<p>Para cuantificar la degradación <em>in situ </em>de tejidos vegetales y su relación con la composición química de especies forrajeras se seleccionaron láminas foliares de las gramíneas <em>Bouteloua repens </em>y <em>Bothriochloa pertusa </em>y foliolos de las leguminosas <em>Stylosanthes scabra</em>, <em>Desmodium barbatum </em>y <em>Tephrosia cinerea</em>. En el rumen se incubaron muestras de 10 mm de largo durante 0, 12, 24, 48 y 72 horas; para la lectura del área residual se digitalizaron ocho tejidos. A las 24 y 48 h de incubación <em>B. pertusa </em>presentó relaciones positivas entre la degradación de la epidermis adaxial (EA) y la FDN (R2= 90,2), entre el mesófilo y la DIVMS (R2=80,1), y entre los tejidos lentamente degradables (TLD) y la FDA (R2= 83,9); y relaciones negativas entre EA y DIVMS (R2= –73,1), los TLD con la DEF (R2= –74,3), la EA con la PC (R2= –87,6), y el esclerenquima con la DIVMS y la PC (R2= –84,3 y R2= –90,8). Después de 72 horas de incubación las mayores áreas residuales en gramíneas fueron la estructura kranz entre 34,2% y 36,5%, el mesófilo entre 20,9% y 21,4%, el xilema de 11,5% a 17,1% y la EA entre 11,8% y 13,9%; no obstante el esclerénquima se relacionó estrechamente con indicadores de calidad nutricional. Las gramíneas obtuvieron mayores porcentajes de tejidos no degradables que las leguminosas; por lo tanto, la DIVMS y la DEF fueron menores. Entre las leguminosas, <em>S. scabra</em>, presentó la mayor digestibilidad de los tejidos rápidamente degradables, tanto en sequía como en lluvia; en la época seca se incrementó en 134,6% la degradación de estos tejidos. Adicionalmente, en esta especie el área residual de los taninos no presentó relación directa con la degradación del mesófilo. En leguminosas la tasa y extensión de la degradación de las epidermis se incrementaron marcadamente a mayor tiempo de incubación, presentándose diferencias entre especies, situación que no exhibieron las gramíneas. En <em>D. barbatum </em>la degradación de la EA se asoció con la DIVMS y la DEF, explicando en 77,4% y 72,95% estos porcentajes; de igual manera se reportaron relaciones negativas entre la degradabilidad de la epidermis y los porcentajes de FDN, FDA, lignina y taninos (R2= –0,76; R2= –0,79; R2= –0,53 y R2= –0,76, respectivamente).</p><p> </p><p><strong>Microbial <em>in situ </em>degradation of grasses and legumes leaf tissues and its realtionship with nutritional quality and precipitation</strong></p><p>A study was conducted to measure the <em>in situ </em>de- gradation of plant tissues and its relationship with forage chemical composition in leaves of the grasses <em>Bouteloua repens </em>and <em>Bothriochloa pertusa </em>and of the legumes <em>Stylosanthes scabra</em>, <em>Desmodium barbatum </em>and <em>Tephrosia cinerea</em>. Forage samples of 10 mm in length were incubated in the rumen for 0, 12, 24, 48 and 72 h and tissue residual area was determined by reading the residual area in eight digitalized leaf samples per forage. In <em>B. pertusa</em>, after 24 and 48 h of incubation there were positive relations between the degradation of adaxial epidermis (AE) and the neutral detergent fiber (NDF, R2 = 90.2), between mesophyll and in vitro dry matter degradability (IVDMD, R2 = 80,1), and between slowly degradable tissues (SDT) and the acid detergent fiber (ADF, R2 = 83.9); and negative relationships between AE and IVDMD (R2 = -73,1), between SDT and effective dry matter degradation (EDMD, R2 = -74,3), between AE and crude protein (CP, R2 = -87,6), and between sclerenchyma and IVDMD with CP (R2 = -84,3 and R2 = -90,8, respectively). After 72 h of incubation, the greater residual areas in grasses were the Kranz structure (34.2% to 36.5%), mesophyll (20.9% to 21.4%), xylem (11.5% to 17.1%) and AE (11.8% to 13.9%). The area of sclerenchyma was related closely to indicators of nutritional quality. Overall, the grasses had greater content of non-degradable tissues than the legumes; therefore, their IVDMD and EDMD were lower. Among the legumes, <em>S. scabra </em>showed the greater digestibility of rapidly degradable tissues, both during the rainy and dry seasons. At the dry season, the degradation of these tissues increased by 134.6%. Additionally, in this legume, the content of tannins did not have a direct relationship with the degradation of mesophyll. The rate and extension of the degradation of epidermis in legumes, were noticeably increased with increasing length of incubation, existing differences between species, a situation which was not observed in the grasses. In <em>D. barbatum</em>, the degradation of the AE was positively associated with both the IVDMD and EDMD (R2 = 77,4% and 72,95%, respectively). On the other hand, the degradability of epidermis and the percentage of NDF had negative relations with ADF, lignin and tannins (R2 = -0,76; R2 = -0,79; R2 = -0,53 and R2 = -0,76, respectively).</p>
Validation of an in vitro model for predicting rumen and total-tract fiber digestibility in dairy cows fed corn silages with different in vitro neutral detergent fiber digestibilities at 2 levels of dry matter intake
