RUMINAL DIGESTION AND CHEMICAL COMPOSITION OF NEW GENOTYPES OF BUFFELGRASS (Cenchrus ciliaris L.)

Guillermo Juan García Dessommes, Roque Gonzalo Ramírez Lozano, Rahim Foroughbackhch P., Rocío Morales Rodríguez y Graciela García Díaz

Guillermo Juan García Dessommes. M.Sc. in Ruminal Nutrition. Researcher, Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP), Mexico. Doctoral Candidate, Universidad Autónoma de Nuevo León (UANL), México. Address: Apartado Postal No 3, General Terán, N. L: CP 67400. México. e-mail:dessommes57@hotmail.com

Roque Gonzalo Ramírez Lozano. D.Sc. in Ruminal Nutrition, UANL. Professor-Researcher, School of Biological Sciences, UANL, Mexico. Address: Apartado Postal, 142, Suc. F, Cd. Univesitaria, San Nicolás de los Garza, N.L., 66451, México. e-mail: roqramir@fcb.uanl.mx.

Rahim Foroughbackhch P. Doctor in Biological Sciences, UANL. Professor-Researcher, Department of Botany, School of Biological Sciences, UANL, Mexico. Address: Pedro de Alba y Manuel Barragán S/N, Cd. Universitaria, San Nicolás de los Garza, N.L., 66451, México.

Rocío Morales Rodríguez. Candidate to M. Sc. in Food Sciences, UANL. Address: Pedro de Alba y Manuel Barragán S/N, Cd. Universitaria, San Nicolás de los Garza, N.L., 66451, México.

Graciela García Díaz. D.Sc. in Biotechnology, UANL. Professor-Researcher, School of Biological Sciences, UANL, Mexico. Address: Pedro de Alba y Manuel Barragán S/N, Cd. Universitaria, San Nicolás de los Garza, N.L., 66451, México.

Resumen

Este trabajo evalúa y compara la producción de materia seca (MST), el contenido nutrimental y degradabilidad efectiva de la materia seca (DEMS), proteína cruda (DEPC) y pared celular (DEFDN) de cinco nuevas líneas y un híbrido de pasto buffel en el noreste de México. El consumo potencial de minerales contenidos en los nuevos genotipos por bovinos también fue estimado. Todos los pastos se establecieron bajo condiciones de temporal usando un diseño completamente al azar con tres repeticiones. La colecta manual de plantas fue llevada a cabo el 14 nov., 2000, en Nuevo León, México. La producción de MST no fue significativamente diferente entre zacates. Sin embargo, la proteína cruda, pared celular y sus componentes (celulosa, hemicelulosa y lignina) fueron significativamente diferentes entre los pastos evaluados. Asimismo, DEMS, DEPC y DEFDN fueron significativamente diferentes entre pastos. El híbrido Nueces tuvo los valores más altos para degrabilidad, mientras la línea PI 2 tuvo los valores más bajos. Al parecer el alto contenido de lignina en los nuevos genotipos pudo haber influido en la baja degradación de los nutrientes en el rumen de los borregos. Solo K, Fe y Co, en todos los zacates, tuvieron concentraciones suficientes para satisfacer los requerimientos de ganado de carne. Los resultados de producción de materia seca y dinámica nutricional sugieren que las nuevas líneas PI 1 y PI 4 pueden ser consideradas como buenos substitutos del híbrido Nueces para rumiantes en pastoreo en el noreste de México.

