Forage yield and nutritive value of Eragrostis curvula and Digitaria eriantha in central-south semi-arid Argentina

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1 Tropical Grasslands (21) Volume 35, Forage yield and nutritive value of Eragrostis curvula and Digitaria eriantha in central-south semi-arid Argentina A.O. GARGANO 1,2, M.A. ADÚRIZ 1, H.M. ARELOVICH 1,3 AND M.I. AMELA 1 1 Departamento de Agronomía, Universidad Nacional del Sur, Argentina 2 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) 3 Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (CIC) Abstract A clipping experiment was conducted over 4 consecutive years at Bahía Blanca (Argentina) to evaluate dry matter (DM) yield, crude protein (CP) concentration and in vitro DM digestibility (IVDMD) in Digitaria eriantha cv. Irene and Eragrostis curvula cv. Tanganyika. During spring and summer (mid-october to mid-february) the plants were cut whenever they reached 25 3 cm height. The same measurements were also performed for the autumn regrowth on June 1 and July 15 each year. The 4-year average of total DM yield for the spring summer and deferred growth periods were 2812 and 798 kg/ha for D. eriantha, and 3591 and 729 kg/ha for E. curvula, respectively. The CP and IVDMD values were: D. eriantha 12.1 and 5.%; 64. and 5.8%; and E. curvula 9.2 and 3.9%; 51.6 and.4%, for the spring summer and deferred periods, respectively. Differences in spring summer yields between species depended upon the initiation of the growing cycle. Quality of deferred forage was significantly higher for D. eriantha. However, a large proportion of leaf lodging was observed for its deferred forage mass. This problem may substantially reduce the ability of grazing animals to harvest this forage. Correspondence: A.O. Gargano, Departamento de Agronomía, Universidad Nacional del Sur, 8 Bahía Blanca, Argentina. agargano@uns.edu.ar Introduction Cow calf operations predominate in the semiarid region of Argentina, comprising a large portion of beef cattle operations in 4 central provinces. Common environmental constraints to forage production for beef cattle are low rainfall, extreme winter summer temperatures and shallow eroded soils. Because of its adaptability to these constraints, Eragrostis curvula became the most important summer perennial grass for supporting cow calf operations in this area (Covas 1991). The same author indicated that the 7 ha planted with E. curvula in Argentina was mainly concentrated in this region. The main limitation of E. curvula is a sharp decrease in nutritive value as the plant reaches maturity (Vera et al. 1973; Voigt et al. 1981). This loss of quality is exacerbated when summer and autumn biomass is deferred for grazing until the end of autumn or winter (Marchi and Giraudo 1973; Gargano and Adúriz 1984). The resultant fibrous, low protein forage is poorly consumed (NRC 1987), being inadequate for the nutritive requirements of pregnant cows (NRC 1996). South African studies indicate that Digitaria eriantha has a growth cycle and adaptation to semi-arid environments similar to that of E. curvula, as well as a high production potential (Brockett and Gray 1984; Hardy and Gray 1989; Hardy et al. 199; Dannhauser 1991). Research on D. eriantha revealed better nutritive value and animal performance than for E. curvula (Grunow et al. 1984; Rabotnicof et al. 1986a, 1986b; Dannhauser 1988; Rethman and de Witt 1991; Snyman 1994; Rethman et al. 1997). A large portion of the south and south-west of Buenos Aires Province is semi-arid, and D. eriantha is being promoted as an alternative forage to E. curvula. However, little information is available on the performance of this species in the environmental conditions of semi-arid Argentina. Experimental results on yield and forage quality for D. eriantha were recently reported (Gargano

