The effect of weaning age on gastrointestinal epithelial development of calves in New Zealand : a histomorphological analysis : a thesis presented in partial fulfilment of the requirements for the degree of Master of Animal Science at Massey University, Palmerston North, New Zealand

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Dairy-beef calves born on New Zealand dairy farms are artificially reared and often weaned earlier than would occur under cow-calf beef production systems. Most weaning studies have focused on calves reared on concentrated diets for indoor systems and early (6-12 weeks of calf age) weaning systems. Weaning studies on artificially reared calves destined for grazing are limited and the effects of weaning age on gastrointestinal tract (GIT) development of dairy-beef calves reared on pasture are not known. This study evaluated the effects of milk-weaning at 10 (early) vs 20 (late) weeks of age on the histomorphology of the GIT of dairy-beef calves until 30 weeks of their age. The objectives of this study were to (1) evaluate differences in GIT epithelial development of early and late weaned calves and (2) evaluate whether differences (if any) persisted post-weaning. Male Hereford-Friesian calves (n=72) were sourced from two commercial dairy farms at three weeks of age and allocated to two treatments: early and late weaned. From the beginning of the study, all calves were kept outside (six calves per paddock; six paddocks per treatment) with free access to pasture (ryegrass clover mixed). A commercially purchased milk replacer (1 kg/day/calf; diluted in water to 7 L) was divided into two equal portions and fed twice daily using group feeders. Calves were weaned with a gradual reduction in milk replacer allowance beginning three weeks before designated weaning age. All calves were kept in their respective paddocks until humanely killed. Calves from each treatment were slaughtered across three different time points: 10 weeks, 20 weeks, and 30 weeks of age. At each time point 12 calves from each treatment (n=24) were slaughtered. Tissue samples from the rumen, duodenum, jejunum, and caecum were collected, processed to make histological slides, and analysed for histomorphological differences. The effect of slaughter age was significant for rumen ventral sac papillae length, duodenum villus width, jejunum crypt depth, and jejunum layer depth (essentially a combination measurement of villi length and crypt depth). The effect of weaning age was significant for caecum crypt depth, with early weaned calves having increased crypt depth across slaughter age. The interaction of weaning age and slaughter age was significant for rumen ventral sac papillae length, with early weaned calves having significantly greater papillae lengths (P<0.05) at slaughter week 20, but this difference did not persist post weaning (30 weeks). This study successfully reared and weaned calves at 10 and 20 weeks of age onto a pasture only diet without any major influence on histomorphology of their GIT observed.
Figure 3.2 (= Van Keulen et al., 2020 Fig 1) & Figures 3.3 and 3.4 (=Adelman et al., 2018 Fig 2 c & d and Fig 2 f respectively) were removed for copyright reasons.