Summary

This study evaluates and compares the dry matter production (TDM), chemical composition and effective degradability of dry matter (EDDM), crude protein (EDCP) and neutral detergent fiber (EDNDF) of the Nueces hybrid and five new genotypes of buffelgrass growing in Northeastern Mexico. Potential intake of minerals by cattle consuming the new genotypes was also estimated. All grasses were established in a completely randomized design with three replicates in a rain fed experiment. Plants were hand harvested on Nov. 14, 2000 at Nuevo Leon, Mexico. TDM was not significantly different among genotypes. Crude protein content and cell wall and its components (cellulose, hemicellulose, and lignin) were significantly different among grasses. Also, EDDM, EDCP, and EDNDF were significantly different among the buffelgrass genotypes. The Nueces hybrid had the highest degradability values; in contrast, PI 2 had the lowest values. It seems that high lignin content in new genotypes may negatively influence nutrient digestion in the rumen of sheep. Only the K, Fe and Co contained in all grasses would be sufficient to meet the requirements of grazing cattle. Data of dry matter production and nutritional dynamics, suggest that the new genotypes PI 1 and PI 4 could be considered as good substitutes of the Nueces hybrid for grazing ruminants in northeastern Mexico.

Resumo

Este trabalho avalia e compara a produção de matéria seca (MST), conteúdo nutricional e capacidade de degradação efetiva da matéria seca (DEMS), proteína crua (DEPC) e parede celular (DEFDN) de cinco novas líneas e um híbrido de pasto buffel no nordeste do México. O consumo potencial de minerais contidos nos novos genótipos por bovinos também foi estimado. Todos os pastos se estabeleceram sob condições de temporal usando um desenho completamente ao azar com três repetições. A colheita manual de plantas foi levada adiante em 14 nov., 2000, em Nuevo León, México. A produção de MST não foi significativamente diferente entre pastos. No entanto, a proteína crua, parede celular e seus componentes (celulosa, hemi-celulosa e lignina) foram significativamente diferentes entre os pastos avaliados. Assim mesmo, DEMS, DEPC e DEFDN foram significativamente diferentes entre pastos. A híbrida "Nozes" teve os valores mais altos para degrabilidade, enquanto que a línea PI 2 teve os valores mais baixos. Ao parecer o alto conteúdo de lignina nos novos genótipos pode ter influído na baixa degradação dos nutrientes no rúmen dos borregos. Só K, Fe e Co, em todos os pastos, tiveram concentrações suficientes para satisfazer os requerimentos de gado de carne. Os resultados de produção de matéria seca e dinâmica nutricional sugerem que as novas líneas PI 1 e PI 4 podem ser consideradas como bons substitutos do híbrido Nozes para ruminantes em pastoreio no nordeste do México.

KEYWORDS / Buffelgrass / Grass Genotypes / Nutrient Digestibility / Northeastern Mexico / Ruminants /

Received: 10/29/2002. Modified: 03/20/2003. Accepted: 03/25/2003

Introduction

Common buffelgrass (T-4464) was introduced into Texas in the late 1940s and it is currently grown on 8 to 10 million acres in Southern Texas, USA, and the North of Mexico. Since introduction, buffelgrass has had a marked impact on the livestock industry of these regions since, as a range grass, it is highly productive and has allowed an increase in cattle stocking rates from one animal unit (AU) for every 12ha to 1 AU for 4ha (Hanselka, 1985). Buffelgrass reproduces by obligated apomixis, in which seeds are formed without sexual fertilization. Consequently, the progeny are genetically identical to the maternal parent. The monoculture of this grass with its unique type of reproduction encompasses millions of ha with genetically identical plants, and represents a high risk due to the susceptibility to diseases or pests. Recently, a blight of epidemic proportions on common buffelgrass has been reported in Mexico and South Texas. The causal agent has been identified as Pyricularia grisea (Cook) Sacc. (Rodríguez et al., 1999). Because of this and other potential problems, new strains, cultivars, and hybrids of buffelgrass have been tested in order to increase the genetic pool of this grass in the region.

