Background: Forage inclusion in starters has become an acceptable strategy in the last decade. To compensate for the lower energy provided by forage, concurrent lipid supplementation can be proposed. As indicated previously, forage digestibility may be negatively influence with lipid supplementation mainly due to the lower ruminal microbial activity. We hypothesized that this may be modulated by alteration of forage particle size and lipid supplement type in dairy calves. 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, 6 males and 7 females per each) were offered to Holstein calves (3 d old) with equal amounts of lipid (25 g/kg DM) throughout the experimental period. A constant amount of milk (d 1 to 53) was offered pre-weaning. 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-hydroxy butyrate, and insulin during the pre-weaning period. During both pre- and post-weaning periods, the highest and lowest urinary excretion of allantoin and total purine derivatives, 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 and blood urea nitrogen during the pre-weaning and reduced feed efficiency and hip height during post-weaning periods. 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 unsaturated fatty acids should not be provided to milk-fed Holstein calves together with long particle forage. Feeding soybean oil 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.
