Two experiments using the Cornell Net Carbohydrate and Protein System were conducted to characterize the carbohydrate and protein fractions and corresponding rates of digestion of 15 tropical pasture grasses and to evaluate their ability to support milk production by dual-purpose cows. In the first experiment, ranges in carbohydrate and protein fractions of 15 grasses at 35 to 42 d of regrowth were: neutral detergent fiber (NDF) 63.5 to 74.9% of DM; permanganate lignin 4.7 to 7.8% of NDF; CP 5.5 to 11.9% of DM; and soluble protein 15.1 to 44.1% of crude protein (CP). The ranges of rates of digestion expressed as percent per hour were neutral detergent solubles (7.5 to 27.4); NDF (3.8 to 8.4); and neutral detergent insoluble protein (2.9 to 9.5). Predictions of the amount of milk that could be produced based on the amount of metabolizable energy supplied by the diet decreased 35% when NDF increased from 60 to 80%, and increased 88% when the rate of digestion of NDF increased from 3 to 6%/h. The milk production that could be sustained based on metabolizable protein in the diet doubled as CP increased from 4 to 12%. In the second experiment, nitrogen fertilization reduced NDF 7.3% and increased CP 84% without changing protein solubility, resulting in increased rumen nitrogen and metabolizable protein balances. With all forages, the Cornell Net Carbohydrate and Protein System predicted that availability of metabolizable protein would limit milk production. Predicted microbial growth was limited by ruminally available protein rather than by available carbohydrate.

Nota de investigación In vitro ruminal degradation of neutral detergent fiber insoluble protein from tropical pastures fertilized with nitrogen Degradación ruminal in vitro de la proteína insoluble en fibra detergente neutro de pastos tropicales fertilizados con nitrógeno Francisco Indalecio Juárez Lagunes a*

Constantemente los forrajes deben realizar procesos de adaptación a los diferentes cambios en los factores ambientales lo cual conlleva a que sus variables fisiológicas se vean afectadas. El objetivo fue evaluar el comportamiento fisiológico de siete especies forrajeras tropicales en diferentes condiciones ambientales durante 2019 en Palmira, Colombia. Se implementó un diseño de bloques completos al azar, en tres localidades con diferentes altitudes, con cuatro repeticiones por especie para un total de 28 unidades experimentales por localidad. Se utilizó un analizador de medición fotosintético portátil Lcpro+, para la toma de datos de la tasa de fotosíntesis, conductancia estomática, tasa de transpiración y CO2 interno. Los datos recolectados fueron analizados mediante una comparación de medias con la prueba de Duncan (p≤ 0.05). Los indicadores fisiológicos de la especie Estrella, Kikuyo y la leguminosa Centrosema molle en las tres altitudes y las dos épocas demostrarían los mecanismos de adaptación que estas desarrollan en lugares adversos a los que se establecen.