Because of its wide adaptation to semiarid regions and relatively good nutritional quality, buffelgrass is considered as a South Texas and Northeastern Mexico wonder grass (Hanselka, 1988). However, seasonality of rainfall and temperature are major influences on nutritional quality of buffelgrass (White and Wolfe, 1984). Silva and de Faria (1995) reported significant differences in the nutritional value among new cultivars and hybrids of buffelgrass; moreover, the rate and extent of ruminal digestion of the nutrients contained in forage of new genotypes has not been reported in the scientific literature. Thus, effective degradability and the rate of digestion are important characteristics of forage that may be used to predict the nutritive value more accurately and compare the utility of this kind of forages in the diets for ruminants (Ørskov, 1991). Grasses are important sources of organic and inorganic nutrients for ruminants; however, in some circumstances, they are deficient in one or more of these nutrients. Minerals are required to meet the animal needs for optimum development and health (Spears, 1994), as they are essential nutrients and influence animal performance (McDowell, 1997). The object of this study was to evaluate and compare the nutrient content and ruminal fermentation of forage of five strains and one hybrid of buffelgrass under rain feed conditions in Northeastern Mexico

Materials and Methods

Research was carried out at the Experimental Station "General Terán", Instituto Nacional de Investigaciones Forestales, Agricolas y Pecuarias (INIFAP) and at the Universidad Autónoma de Nuevo León (UANL). General Terán, N.L., Mexico, is located at 25º18'N and 99º35'W, with an altitude of 332masl. The climate is typically semitropical and semiarid with warm summers. The predominant type of vegetation is known as the Tamaulipan Thorn scrub or subtropical Thorn scrub woodlands. The dominant soils are deep, dark-gray, lime-clay Vertisoles resulting from alluvial processes. These soils are characterized by high Ca carbonate (pH 7.5-8.5) and relatively low organic matter content. Annual mean temperature is 22.4°C, average yearly rainfall 784 mm and evaporation 1622mm.

Under rain fed conditions, five strains of buffelgrass (Cenchrus ciliaris L.) identified as PI-307622 (PI 1), PI-409252 (PI 2), PI-409375 (PI 3), PI-409443 (PI 4), PI-409460 (PI 5), and the hybrid Nueces were established in experimental plots, using a completely randomized design with three replicates. The plots consisted of 5m long rows, with 0.8m between rows. With the purpose to achieve a uniform grass growth, all grasses were cut prior to the experiment, in March 2000. The first significant rainfall of that year occurred on Sept. 14 (66mm) and provided the conditions to sustain grass growth. In Sept. and Oct., 452mm of rainfall were recorded, which allowed the grasses to reach full blossom by Nov. 14, when all grasses were hand harvested to a height of 0.15m above ground. Partial dry matter was determined after drying in an oven at 55ºC for 72h. Blades and stems were split and weighed individually, and the proportion of blades (H%) for each sample was obtained. Then, samples were ground in a Wiley mill (1mm screen) and stored in plastic containers.

Samples were analyzed for dry matter (DM), organic matter (OM), crude protein (CP; AOAC, 1990), neutral detergent fiber (NDF), acid detergent fiber (ADF; Goering and Van Soest, 1970) and acid detergent lignin (ADL; AOAC, 1990). Hemicellulose (NDF-ADF) and cellulose (ADF-ADL) were estimated by difference. Estimation of insoluble N₂ in NDF (INNDF) and insoluble N₂ in acid detergent fiber (INADF), which corresponds to the non-degraded N₂, was performed according to Van Soest et al. (1991), and the slowly degraded N₂ associated to the cell wall components was calculated as INNDF minus INADF (Goering and Van Soest 1970; Krishnamoorthy et al., 1982).

Mineral content was estimated by incinerating the samples in a muffle oven at 550ºC, during 4h. Ashes were digested in a solution containing HCl and HNO₃, using the wet digestion technique (Diaz-Romeau and Hunter, 1978). Concentrations of Ca, Na, K, Mg, Cu, Fe, Zn, Mn, Co, and Mo were obtained using an atomic absorption spectrophotometer, and the P content was determined in a colorimeter (AOAC, 1990).

