Abstract The goal was to determine the digestion rates of carbohydrate fractions A (sugars, oligosaccharides and organic acids), B1 (starch and soluble fiber), NSC (non-structural carbohydrates) and B2 (available NDF) in four tropical grasses using the gas production technique. Samples of whole forage (WF), residue insoluble in 90% ethanol (EIR) and isolated NDF (iNDF) were fermented in vitro and gas production measured. Gas volumes were determined from the following fractions, A = WF minus EIR; B1 = EIR - ND; NSC = WF - iNDF; and B2 = iNDF. Grasses were Andropogon gayanus, Urochloa brizantha, Cynodon plectostachyus, and Megathyrsus maximus each grown in Veracruz, Mexico on four plots (5×5 m), fertilized (relationship equivalent to 0 and 100 kg N/ha) and clipped 35 d after the N fertilization. A complete randomized block design with factorial arrangement 4×2 and two replicates per treatment was used. Factors were grass species and N fertilization. Data were fit using a single-pool exponential model with lag. The volume (mL gas/100 mg OM), rate (%/h) and lag (h) were: WF (22.8; 5.3; 2.1); A (3.2; 15.7; 0.5); B1 (1.5; 15.7; 0.2); and B2 (18.3; 6.6; 5.2). Andropogon and Urochloa had higher NSC content compared to Megathyrsus and Cynodon but lower gas yield per unit of NSC. Rates of digestion for the B2 fraction ranged from 4 to 8 %/h; and NSC digestion rate averaged 15.7 %/h. Nitrogen fertilization reduced carbohydrate pool sizes but did not affect rates of digestion. It is concluded that the rates of digestion of the carbohydrate fractions differs by grass specie.

Resumen El objetivo consistió en determinar las tasas de digestión de las fracciones de carbohidratos A (azúcares, oligosacáridos y ácidos orgánicos), B1 (almidón y fibra soluble), CNE (carbohidratos no estructurales) y B2 (FDN disponible) en cuatro pastos tropicales utilizando la técnica de producción de gas. Las muestras de forraje completo (FC), el residuo insoluble en etanol al 90% (RIE) y FDN aislada (FDNa) se fermentaron in vitro y se midió la producción de gas. Los volúmenes de gas fueron determinados a partir de las siguientes fracciones, A= FC menos RIE; B1= RIE - DN; CNE= FC - DN; y B2= DN. Los pastos fueron Andropogon gayanus, Urochloa brizantha, Cynodon plectostachyus y Megathyrsus maximus, cada uno cultivado en Veracruz, México, en cuatro parcelas (5 × 5 m), fertilizadas (relación equivalente a 0 y 100 kg N / ha) y recortadas 35 días después de la fertilización con nitrógeno. Se utilizó un diseño de bloques completamente aleatorizado con arreglo factorial 4 × 2 y dos repeticiones por tratamiento. Los factores fueron las especies de gramíneas y la fertilización con nitrógeno. Los datos se ajustaron utilizando un modelo exponencial simple con retraso. El volumen (ml de gas / 100 mg de MO), la tasa (% / h) y el retraso (h) fueron: FE (22.8; 5.3; 2.1); A (3,2; 15,7; 0,5); B1 (1.5; 15.7; 0.2); y B2 (18.3; 6.6; 5.2). Andropogon y Urochloa tuvieron mayor contenido de CNE en comparación con Megathyrsus y Cynodon, pero menor rendimiento de gas por unidad de CNE. Las tasas de digestión para la fracción B2 oscilaron entre 4 y 8% / h; y la tasa de digestión CNE promedió 15.7% / h. La fertilización nitrogenada redujo el tamaño de las reservas de carbohidratos, pero no afectó las tasas de digestión. Se concluye que las tasas de digestión de las fracciones de carbohidratos difieren según la especie de pasto.

Abstract: The objective was to determine in vitro the NDF insoluble protein (NDIP) extension and degradation rate of four tropical grasses by the potential effect of N fertilization. The grasses (Andropogon gayanus, Brachiaria brizantha, Cynodon plectostachyus and Megathyrsus maximus) that grow in Mexico were used. Each grass was grown in four plots (5×5 m), fertilized (relationship equivalent to 0 and 100 kg N/ha) and clipped 35 d after the N fertilization. A complete randomized block design with factorial arrangement 4×2, and two replicates per treatment was used, where the factors were grass species and N fertilization. Non-protein nitrogen (NPN), buffer insoluble protein (IP), NDIP and acid detergent insoluble protein (ADIP) were performed. Freeze-dried samples were incubated at 0, 1.5, 3, 6, 9, 12, 24, 48 and 96 h. After fermentation, the CP content of the NDF residues was determined. An exponential equation was used to determine the rate of the NDIP disappearance. There was no detectable interaction between type of grass and fertilization level. The NDIP (as %CP) averaged 35 % with a range of 10 to 60 %. The NDIP variation was primarily due to species. The extent and rates of degradation of the NDIP were 70.6 % and 7.1 %/h respectively, with no N-fertilization effect. The NDIP was degraded faster (P≤0.05) than NDF (7.7 vs 5.0 %/h). These data show that the NDIP is ruminally degraded and that this fraction significantly contributes to the rumen nitrogen supply.

