School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemThe nutritive value of white clover pasture for cows in mid-lactation when supplemented with maize silage( 1992)In a completely randomised grazing trial, 8 cows in mid-lactation were offered white clover pasture (35kg DM/cow/day) with 4 of the cows given a supplement of maize silage (8 kg DM/cow/day). Factors evaluated over a 28 day period were total DMI, pasture DM/maize silage DM substitution rate, herbage selection in the pasture, milk yield, fat and protein concentration. Supplementing cows with maize silage increased total DMI by 22% and lowered pasture herbage DMI by 21%; equivalent to a substitution rate of 0.5 kg pasture DM /kg of maize silage DM. Selection of herbage in the pasture was not affected by supplementation, but cows on both treatments selectively grazed the white clover. Supplementing cows with maize silage increased milk yield by 34%, fat yield by 35% and protein yield by 35% (P<0.05), but did not affect milk fat and protein concentration. In a metabolism trial, 6 rumen fistulated cows in mid-lactation were fed similar diets in a crossover experiment but with white clover fed ad libitum. Digestibility, nitrogen balance, milk yield, fat and protein concentration were evaluated over a 7 day period. Maize silage DM and white clover DM digestion in the rumen were evaluated by DM disappearance rates from nylon bags over a 96 hr period. Measurments of rumen fluid volume, fluid mean retention time, fluid outflow, pH, ammonia and VFA concentration and the acetate/propionate ratio were estimated over a 24 hr period. Supplementing cows with maize silage increased total DMI by 34%, without affecting DM digestibility (DMD). However, OMD was reduced by 5% and fibre digestion by 18% (P<0.05). Supplementing cows with maize silage did not affect the rate of digestion of maize silage DM or white clover DM in nylon bags, but white clover was digested at a faster rate than maize silage (P<0.05). The increased DOMI by cows supplemented with maize silage was highly correlated with a 24% increase in milk yield (P<0.05; iI=61 ) but had no effect on milk fat and protein concentration. At similar nitrogen intakes cows supplemented with maize silage produced 2.3 kg more milk (P<0.050='66) but there was no effect on milk fat and protein concentration. Rumen fluid volume, mean retention time and fluid outflow rate were not significantly altered by the maize silage supplement, although rumen pH and ammonia concentration were significantly lower (P<0.05). Total VFA concentration was not significantly different between the two diets, but acetate concentration was significantly lower (P<0.05), while propionate concentration was significantly higher (P<0.05) in supplemented cows. The reduction in acetate and the corresponding increase in propionate, lowered the acetate/propionate ratio. Butyrate concentration was not affected. It was concluded that for cows in mid-lactation fed on good quality white clover dominant pasture, maize silage supplementation can increase milk yield, without affecting milk fat and protein concentration. The increase in milk yield was attributed to increased apparent energy intake as nitrogen intake did not change markedly. However further work is required to quantify whether increased nitrogen utilization occurred and whether the lower ammonia concentration in maize silage supplemented cows were due to improved rumen microbial synthesis. The possibility also exists that improved milk yields were due to by-pass rumen starch, thereby providing glucose to the animal, after enzymatic digestion in the small intestine.