The rate and extent of DM, CP and NDF digestibility in gasses were measured using the nylon bag technique. Four rumen fistulated Pelibuey x Rambouillet sheep (weighing 45.2 ±2.3kg, BW) were used to incubate bags (5 x 10cm, 531/4µ pore size) containing ground samples (4g) of each grass replication and suspended in the ventral part of the rumen of each sheep. Throughout the experiment, sheep were fed alfalfa hay ad libitum. For each grass replication six bags were incubated for 4, 8, 12, 24, 36, and 48h. Upon removal from the rumen, bags were washed in cold running water. Zero time disappearance was obtained by washing non-incubated bags (0h bag). All bags were dried at 60ºC in an oven during 48h. Weight loss of DM, CP and NDF was recorded. Disappearance of DM, CP, or NDF for each incubation time was calculated as

The disappearance of DM, CP, or NDF for each incubation time was used to estimate the digestion characteristics of DM, CP and NDF, using the equation of Ørskov and McDonald (1979)

p = a+b (1-e^-ct),

where p: disappearance rate at time t; a: an intercept representing the soluble portion of DM, CP or NDF at the beginning of incubation (time 0); b: portion that is slowly degraded in the rumen; c: rate constant of disappearance of fraction b; and t: time of incubation.

The nonlinear parameters a, b, and c and effective degradability of DM (EDDM), CP (EDCP) or NDF (EDNDF) = (a+b)c/(c+k)(e^-(ct)LT), were calculated using the Neway computer program (McDonald, 1981); k is the estimated rate of outflow from the rumen and LT is the lag time. The EDDM, EDCP and EDNDF values were estimated assuming a rumen outflow rate of 2.0%/h. Total digestible DM/ha (DDM) was calculated as TDM x EDDM and total digestible CP/ha (DCP) as TDM x EDCP.

Data of chemical composition, a, b, c, EDDM, EDCP, EDNDF, DDM and DCP were statistically analyzed using a one-way analysis of variance. Means were separated using the Least Significant Difference technique. Simple linear correlation analyses were performed between %H, chemical composition and EDDM, EDCP and EDNDF (Snedecor and Cochran, 1980).

Results and Discussion

The total dry matter (TDM) production was not significantly different among the evaluated genotypes (Table I). However, PI 4 and PI 2 yielded more dry matter than other grasses including the Nueces buffelgrass, which is recognized as highly productive and with good nutritional quality. The %H was significantly different among grasses, being Nueces and PI 4 the leafier grasses.

The CP content was significantly different among genotypes. PI 5 had the highest value and PI 3 the lowest. The mean value of CP in all genotypes was 8.3% (Table I). This value is about one percent above the minimal (7%) required to sustain rumen functionality (NRC, 1996). It appears that low N content in the forage is related with the available N₂ in the soil (Ramos and McDowell, 1994). The soil where this experiment was located has been characterized for its low N₂ availability (INIFAP, 1991). However, higher CP values were reported, only at the end of summer and the beginning of autumn, for the total plant in common buffelgrass (Ramírez et al., 2001a) and the hybrids Nueces (Ramírez et al., 2001b) and Llano (Foroughbackhch et al., 2001) growing in the same region but in different soil types.

The cell wall (NDF) content and its components (ADF, ADL, cellulose, and hemicellulose) were significantly different among grasses (Table I). Nueces resulted with the highest cell wall contents and PI 5 with the lowest. Moreover, PI 5 had the largest fully digestible portion of the DM (100-NDF= 34.1%), and Nueces the lowest (28.2%). Also, Nueces contained less lignin (Table I). Lignification of the cell wall has been related to low degradability of nutrients contained in plants (Van Soest, 1994). The INNDF was not significantly different among grasses. Conversely, INADF and INNDF-INADF were significantly different. The N₂ non-available for ruminants, estimated by the faction INADF, was high (29.8%; mean value of all grasses) as compared with previous reports (3 to 15%; Van Soest, 1994).

