Spatio-temporal evaluation of the nutritive value of Croton cortesianus and Leucophyllum frutescens through in vitro fermentation kinetics
The aim of the study was to evaluate, seasonally, two native shrubs which are commonly selected by white-tailed deer in a semiarid and subtropical area of Northeastern Mexico. Foliage from Croton cortesianus and Leucophyllum frutescens was sampled from Summer 2004 to Spring 2005 in two country sites: China and Linares in the state of Nuevo Leon, Mexico. In vitro gas production was recorded at 3, 6, 9, 12, 24, 48, 72 and 96h. Kinetic parameters such as the asymptotic gas production (B), rate of gas production (k) and lag phase (L) were estimated by the exponential model G = b × (1 − e−k(t−L)). Microbial protein synthesis, ME content and in vitro organic matter digestibility (IVOMD) were also evaluated. Data were analyzed according to a completely randomized design with factorial arrangement. The factors were sampling sites, shrub species and seasons. Kinetic parameters significantly varied among shrubs, sites and seasons (P < 0.001). The asymptotic gas production (B) ranged from 127 ml of gas/g DM in C. cortesianus in summer 2004 in China country to 237 ml of gas/g DM in C. cortesianus in spring 2005 in Linares country. The rate of gas production (k) was the lowest (P < 0.05) in C. cortesianus in summer 2004 (0.035 %/h) while the highest (0.144 %/h) was collected in autumn 2004 in Linares. Values regarding L ranged from 0.05 to 2.89 h, and were superior (P < 0.05) in C. cortesianus during autumn 2004 in Linares country. Interactions sites x shrub species x seasons were significant (P < 0.001) for all kinetic parameters. Microbial protein synthesis measured as purines varied significantly among shrubs and seasons. Mean values ranged from 1.51 to 11.36 µmol, and were lowest (P < 0.05) in C. cortesianus in spring 2005 in Linares and highest in L. frutescens in autumn 2004 in China. The ME content varied from 0.36 to 2.71 Mcal/kg DM (P < 0.05). Estimates of IVOMD ranged from 46.4 to 89.7%. C. cortesianus had the greatest IVOMD values in summer 2004 in China (P < 0.05). Data suggested that although spatio-temporal fluctuations, variables such as the constant rate of gas production, which might indicate nutrient availability for rumen microorganisms (overall mean = 0.079 %/h), digestibility (overall mean = 70.2 %) and microbial protein synthesis (overall mean = 7.2 µmol) support the nutritive potential of C. cortesianus and L. frutescens for white-tailed deer in marginal semiarid regions of northeastern Mexico.
Keywords: Gas production, native shrubs, semiarid regions