Voluntary intake and rumen function of Zebu bulls (Bos indicus) given tropical forages
Three experiments carried out in the southeast of Mexico (Yucatan) are reported. In the first, yield of P. purpureum increased from 3,760 to 14,526 kgDM/ha with age 6 weeks (young) and 28 weeks (mature). Maturity increased lignin from 43 to 108g/kgDM. Potential degradabilities of leaf and stem were 788g & 782g/kgDM with young, and 725 & 403g/kgDM with mature. Young yielded more potentially degradable DM (70 & 35kgDM/ha/d) than mature. In the second, molasses urea block (MUB) supplementation of bulls grazing during the dry season, did not improve the potential degradability of pasture (525g/kgDM). Estimated pasture intake was 68gDM/kgW0.75 /d, and measured rumen pool size of grazing bulls was 79 gDM/kgW 0.75. Intake of MUB was low (236gDM/d) and only increased rumen NH 3-N from 100 to 128mg/l. Availability of pasture was 255kg DM/animal. It is suggested that need of supplementation could be low if there is sufficient pasture to allow selective grazing. In the third, hay diets of young or mature P. purpureum were studied in six zebu (Bos indicus) bulls fed ad libitum. Intakes were, 91 and 44gDM/kgW0.75/d, digestibilities, 591 and 453g/kgDM, estimated energy intakes 768 and 307kJME/kgW0.75/d and Metabolizable Protein, 4.1 and 1.3g/kgW0.75/d with young and mature forages respectively. Forage maturity resulted in negative N balance (352 vs -113mgN/kgW0.75/d), decreased potential (756 vs 568g/kgDM) and fractional rate (0.07 vs 0.03%) of rumen DM degradation, but gave higher DM degradabilities of the non-soluble fraction (378 vs 283g/kgDM). Forage maturity reduced rumen outflow rates of liquid (0.09 vs 0.05%), and solid (0.038 vs 0.015%) phases, increasing mean retention times of digesta (22 vs 65h), and the rumen pool size (84 vs 120gDM/kgW0.75) and reduced the eating (380 vs 273min) and rumination daily times (433 vs 398min). The Purine Derivatives (PD) excretion in urine reflected differences between diets (651 vs 354mMol/kgW0.75/d). However, rumen Microbial N Supply estimated by the PD method was very low in comparison with estimates from digestibility and N balance measurements. It is suggested this effect could be explained by greater recycling of PD to the rumen and a lower endogenous excretion by zebu cattle.