Fraction a (lost during washing of bags) of DM, CP, and NDF was significantly different among the grasses. Conversely, fraction b (slowly degraded in the rumen of sheep) was not different (P<.05). The degradation rate (c, %/h) in DM and PC was different (P<0.05), but was not different (P>0.05) in NDF. The EDDM, EDNDF, and EDCP were significantly different among grasses (Table II). The hybrid Nueces had the highest value of EDDM, EDNDF, and EDCP while the strain PI 2 had the lowest values. Higher lignin content in the new genotypes might have decreased rumen fermentation of nutrients in the rumen of sheep. The EDCP overall mean was estimated to be 64.2% and the potential available CP (100-INADF) was 70.2%. The difference of 6% between these two values might be the amount of INNDF-INADF which is slowly available in the rumen of the animal, but could be fully digested in the abomasum (Van Soest, 1994; Van Soest, et al., 1991). Moreover, it seems that INADF negatively affected EDCP (r = -0.47, P<0.05).

The degradability values found in Nueces were higher than those reported by Ramírez et al. (2001b), who evaluated Nueces growing in these regions but harvested in different dates. High degradability values reported in the present study may be associated to the positive effects of high precipitation (Foroughbackhch et al., 2001; Ramírez et al., 2001a). In this experiment, more than 400mm fell during the growing season. The %H also influenced the effective degradability of nutrients in the grasses, since it correlated with EDDM (r= 0.63, P<0.05), and EDCP (r= 0.53, P<0.05). The EDCP was also positively correlated to CP content (r= 0.50, P<0.05), and negatively to INADF. This means that when CP increased, EDCP did as well.

There were no differences (P>0.05) in DDM among the grasses tested (Table II). The differences found in EDDM were overriden by the non-significant differences in forage production (TDM) among the genotypes. In contrast, there were significant (P>0.05) differences in DCP among the genotypes evaluated. The strain PI 4 had the highest value and PI 2 the lowest (Table II).

With exception of Ca, Fe, Mn, and Co, all minerals evaluated were significantly (P<0.05) different among genotypes (Table III). In general, most minerals had concentrations that are low for the needs of adult grazing cattle. It appears that soil characteristics did influence forage concentration of specific minerals. Forages growing in soils with high values of Ca carbonates and pH, and low organic matter content, such as the ones used in this experiment, tend to have lower content of most essential minerals (INIFAP, 1991). White and Wolfe (1985) also reported low values for P (0.23%), Ca (0.30%), K (1.6%) and Mg (0.18%) in common buffelgrass harvested during the autumn season in Cotula, Texas, USA.

Table IV shows the potential mineral intake, calculated for each mineral appearing in Table III, by a cow of 400kgBW, with a daily intake of 10.2kgDM of the evaluated grasses. The potential intake of K, Fe, Co, Mn (only in PI 4), and Mo (only in Nueces), would be sufficient to meet the requirements of these minerals for a growing cow of 400kg grazing any of the genotypes tested in these soils. However, Ca, P, Na, Mg, Cu, and Zn, were lower than required. Deficiencies of P and Na have been reported and they occur in many grass species that grow in warm climates (McDowell, 1997). Thus, to obtain an optimal productivity of cattle grazing these grasses growing in this type of soil, the animals have to be supplemented with Ca, P, Na, Mg, Cu, Zn, Mn (except for PI 4), and Mo (except for Nueces).

Conclusions

Forage dry matter production and total digestible dry matter were not significantly different among evaluated grasses. However, there were significant differences for most of the chemical components and digestion parameters. The hybrid Nueces had higher values of EDDM, EDNDF, and EDCP than the other grasses. High lignin content in the forage of new genotypes may decrease the amount of nutrients degraded in the rumen of sheep. In general, the mineral content in all grasses was low for the needs of grazing ruminants, an effect probably caused by the low mineral content in the soil. Only K, Fe, and Co would be sufficient to fulfill the requirements of a mature cow grazing the evaluated grasses, while other minerals have to be supplemented for an optimal cattle growth. As new genotypes had very similar dry matter production and nutritional dynamics, it is suggested that they could be used as forage substitutes of Nueces buffelgrass growing in the region.

Acknowledgments

The authors acknowledge financial support from CONACYT-SIREYES (Project 2000-60-1006) and Fundación Produce N.L.

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