摘要Background Forage inclusion in starters of young dairy calves has become an acceptable strategy in the last decade.To compensate for the lower energy provided by forage,concurrent lipid supplementation can be proposed.How-ever,ruminal microbial activity and forage digestibility may be decreased by lipid supplementation.We hypothesized that the composite effect of forage and lipid supplements may be dependent on forage particle size and the type of lipid supplement.Therefore,we evaluated the effect of long(LP;geometric mean,4.97 mm)vs.short alfalfa hay particle sizes(SP;geometric mean,1.26 mm)with either soybean oil(SBO)or palm fatty acids(PLF)as lipid source in a 2×2 factorial design with treatments SP-SBO,SP-PLF,LP-SBO,and LP-PLF.Treatments(n=13 with 6 males and 7 females each)were offered to Holstein calves(3 days old)with equal amounts of lipid(25 g/kg DM)through-out the experimental period.The milk offering scheme(d 1 to 53)was equal for all groups.Data collection continued until 20 d post-weaning.Results Interaction between forage particle size and lipid supplement was significant for the following readouts:the highest and lowest starter intakes during the pre-weaning period occurred in LP-PLF and LP-SBO,respectively.This was associated with similarly contrasting changes in average daily gain(ADG)during the post-weaning period,body weight at the end of experiment,withers height,digestibility of organic matter and neutral detergent fiber,and blood serum concentrations of glucose,beta-hydroxybutyrate,and insulin during the pre-weaning period.Dur-ing both pre-and post-weaning periods,the highest and lowest urinary excretion of allantoin and total purine deriva-tives,representing microbial protein synthesis,were observed in LP-PLF and LP-SBO,respectively,indicating that those diets were most and least favorable for rumen development.Irrespective of forage particle size,supplemental SBO vs.PLF increased serum malondialdehyde as an oxidative stress indicator across periods,increased blood urea nitrogen and feed efficiency in the pre-weaning period,and reduced hip height during the post-weaning period.Conclusions It can be concluded that feeding a rumen-inert,mostly saturated fatty acid source with alfalfa hay as long particle size is recommended with view on performance,whereas a combination soybean oil rich in unsatu-rated fatty acids should not be provided to milk-fed Holstein calves together with long particle forage.Feeding soybean oil and alfalfa hay as long particles is not advisable mainly due to lower starter consumption and impaired development of ruminal function. If dietary supplementation of soybean oil is applied, incorporation of forage as small particles should be preferred to support rumen development.