based on the Schwarz Bayesian Information Criterion. Dry matter intake was 17.7 ± 44 1.83 kg/day, milk production was 27.0 ± 4.64 kg/day, and methane production was 45 21.5 ± 1.69 g/kg DM. Milk C8:0, C10:0, C11:0, C14:0 iso, C15:0 iso, C16:0 and 46 C17:0 anteiso were positively related (P<0.05) to methane (g/kg DM intake), whereas 47 C17:0 iso, cis-9 C17:1, cis-9 C18:1, trans-10+11 C18:1, cis-11 C18:1, cis-12 C18:1 48 and cis-14+trans-16 C18:1 were negatively related (P<0.05) to methane. Multivariate 49 analysis resulted in the equation: methane (g/kg DM) = 24.6 ± 1.28 + 8.74 ± 3.581 × 50 C17:0 anteiso -1.97 ± 0.432 × trans-10+11 C18:1 -9.09 ± 1.444 × cis-11 C18:1 + 51 5.07 ± 1.937 × cis-13 C18:1 (individual FA in g/100 g FA; R 2 = 0.73 after correction 52 for experiment effect). This confirms the expected positive relationship between 53 methane and C14:0 iso and C15:0 iso in milk FA, as well as the negative relationship 54 between methane and various trans-intermediates, particularly trans-10+11 C18:1. 55However, in contrast with expectations, C15:0 and C17:0 were not related to methane 56 production. Milk FA profiles can predict methane production in dairy cattle. 57

The ruminant gastrointestinal tract (GIT) faces the challenge of protecting the host from luminal contents and pathogens, while supporting the absorption and metabolism of nutrients for growth and maintenance. The GIT of the calf in early life undergoes some of the most rapid microbial and structural changes documented in nature, and these adaptations in GIT function make the young calf susceptible to GIT diseases and disorders. Despite these challenges, the calf's GIT has a certain degree of plasticity and can sense nutrient supply and respond to bioactive ingredients. Calf GIT research has historically focused on the transition during weaning and characterizing ruminal papillae development using microscopy and digesta metabolite responses. Through the use of new molecular-based approaches, we have recently shown that delaying the age of weaning and providing a step-down weaning protocol is associated with a more gradual shift in ruminal microbiota to a postweaned state. In addition to ruminal adaptations during weaning, nutrient flow to the lower gut changes dramatically during weaning, coinciding with a wide array of structural and microbiological changes. Structural and gene expression changes suggest that the lower gut of the dairy calf undergoes alterations that may reduce barrier function when solid feeds are consumed. More recently, in vivo data revealed that the weaning transition increases total gut permeability of the calf. Interestingly, the lower gut may be able to communicate with the forestomach, meaning that a nutrient can be sensed in the lower gut and cause subsequent adaptations in the forestomach. An improved understanding of how diet, microbiota, and functional ingredients interact to affect growth and barrier function of the intestinal tract would greatly benefit the dairy calf industry. A mechanistic understanding of such adaptations would also aid in the formulation of specific management regimens and provision of functional ingredients required to characterize and enhance gut function in young calves.

The objective of this study was to investigate how preweaning plane of milk replacer intake and age can affect insulin and glucose kinetics as well as abomasal emptying rate in dairy calves fed twice a day. A total of 12 female Holstein Friesian calves were blocked by cow parity, paired by colostrum origin, and were randomly assigned to a high plane of milk replacer intake (8 L/d, 1.2kg of milk replacer/d; n=6) or a low plane of nutrition (4 L/d, 0.6kg of milk replacer/d; n=6). All calves received 4 L of colostrum over 2 meals (1 and 6h after birth) and were then directly transferred to their assigned feeding plans until they were stepped-down from milk by 50% during wk 7 and weaned on wk 8. Milk replacer (24% crude protein, 18% crude fat) was fed at 150g/L twice daily (0700 and 1700h) and all calves had ad libitum access to pelleted calf starter, chopped wheat straw, and water. Jugular catheters were placed in all calves at 4, 7, and 10wk of age. Then, postprandial response to plasma glucose, insulin, and acetaminophen (supplied with the meal) were determined to measure abomasal emptying. The next day, a glucose tolerance test was conducted by infusing glucose via the jugular catheter. At 4 and 7wk of age, the rate constant (%/h) for abomasal emptying of the meal was lower in high calves (0.21±0.02 in wk 4; 0.27±0.02 in wk 7) compared with low (0.34±0.02 in wk 4; 0.47±0.02 in wk 7). The postprandial plasma insulin area under the curve over 420min was greater in high calves (18,443±7,329; low=5,834±739 µU/mL) compared with low. We found no differences in glucose tolerance test kinetics between the high and low dairy calves at 4, 7, or 10wk of age. The findings from this study suggest that feeding dairy calves an elevated plane of nutrition in 2 meals of milk replacer per day does not decrease insulin sensitivity.

