NUTRITIVE VALUE, In Vitro FERMENTATION AND SECONDARY METABOLITES OF WEEDS AND MAIZE STRAW USED FOR FEEDING DAIRY CATTLE

Abstract

In the highlands of Central Mexico a surplus of different forages is observed during the rainy season particularly weeds, which grow in maize fields. Weeds are widely used by farmers to feed dairy cattle. The objective of the present work was to determine the nutritive value of weeds, their content of secondary metabolites, and their effect on in vitro fermentation kinetics when included (at different levels of inclusion) in a diet based on maize straw. The present study was carried out in two regions of the Toluca valley from August to October 2007. A split plot design was used to evaluate the variables associated with the nutritive value and a randomized design was employed to evaluate the content of secondary metabolites in the different weed species. Significant differences (P<0.001) were observed in the CP content associated with the maturity stage of the plants, P3 being different from the other two periods. Significant differences (P<0.01) were observed in NDF content for both species and treatments. The highest NDF content was observed in Echinochloa oplismenoides. For the case of treatments the content of NDF increased linearly as the inclusion levels of maize straw augmented. The soluble carbohydrate fraction (a) was highest (P<0.05) in Tridax coronopifolia, Tripogandra purpuracens y Drymaria laxiflora. The insoluble but potentially degradable fraction (b) was significantly different for species (P<0.01), treatments and periods (P<0.001). Gas production increased linearly with growing inclusion levels of maize straw, on the contrary the fermentation rate of fraction b (cb) decreased (0.04 to 0.02) at high inclusion levels. The cb rate increased (P<0.001) from period 1 to period 3. The lag period by species ranged from 6.45 h in Bidens odorata to 12.84 h in Tripogandra purpuracens. Weed species like Drymaria laxiflora, Tithonia tubiformis, Oxalis divergens, Tripogandra purpuracens y Simsia amplexicaulis showed low tannins phenolics content, so it is suggested that they can be used as forage to feed dairy cattle. It is concluded that the inclusion of weeds in dairy cattle diets based on maize straw increases CP content and improves fermentation kinetics, particularly the a fraction and the fermentation rate of the b fraction.