2 162 A.O. Gargano, M.A. Adúriz, H.M. Arelovich and M.I. Amela et al. 1997a; 1997b). The objective of this experiment was to improve knowledge of seasonal variation in forage yield and nutritive value of D. eriantha as compared with E. curvula. Materials and Methods A clipping experiment was carried out at the experimental field of Universidad Nacional del Sur at Bahía Blanca, SW of Buenos Aires Province, Argentina (38 44 S, 62 1 W). This study was conducted for 4 consecutive years in the following periods: , , and The soil was a typical Ustipsamment, with a petrocalcic phase and loamy-sand texture (USDA 1999); it had low organic matter content and was susceptible to wind erosion (Sánchez and Kruger 1981). Annual rainfalls between May and April for these periods were 414, 71, 97 and 495 mm, respectively, compared with a long-term (25 years) mean of 693 mm. Monthly precipitation for the 4 annual periods is shown in Figure 1. The species Digitaria eriantha cv. Irene and Eragrostis curvula cv. Tanganyika were compared under different clipping treatments. The experiment was a completely randomised block design with 3 replications and a split-plot in time. The treatments were: (1) clipping whenever growing plants reached 25 3 cm height during spring summer (from about October 15 February 2); and (2) clipping deferred forage at 2 fixed dates, June 1 and July 15. The latter included forage accumulated from February 2 until growth ceased in April. Clipping treatments were applied sequentially to the same plots. The experimental plots ( m) were sown in October 1994, with 4 rows 35 cm apart. Each block had 4 plots. Thus, the design included 2 observations per replicate for the spring summer treatment but only one observation per replicate for the deferred forage treatment. The experimental period started in spring The forage was hand clipped at a height of approximately 5 cm from a central area of 3.5 m.7 m comprising 2 rows of each plot. The harvested forage was immediately dried in an oven at 6 C to constant weight. Dry matter (DM) yield was computed from the forage DM harvested from the 2.45 m 2 of each plot at each clipping date. After drying, all samples were ground through a 2 mm screen in a Wiley mill (Standard Model 3, Arthur H. Thomas Co., Philadelphia, PA) and stored for lab determinations. In vitro DM digestibility (IVDMD) was determined by using the Barnes (1966) modification of the original Tilley and Terry (1963) technique. Total N was determined by semi-micro Kjeldhal (Nelson and Sommers 199), and N data multiplied by 6.25 to obtain crude protein (CP) values. Total DM yield for the spring summer growth was obtained by adding the DM harvested at each clipping from October 15 February 2. Total annual DM yield was computed from the sum of spring summer growth plus additional DM yield harvested between February 2 and deferred clipping dates. The dead leaf fraction from deferred forage was determined by manual separation of dead and green material, and the results were expressed as percentage of total biomass. The IVDMD and CP values of spring summer cuts were obtained on a composite sample from the harvests of each plot. Annual IVDMD and CP means were computed from weighted averages for spring summer and deferred forage. Data on forage production and quality were subjected to analysis of variance. For spring summer data, the model included the effects of block, plant species, year as a split-plot factor and interaction. For deferred forage, the same model was used, but treatments were plant species and clipping dates. Therefore, analysis of variance for spring summer and deferred winter was a 2-factor and a 3-factor analysis, respectively. When treatment year interaction was apparent, a combined mean square was used to test the effect of treatment combinations (Steel and Torrie 198); otherwise the treatment effects were tested against the residual error. Means were separated by a classic t test, when only 2 means were compared; or Tukey s test for 4-mean comparisons. Results and Discussion Dry matter yield The DM yields for each species in the 4-year cycle are shown in Table 1. A significant interaction (P <.5) of clipping treatments year was found for spring summer, deferred winter and total forage yield. The low comparative yield of the first year was mainly due to reduced and unfavourable distribution of precipitation. However, higher precipitation in the third year was not reflected in increased DM yield of both

3 Eragrostis curvula and Digitaria eriantha yield and quality Rainfall (mm) Month Figure 1. Monthly rainfalls in each of 4 years of study.

4 164 A.O. Gargano, M.A. Adúriz, H.M. Arelovich and M.I. Amela species compared with yields in the second and fourth years, probably because 26 mm (28.7% of the total) fell during winter dormancy (June July). As expected, biomass production of both forages was highest in the spring summer. During this period (October 15 February 2), DM yield was 83% and 78% of the annual total for E. curvula and D. eriantha, respectively. Similar results were found in previous studies (Gargano and Adúriz 1984; Veneciano and Terenti 1997). For the 4 years of the study, spring summer biomass production for D. eriantha was 21.7% less than that of E. curvula (P <.5). As an average for the 4-year study, E. curvula plots were clipped 5 times during spring summer and D. eriantha 3.8 times. This can be attributed to the higher temperature required by D. eriantha to start its growing cycle compared with E. curvula. Every year E. curvula initiated regrowth at the beginning of August and D. eriantha 2 3 weeks later. Regrowth for both species was damaged by late frosts; however, the damage was always greater for D. eriantha. After spring regrowth, D. eriantha was completely dry and yellow, while E. curvula was mostly green. As a consequence, the mean date for the first cut was October 19 for E. curvula vs November 21 for D. eriantha, which produced an average of 1.2 fewer cuts than E. curvula. In agreement with previous findings (Rabotnicof et al. 1986b; Rethman et al. 1997; Gargano et al. 1997a), the larger DM yield and a 4- to 5-week advance in the availability of the first spring regrowth favours E. curvula, marking substantial differences between species. The difference of 45 days between clipping dates for deferred grass yield had no effect on E. curvula DM yield (P >.5). Even prolonged intervals between clipping dates for deferred E. curvula in winter have not affected biomass production (Vera et al. 1972). A similar response for both species was found by Rabotnicof et al. (1986b). In contrast, there was a trend for deferred DM yield of D. eriantha to decline between clipping dates in and These differences between clipping dates in DM yield were significant only for (P <.5), which can be attributed, at least partially, to the comparatively lower temperatures registered in the cold months. This can be further illustrated by the mean temperatures observed between April 1 July 15, which were 12.3, 12.1, 11.3 and 1.2 C for each of the 4 years of study, respectively. Average deferred forage DM was different across years, particularly for the last cycle in which DM yield was the greatest (P <.1). Rainfall amount and distribution in February and March 1999 was an important factor in this increase in DM yield. Cumulative rainfalls for these 2 months in 1996, 1997, 1998 and 1999 were: 73, 144, 183 and 214 mm, respectively, compared with a mean for the last 25 years of 143 mm. April rainfall was not considered because growth of both forages practically ceases Table 1. Dry matter yields (kg/ha) in spring summer, deferred forage and total biomass productivity per year for Eragrostis curvula and Digitaria eriantha in each of 4 years of study. Cycle Species Spring summer clipping Deferred forage clipping dates Total yield/year 1 Means followed by different letters in each cell differ (P <.5). 2 Significant species clipping date interaction within cell (P <.5). (1) June 1 (2) July 15 (3) (1 + 2) (1 + 3) E. curvula a 1,2 285 ab D. eriantha b 2931 b SE ± 92.4 ± 52.1 ± E. curvula 4736 b b b D. eriantha 335 a a 4134 a SE ± 48.9 ± 46.7 ± E. curvula 3786 b 5 a a 4286 b b D. eriantha 266 a 872 b 65 a 3532 a 331 a SE ± 32.1 ± 49.5 ± E. curvula 3543 b b b D. eriantha 2838 a a 3851 a SE ± 43.6 ± 46.7 ± 61.9

