Regrouping in commercial dairy herds often involves integrating new animals into an established group. This can be particularly stressful for first-lactation dairy heifers when integrated with the main dairy herd after calving as they are often subject to high levels of aggression and bullying. This, in turn, can have a negative effect on production performance. The aim of the research within this thesis was to identify the effect of different re grouping regimes on the welfare and productivity of first lactation" heifers when introduced to a group containing older cows. Experiment 1 examined the effects of introducing heifers to an established group after evening rather than morning milking, and found welfare benefits (in terms of reduced received aggression) in this approach. Experiment 2 determined the effects of mixing heifers with non-lactating cows during the pre-calving period on welfare and performance in the post-calving period. This study showed that these animals were subject to less aggression in the post calving period than those not mixed with older cows prior to calving. Experiments 3 and 4 examined the impact of giving heifers different levels of 'recovery' period after calving before integrating with an established group. Both keeping heifers in a separate heifer group for two weeks after calving, and even just extending the time period in a straw-calving pen, led to improvements in welfare. No effect on stress hormone levels or on productivity were shown in any of the studies. In all studies heifers lay for relatively short periods (i.e. less than 6 hours) during the first 24 hours after introduction to cubicle accommodation if this occurred relatively soon after calving. Lying time was increased if heifers were allowed to stay in straw pens during this period. It is suggested that future research should determine if pain causes reduced lying behaviour in freshly-calved heifers in cubicle accommodation.

The aim of this study was to evaluate the effects of M. elsdenii (Me) dosing on dairy heifer performance. A secondary set of data (feed intake, heifers birth weights, age and Weight at insemination, and first lactation milk performance) of heifers (dosed and not dosed with Me) was obtained from the dairy herd of the Agricultural Research Council – Animal Production. Data were arranged in a complete randomised design and analysed as repeated measures. Milk, pre-weaning starter and metabolised energy intake did not differ between the control and the Me groups. Post-weaning starter feed intake was higher (p=0.03) for Me fed heifers than control heifers. The post-weaning metabolisable energy intake was also higher (p=0.03) for heifer fed Me than control heifers. The average daily weight gain of heifers dosed with Me was higher during the pre-weaning period (0.66 kg/day; p=0.04) and after weaning (1.12 kg/day; p=0.03) compared to control (0.60 and 0.65 kg/day, respectively). At 42 and 70 days old, the BW of Me-heifers was greater (75.8 ± 2.6 and 91.2 ± 4.6 kg) than control heifers (61.9 ± 2.6 and 77.2 ± 4.6 kg) (p<0.05). There was no difference (P>0.05) in BW at insemination, number of insemination and milk yield between the two groups of cows (p>0.05). Early feeding of Me to heifers in the present study positively affect heifer growth during and early after milk feeding period, confirming previous report. Animal weight at puberty and the subsequent milk production were not influenced by feeding Me. It is possible that Me did not survive long after weaning to continue to express its influence on animal performance. / Agriculture and  Animal Health / M. Sc. (Agriculture)