Scotland's Rural College Short- and long-term effects of conventional and artificial rearing strategies on the health and performance of growing lambs Belanche,

15 Artificial rearing of young animals represents a challenge in modern ruminant production 16 systems. This work aims to evaluate the shortand long-term effects of the type of 17 rearing on the animal ́s health, growth, feed utilization and carcass performance. 18 Twenty-four pregnant ewes carrying triplets were used. Within each triplet set, lambs 19 were randomly allocated to one experimental treatment: natural rearing on the ewe 20 (NN); ewe colostrum for 24h followed by artificial rearing with milk replacer (NA); and 21 50g of colostrum alternative supplementation followed by artificial rearing (AA). Milk 22 replacer, ryegrass hay and creep feed were offered ad libitum and each experimental 23 group was kept in independent pens until weaning at 45d of age. After weaning all 24 lambs were placed together on the same pasture for fattening for 4 months. Blood 25 samples were taken at 24h after birth, at weaning and at the end of the fattening period 26 (23 weeks). Results showed that no failure in the passive immune transfer was detected 27 across treatments. Although artificially reared lambs at weaning had lower plasma 28 levels of β-hydroxy-butyrate (-62%), HDL (-13%) and amylase (-25%) and higher levels 29 of LDL (+38%) and alkaline phosphatase (+30%), these differences disappeared during 30 the fattening period. Only the greater levels of calcium and the lower levels of 31 haemoglobin and white blood cells detected at weaning in artificially reared lambs 32 (+7.2%, -2.8% and -17.8%) persisted by the end of the fattening period (+4.3%, -3.3% 33 and -9.5%, respectively). Minor diarrheal events from weeks 2 to 5 were recorded with 34 artificial rearing, leading to lower growth rates during the first month. However, these 35 artificially reared lambs caught up towards the end of the milk feeding period and 36 reached similar weaning weights to NN lambs. During the fattening period NN lambs 37 had a greater growth rate (+16%) possibly as a result of their greater early rumen 38 development which allowed a higher feed digestibility during the fattening period in 39 comparison to NA lambs (+5.9%). As a result, NN lambs had heavier final body weights 40 (+7.0%), but tended to have lower dressing percentage (-5.7%) than artificially reared 41 lambs, thus no differences were noted in either carcass weight or in carcass 42 conformation across treatments. In conclusion, the use of a colostrum alternative and 43 milk replacer facilitated the successful rearing of lambs, reaching similar productive 44 parameters; however special care must be taken to maximize the rumen development 45 before weaning. 46