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The experiment was designed to study the importance of early rumen development and of the composition of solid feed intake on growth performance and abomasal health in milk-fed veal calves. One hundred and six Holstein-Friesian male calves were included in the experiment, and studied during 2 successive 12-wk periods (period 1 and period 2). In a 2 × 2 factorial arrangement, effects of partially replacing milk replacer by solid feed during period 1 and partially replacing dry matter (DM) intake from maize silage and barley straw by concentrate during period 2 were tested. Solid feed during period 1 consisted of maize silage, barley straw, and concentrate (25:25:50 on a DM basis). Solid feed during period 2 consisted of maize silage and barley straw (50:50 ratio on DM basis) for the nonconcentrate groups, and maize silage, barley straw and concentrates (25:25:50 on a DM basis) for the concentrate groups. At the end of period 1 (n=16) and at the end of period 2 (n=90), parameters of animal performance, rumen development, rumen fermentation, ruminal drinking, and abomasal damage were examined. Partially replacing milk replacer by solid feed during period 1 resulted in early rumen development (ERD) at the end of period 1, characterized by increased rumen weight, and an increased epithelial and absorptive surface area. Both ERD and partially replacing roughage by concentrates in period 2 increased the rumen development score at the end of period 2. Although ERD calves consumed more solid feed and less milk replacer during period 1 and 2 than non-ERD calves, carcass weight gains at 25 wk were identical, and utilization of the solid feed provided appeared similar to that of milk replacer. Partially replacing roughage by concentrates in period 2 increased dressing percentage and warm carcass weight. Plaque formation at the rumen mucosa was unaffected by ERD or partially replacing roughage by concentrates and generally low in all calves. The prevalence of large scars in the abomasum in ERD calves was decreased compared with non-ERD calves. This may indicate that ERD provided protection against abomasal lesions. In conclusion, early compared with late rumen development improves feed utilization and may be beneficial for abomasal health.

Current calf milk replacer (CMR) compositions significantly differ from whole milk in their levels of energy, protein, and minerals. Energy source is one of the major differences, as CMR contains high levels of lactose, whereas whole milk contains higher levels of fat. The aim of this study was to determine the effect of partially exchanging lactose for fat on performance, digestibility, and gut permeability in calves fed twice daily on a high feeding plane. Lactose and fat were exchanged in the CMR formulation on a weight-weight basis. The CMR were isonitrogenous but not isoenergetic. A total of 60 male Holstein-Friesian calves were assigned to 1 of 30 blocks based on serum IgG, body weight, and date of collection after birth. Within each block, calves were randomly assigned to 1 of 2 treatments: high fat and high lactose. The CMR was provided twice daily until 49 d of age, followed by a gradual weaning period of 14 d. Starter, straw, and water were available ad libitum throughout the complete study. Exchanging lactose for fat did not affect growth; intakes of starter, straw, water, crude protein, or total energy; or apparent total-tract digestibility of nutrients. Gastrointestinal permeability was assessed by measuring the recovery of lactulose and Cr in 24-h urine and the Cr concentration and lactulose: d-mannitol ratio in serum following an oral pulse dose. Urinary recoveries of Cr and lactulose were generally low in both treatments but were higher in calves fed the high-fat CMR. Accordingly, the serum lactulose: d-mannitol ratio and serum Cr concentrations were higher in calves fed the high-fat CMR. In wk 1 and during the weaning transition, calves fed the highfat CMR had significantly fewer abnormal fecal scores. In conclusion, exchanging lactose for fat in the CMR did not affect growth performance, total feed intake, or nutrient digestibility. The high-fat CMR was associated with an increase in permeability markers but positively influenced fecal scores in calves.

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Fermentation, rather than enzymatic digestion, is the main reason for small intestinal starch disappearance in milk-fed calves.