula.
Fig. 4. Efecto del estrés hídrico en la A) Relación raíz / parte aérea (R/S), y B) área foliar específica (SLA, cm²/g) en T. dregei, P. coloratum y E. curvula.

Leaf thickness of P. coloratum increased significantly (p<0.05) under water stress (Fig. 4b). This finding is similar to that reported by Pugnaire et al. (1994). However, no significant differences (p>0.05) were found in the other species.
Field trial. During the first year of growth (2002), biomass production of E. curvula and P. coloratum was 2.6 times higher (p<0.05) than that in T. dregei on average. This shows a slower early growth in T. dregei (Fig. 5). Panicum coloratum reached the highest (p<0.05) biomass production among the three species during summer (October 2001 to April 2002). It was followed by E. curvula, while T. dregei produced two thirds of P. coloratum biomass production. In spring (November 2002), E. curvula showed more forage production than the other two species. Slow growth in T. dregei was previously reported by Zacharias (1990); plants of this species reach maturity in a tenyear-period. In the present study, although T. dregei showed low initial growth, its forage production increased during the following years.

Fig. 5. Forage production (kg/ha) of T. dregei (Td), P. coloratum (Pc) and E. curvula (Ec) during 2002 - 2005. Anguil, La Pampa. Different letters among bars indicate significant differences, LSD (p<0.05).
Fig. 5. Producción de forraje (kg/ha) de T. dregei (Td), P. coloratum (Pc) y E. curvula (Ec), durante 2002 - 2005. Anguil, La Pampa. Entre barras, letras diferentes expresan diferencias significativas, LSD (p<0,05)

The second year (2003) was rather dry for this region (449 mm, Fig. 1). Forage production of E. curvula was higher than that in T. dregei, and forage production of T. dregei was significantly higher (p<0.05) than that in P. coloratum (Fig. 5). In E. curvula, forage production was half that in the previous year. Forage production in P. coloratum was 4.5 times lower in 2003 than in 2002. However, T. dregei had similar forage production in both years. In the first clipping (April 2003), E. curvula had more (p<0.05) forage production than P. coloratum, while T. dregei showed an intermediate forage production. In November, forage production of E. curvula was higher (p<0.05) than that in T. dregei, which reached a greater yield than P. coloratum.
In 2004, forage production of E. curvula and T. dregei was similar, and significantly higher (p<0.05) than that in P. coloratum (Fig. 5). Although rainfall was higher in 2004 (814 mm) than in 2003, yields continued decreasing. During the summer 2004, growth of E. curvula and T. dregei was similar (p>0.05), and their forage production was higher (p<0.05) than that in P. coloratum. In spring of the same year, however, growth was similar in all three species.
In 2005, a dry year (543 mm), E. curvula showed the highest yield (p<0.05) among all the study species. This is similar to results in 2003 (Fig. 5). It has been reported that cultivars of E. curvula can differ in their response capacity to water stress (Echenique & Curvetto, 1986).
During spring (September to November), forage digestibility was higher in P. coloratum than in E. curvula and T. dregei, which had similar values for this parameter (Table 1). Panicum coloratum and T. dregei showed higher protein concentrations than E. curvula (Table 1).

Table 1. Forage quality of T. dregei, P. coloratum and E. curvula in spring and summer 2003, Anguil, La Pampa. Different letters in the same column, within each season, indicate significant differences, LSD (p<0.05).
Tabla 1. Calidad del forraje de T. dregei, P. coloratum y E. curvula en primavera y verano 2003, Anguil, La Pampa. Diferentes letras en la misma columna, dentro de cada estación, expresan diferencias significativas, LSD (p<0,05)

During summer (December to April), although significant differences in DMD, ADF and metabolizable energy were not detected (p>0.05), E. curvula reached higher fiber concentrations and lower dry matter digestibility than the other two species. Tetrachne dregei showed the highest (p<0.05) protein concentrations. Stritzler et al. (1996) found that T. dregei generally produces better forage quality than other warm-season perennial grasses in semiarid Argentina. In previous experiments at different locations of the pampean semiarid region, Ruiz et al. (2004) reported higher protein concentrations in green and differed forage in P. coloratum than in E. curvula.

Conclusions. Under water stress, E. curvula exhibited a better performance than P. coloratum and T. dregei. However, T. dregei showed more forage quality than E. curvula and it presented good response to water stress. Panicum coloratum exhibited the lowest drought resistance, but its forage quality was superior to that of E. curvula and T. dregei. Also, it showed a higher forage production during the first growing period. Since T. dregei showed a good water stress resistance and forage quality, it should be considered an important forage resource in semiarid regions. However, it is necessary to point out that T. dregei has a slow initial growth, while at the same time the initial growth of P. coloratum is good.

ACKNOWLEDGMENTS

The authors thank to Osvaldo Tuya for helpful suggestions on this paper, Lic. Pamela D. Lerner for correcting a previous version of this manuscript, and the assistant staff of the Animal Production Unit of the Agricultural Experimental Station at INTA Anguil "Ing. Agr. Guillermo Covas" for field and laboratory assistance.

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