5 Eragrostis curvula and Digitaria eriantha yield and quality 165 at the beginning of the month. For this last growing stage, DM yield of D. eriantha was larger than that of E. curvula at the first cutting date but the difference was significant only for (P <.5) and occurred independently of moisture conditions. This difference in DM yield between the species is consistent with previous findings at the same experimental site (Gargano et al. 1997a). In South Africa, Snyman (1994) also reported similar results for the period January June, but in a year with high precipitation. A significant species clipping date interaction was also observed for the dead leaf fraction in deferred forage. Deferred forage of D. eriantha was almost entirely dead (96.5%) by the first cutting date compared with 43.2% for E. curvula (P <.5). The corresponding figures for the July harvest were 99.5% and 68.7% (P <.5). A high proportion of dead leaf material in the deferred winter forage of D. eriantha provides additional evidence of a marked sensitivity of this species to low temperature compared with E. curvula. Differences in growth habit for each species may have also contributed to the differential characteristics observed in the deferred forage. Thus, E. curvula sustained its erect bunch type across winter, but severe leaf lodging was observed in the semi-erect bunches of D. eriantha. Gardner (1958) suggested that lodging favours DM loss. Therefore, better utilisation of deferred forage under grazing conditions can be expected from E. curvula than D. eriantha. In the latter, forage harvest by grazing animals would be more restricted and trampling damage would be greater. Specific studies including grazing as a variable may help to clarify this hypothesis. Indicators of nutritive value The 4-year average treatment responses for CP and IVDMD were consistent with those for each year (P >.5), for spring summer, deferred winter and annual means (Tables 2 and 3). Therefore, treatment comparisons were performed by using these 4-year means. The biomass produced during spring summer had similar percentages of CP in each species for all 4 years of the study (Table 2). For this period, mean CP concentration of D. eriantha was 33% higher than that of E. curvula (P <.5). The CP concentration in the deferred forage was not affected by cutting date. This is a desirable feature because it may allow more flexibility for utilisation at later dates. Again D. eriantha performed better than E. curvula by sustaining higher CP in its deferred forage in all 4 years (P <.5). However, the CP values determined for both species are not sufficient to supply the maintenance requirements (>7% CP) of adult gestating beef cows in winter (NRC 1996). This is an important aspect of the cow-calf systems in the south-western semi-arid region of Argentina, because beef females (mainly Angus breed) are in the beginning of the last trimester of pregnancy during June and July. According to the NRC (1996) model, cows receiving diets up to 7.% CP would be in negative balance for degradable and undegradable protein intake. If dietary CP levels are to be at maintenance levels, additional protein has to be obtained from other N sources for animals grazing both species. It is assumed that animal selectivity within deferred forage in both species is unlikely. In addition, previous research indicated that intake level of unsupplemented E. curvula was only 1.54% of bodyweight, which met around 5% of total protein requirement for maintenance (Arelovich et al. 1992). Annual total CP concentration of D. eriantha was 27% higher than that of E. curvula and this might impact on characteristics of supplementation programs. Like CP patterns, the IVDMD values were similar for the different years (P >.5) in each species during spring-summer. However, D. eriantha was more digestible than E. curvula (P <.5; Table 3) in all years. The IVDMD difference between species in the first spring regrowth was accentuated by the fact that E. curvula was cut approximately one month earlier than D. eriantha. Consequently, D. eriantha was more exposed to the effects of late frosts. Significant differences in IVDMD were also found between species for the deferred forage in the 4-year means. IVDMD of deferred forage was markedly lower than those observed in spring summer. IVDMD was somewhat lower for the second cutting date, but this reduction was not significant. These observations on the variation in nutritive value for both species stress two important aspects: the potential energy balance that can be reached in mature cows through deferred forage utilisation; and the effect of a prolonged forage deferment on CP and IVDMD values. In relation to the first point, Van Soest (1994) suggested a close relationship between IVDMD and TDN (total digestible nutrients) from which Net Energy values for maintenance as well as energy