Resumen: El objetivo fue determinar la tasa y extensión de degradación ruminal in vitro de la proteína insoluble en fibra detergente neutro (NDIP) de pastos tropicales por efecto de la fertilización nitrogenada. Se analizaron los pastos Andropogon gayanus, Brachiaria brizantha, Cynodon plectostachyus y Megathyrsus maximus. Cada pasto se estableció en cuatro parcelas (5×5 m), se fertilizó (a una relación equivalente a 0 y 100 kg N/ha) y se muestreó cada 35 días. El análisis estadístico fue un diseño en bloques completos al azar en un arreglo factorial 4×2 y dos réplicas por tratamiento. Los factores fueron la especie y la fertilización. Se determinó el nitrógeno no proteico, proteína insoluble en buffer, NDIP y proteína insoluble en fibra detergente ácido (ADIP). Las muestras liofilizadas de los pastos se incubaron por 0, 1.5, 3, 6, 9, 12, 24, 48 y 96 h. Después de la fermentación, se determinó PC en la FDN residual y se calculó la tasa de desaparición de NDIP con una ecuación exponencial. No se detectó efecto de interacción entre especie y fertilización. La NDIP (%PC) promedió 35 % con rango de 10 a 60 %. La variación de NDIP fue debido a la especie. La tasa y extensión de la degradación de NDIP fueron del 7.1 %/h y 70.6 % respectivamente, sin efecto de la fertilización. La tasa de degradación fue más rápida (P≤0.05) en la fracción NDIP que en la FDN (7.7 % vs 5.0 %/h). Los resultados muestran que parte del NDIP es degradado en rumen, pudiendo aportar N para la actividad microbiana.

O uso do Sistema de Carboidrato e Proteina Líquidos da Universidade de Cornell (CNCPS) tanto para produção de leite como carne tem aumentado durante o últimos anos nas regiões tropicais. Entretanto, a falta de uma caracterização adequada de alimentos tem restringido o seu uso corretamente. Esse trabalho teve como objetivo principal o desenvolvimento e a avaliação de uma tabela de composição de alimentos utilizados nas condições tropicais. Os dados da composição desses alimentos foram baseados nas informações necessárias para o uso do modelo CNCPS desenvolvido pela Universidade de Cornell, USA. A composição desses alimentos foi obtida através de análises realizadas em laboratórios e de experimentos publicados em revistas científicas. Os nutrientes digestíveis totais (NDT) estimados através da composição de carboidratos e proteina dos alimentos pela equação de Weiss et al. (1992) e pelo modelo CNCPS foram comparados com os valores da tabela. O NDT estimado ao nível de mantença (1x) com o modelo CNCPS obteve valores próximos aos estimados pela equação de Weiss et al. (1992) (r² = 92.7% e bias = 0.8%). Entretanto, o r² da regressão entre os valores de NDT da tabela e o estimado pelo CNCPS e por Weiss foram menores (58.1 e 67.5%, respectivamente). Uma comparação completa entre os valores observados e preditos não foi possível devido a falta de caracterização dos alimentos conforme necessário pelos modelos testados. Quando os valores médios de literatura foram utilizados, a correlação entre o NDT estimado e o observado foi muito baixa. Concluímos que os valores de NDT estimados por Weiss e modelo CNCPS fornecem melhores estimativas de NDT do que os valores de tabela. A maioria dos trabalhos publicados que foram avaliados nesse estudo raramente incluíam informações necessárias para modelos como o CNCPS.

The Cornell Net Carbohydrate and Protein System (CNCPS) model has been increasingly used in tropical regions for dairy and beef production. However, the lack of appropriate characterization of the feeds has restricted its application. The objective of this study was to develop and evaluate a feed library containing feeds commonly used in tropical regions with characteristics needed as inputs for the CNCPS. Feed composition data collected from laboratory databases and from experiments published in scientific journals were used to develop this tropical feed library. The total digestible nutrients (TDN) predicted at 1x intake of maintenance requirement with the CNCPS model agreed with those predicted by the Weiss et al. (1992) equation (r² of 92.7%, MSE of 13, and bias of 0.8%) over all feeds. However, the regression r² of the tabular TDN values and the TDN predicted by the CNCPS model or with the Weiss equation were much lower (58.1 and 67.5%, respectively). A thorough comparison between observed and predicted TDN was not possible because of insufficient data to characterize the feeds as required by our models. When we used the mean chemical composition values from the literature data, the TDN predicted by our models did not agree with the measured values. We conclude using the TDN values calculated using the Weiss equation and the CNCPS model that are based on the actual chemical composition of the feeds result in energy values that more accurately represent the feeds being used in specific production situations than do the tabular values. Few papers published in Latin America journals that were used in this study reported information need by models such as the CNCPS.