Intake of young and adult cattle co-grazed with sheep : A thesis submitted in partial fulfilment of the requirement for the degree of Master of Agricultural Science at Lincoln University

An experiment was conducted to investigate possible differences in DM intake and diet selection between groups of calves and cows grazed alone (YCA and ACA) or with lambs (YC+S and AC+S) in a ratio 1:1 (based on W¾) on a temperate ryegrass/white clover pasture for 11 days. Treatments were compared at similar rates of daily disappearance of pasture mass (PM) (RDPM). Digestible DM intake (DDMI) was measured by faecal output/diet DM digestibility (DMD) method . Faecal output was estimated from chromic oxide dilution. Marker recovery rate was estimated from total faecal co llection obtained in an indoors trial using similar animals . Samples of ingesta for botanical composition and in vitro diet DMD analysis were retrieved from oesophageally fistulated cattle. Herbage was sampled from each treatment every 2 days to determine botanical composition and DM digestibility of the sward. Pasture botanical composition changed differently among treatments with grazing . Data suggest an influence of animal size (cattle) on pasture changing composition , with calves presenting a greater change for grass and dead matter than cows. However, treatment did not show a clear effect on mean pasture DMD. The diet of calves showed a slightly greater change in DMD during the grazing period than cows (-17, -22, -14 and -21 DMD units % from an initial diet DMD of 70, 76 , 72 and 75 t for YCA, YC+S , ACA and AC+S respectively ) which is consistent with the botanical composition change f or these same treatments. Digestible DM intake (DDMI, g/kgW¾) declined with reducing PM for all groups of cattle but ACA showed a greater rate of decline than YCA (12 and 7 g DDMI/kgW¾tDM respectively), suggesting a greater ability of calves to cope with the increasing grazing pressure. The rate of decrease in DDMI with PM for mixed grazed cattle was greater for AC+S than for YC+S (27 and 15 g/kgW¾/tDM respectively). The net effect of mixed grazing on mean DDMI over the period was an increase of 28% for calves (61.7 and 48.3 g DDMI/kgW¾/d for YC+S and YCA respectively) compared with no effect for cows (82 . 7 and 79 . 8 g DDMI/kgW"/d for AC+S and ACA respectively). Calves were found to be better able than cows to sustain a higher feed intake under pasture conditions which were deteriorating in both supply and quality when mixed grazed with shee