Abstract 500

Dixon,-RM; Parra,-R

Inst. Produccion Animal, Facultad de Agronomia, Univ. Central de Venezuela, Maracay, Venezuela.

Tropical-Animal-Production. 1984, 9: 1, 68-80; 30 ref.

Three castrated zebu cattle of 286 to 344 kg were given mature Pennisetum purpureum forage either freshly chopped or after treatment with alkali (DM basis 3.6% NaOH + 0.9% urea mixed in 25 litres water/100 kg fresh forage), each without or with 3.6 kg DM of concentrate (90% maize flour residue + 10% cottonseed meal) in a randomized block experimental design. Animals were given daily 100 g minerals, mainly dicalcium phosphate, and the animals receiving untreated forage had salt 50 g in addition to the minerals. Forage DM intake was decreased by 31% owing to HaOH treatment, and by 34% owing to addition of concentrates. Rates of DM digestion (T 1/2) of 8 feeds (P. purpureum forage without or with 5.2% NaOH, Cenchrus ciliaris hay, maize cobs with none, 4 or 8% NaOH, sorghum grain and maize flour residue) were estimated using nylon bags, and rumen liquid samples were taken before and at 4 h and 8 h after feeding to estimate pH, ammonia and VFA concentration and proportions. The rate of rumen DM digestion was significantly different for each of the 8 feeds incubated in the rumen except for ground sorghum grain (T 1/2 18.2 h) and maize flour residue (T 1/2 15.0 h). NaOH treatment of the incubated feed increased rate of DM digestion of both P. purpureum forage (T 1/2 66.6 h and 35.7 h) and maize cobs (T 1/2 80.5, 54.5 and 28.2 h), while T 1/2 of C. ciliaris hay was 47.2 h. NaOH-treated forage did not affect rate of DM digestion or the other measures of rumen fermentation. Feeding concentrates reduced rumen pH from 6.5 before feeding to 5.9 after feeding, reduced the proportion of acetate, increased the proportions of propionate and butyrate, and increased total VFA concentration. Rates of DM digestion of maize cobs treated with the different levels of NaOH were reduced much more (57%) than those of the other incubated feeds (20%) in response to feeding concentrates. The results are consistent with the frequently observed depression of fibre digestion with reduction in rumen pH owing to addition to the diet of readily fermentable carbohydrate, whereas NaOH treatment of dietary forage was not associated with a rumen environment detrimental for the fibre digestion. Concentrate supplementation had a small but significant effect in increasing the proportion of large-particle DM in the rumen.