6 166 A.O. Gargano, M.A. Adúriz, H.M. Arelovich and M.I. Amela balances can be computed (NRC 1996). Thus, the intake of deferred D. eriantha would sustain a positive energy balance in a typical gestating mature cow, while E. curvula would not. With regard to the second point, the two quality parameters described, CP and IVDMD, were not affected by a prolonged deferment period, which is consistent with previous reports (Rabotnicof et al. 1986b; Gargano et al. 1997b). These results are independent of dead material content. Average IVDMD values of D. eriantha were 24., 25.9 and 23.3% larger than values for E. curvula, for spring summer, deferred forage and annual mean, respectively. These results are in agreement with previous findings in comparative trials, which showed that nutritive value is higher for D. eriantha than for E. curvula, at any stage of plant growth and for deferred forage. However, in the different experiments, the values for quality parameters differ mainly due to different climatic and soil characteristics found in each case (Rabotnicof et al. 1986b; Snyman 1994; Gargano et al. 1997b). In conclusion, the biomass productivity and nutritive value of D. eriantha and E. curvula presented different responses to clipping in 4 consecutive years. D. eriantha seems to require higher temperatures to initiate its growth cycle, and seems to be more sensitive to low environmental temperatures than E. curvula. This can be inferred from the delay of approximately one month in obtaining the first spring summer cut, Table 2. Crude protein (%) in spring summer, winter deferred forage and annual means for Eragrostis curvula and Digitaria eriantha in each of 4 years of study. Cycle Species Spring summer clipping Deferred forage clipping dates Annual mean 1 1 Weighted average. 2 Means followed by different letters in each cell differ (P <.5). 3 Significant species clipping date interaction within cell (P <.5). (1) June 1 (2) July 15 (3) (1 and 2) (1 and 3) E. curvula D. eriantha E. curvula D. eriantha E. curvula D. eriantha E. curvula D. eriantha Mean E. curvula 9.2 a a a 8.3 a a D. eriantha 12.1 b 5.1 b 5. b 1.4 b 1.6 b SE ±.37 ±.74 ±.92 Table 3. In vitro dry matter digestibility (%) in spring summer, winter deferred forage and annual means for Eragrostis curvula and Digitaria eriantha in each of 4 years of study. Cycle Species Spring summer clipping Deferred forage clipping dates Annual mean 1 1 Weighted average. 2 Means followed by different letters in each cell differ (P <.5). 3 Significant species clipping date interaction within cell (P <.5). (1) June 1 (2) July 15 (3) (1 and 2) (1 and 3) E. curvula D. eriantha E. curvula D. eriantha E. curvula D. eriantha E. curvula D. eriantha Mean E. curvula 51.6 a a a 49.9 a a D. eriantha 64. b 51.6 b 5. b 61.2 b 61.2 b SE ±.266 ±.566 ±.367

7 Eragrostis curvula and Digitaria eriantha yield and quality 167 inferior DM yield, shorter growth cycle and larger amounts of dead material in deferred forage in D. eriantha as compared with E. curvula. In contrast, forage nutritive value from D. eriantha was significantly higher across the year, despite extensive frost damage in winter. Neither E. curvula nor D. eriantha deferred forage would supply enough energy and protein for gestating cows. However, the higher quality of the D. eriantha forage would allow a more economic supplementation program. References ARELOVICH, H.M., LABORDE, H.E., VILLALBA, J.J., AMELA, M.I. and TORREA, M.B. (1992) Effects of nitrogen and energy supplementation on the utilisation of low quality weeping lovegrass by calves. Agricoltura Mediterranea, 122, BARNES, R.F. (1966) The development and application of in vitro rumen fermentation techniques. Proceedings of the X International Grassland Congress. pp BROCKETT, G.M. and GRAY, N.N. 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