Rumen Development of Artificially-Reared Lambs Exposed to Three Different Rearing Regimens

SIMPLE SUMMARY: Young ruminants possess an undeveloped rumen when first born. Encouraging good rumen development in early life is vital for young ruminants as they transition to a solid feed diet, to ensure optimum growth post-weaning. This study aimed to investigate the effect of three different rearing regimens on rumen development in lambs reared artificially. Weaning of lambs at 42 d of age improved rumen fluid n-butyric content and also resulted in a thicker rumen dorsal wall. Feeding lambs pellets with low fibre and weaning them early increased rumen fluid n-valeric content. Papillae width, density, and rumen wall muscle thickness were affected by rearing treatment. Empty rumen weight, rumen volume, papillae development, and rumen fluid iso-butyric and iso-valeric content had positive associations with dry matter intake and nutrient intakes from solid feed. The majority of volatile fatty acids in the rumen fluid had a positive association with papillae height on the rumen dorsal wall. These results suggest that lamb diet and age at weaning influenced rumen function and physical development. Further studies examining how rumen microbial composition and rumen gene expression are influenced by rearing regimens are required. ABSTRACT: The objective of this study was to examine the effect of three different rearing regimens on rumen development in lambs reared artificially. Romney ram lambs were randomly allocated to one of three treatments: commercial milk replacer fed to 57 d of age and high fibre concentrate pellets (HFP57); commercial milk replacer, high fibre concentrate pellets, and early weaning from milk replacer at 42 d of age (HFP42); high protein milk replacer from 2–16 d of age followed by commercial milk replacer, low fibre concentrate pellets, and early weaning from milk replacer at 42 d of age (LFP42). Lambs were slaughtered at 57 d of age. Volatile fatty acid content in rumen fluid at slaughter was analysed and rumen tissue samples were collected for histological examination. The rumen n-butyric content was greater (p < 0.05) in both LFP42 and HFP42 treatment lambs compared to HFP57 lambs. The n-valeric content was greater (p < 0.05) in LFP42 lambs compared to both HFP57 and HFP42 treatment lambs. Thickness of the rumen dorsal wall determined by ultrasound scanning at 49 d was greater (p < 0.05) in both HFP42 and LFP42 lambs compared to HFP57 lambs. There was an interaction (p < 0.05) between treatment and site of rumen tissue sampling on papillae width, density, and rumen muscular layer thickness. Collectively, early weaning and the provision of a low fibre pellet leads to improved rumen function and physical development.