The hypothesis of the current study was that a higher protein content (24%) in the starter diet combined with the addition of inert fat source (palm fatty acids; PLF) to rumen microbes rather than soybean oil (SBO) as a fat source consisting of triglyceride would improve growth performance of dairy calves. This study examined the effects of starter protein content [20% vs. 24% crude protein (CP), dry matter] and supplemental fat sources [soybean oil (SBO) vs. hydrogenated palm fatty acids (PLF); both 2.5% DM] on growth performance, digestibility, blood metabolites, ruminal protozoa populations, and urinary nitrogen components in dairy calves. A total of 52 female dairy calves (mean age 3 days and body weight 40.7 kg) were allocated to the following treatments in a completely randomized design: 1) 20% CP starter diet supplemented with SBO (20CP-SBO), 2) 20% CP starter diet supplemented with PLF (20CP- PLF), 3) 24% CP starter diet supplemented with SBO (24CP-SBO), and 4) 24% CP starter diet supplemented with PLF (24CP- PLF). Calves were weaned on day 53, and the study ended on day 73. In comparison to the 20CP, 24% CP improved starter feed, average daily gain (ADG), withers height (d53), hip-width, organic matter (OM) digestibility, and blood glucose concentration (preweaning). Feeding of SBO decreased starter feed intake, ADG, withers height, OM, and CP digestibility and decreased blood concentrations of glucose (pre- and postweaning) and beta-hydroxybutyrate (postweaning) and increased aspartate aminotransferase concentration (preweaning) compared to PLF feed. In addition, SBO supplementation decreased urinary excretion of purine derivatives and microbial protein synthesis in the postweaning period and reduced the protozoa population in the pre- and postweaning periods. Regarding the interaction between the main effects (starter protein content and fat source), starter intake was greater when calves fed diets supplemented with PLF compared to SBO diets when calves fed either 20CP or 24CP diets. Accordingly, the higher and lower average daily gains (ADG) were recorded for the 24CP-PLF and 20CP-SBO diets during the same period. Calves fed the 20CP-SBO diet had lower feed efficiency during the preweaning period and throughout the experimental period. The higher hip height was found at the time of weaning and on d73 in calves fed the 24CP-PLF diet. The lower digestibility for neutral detergent fiber was found in calves fed the 20CP with supplemental SBO diet. The higher postweaning blood urea nitrogen concentration was found in calves fed the 24CP-SBO diet. Feeding the 20CP-SBO diet to dairy calves decreased urinary allantoin excretion and rumen microbial protein synthesis, but increased urinary nitrogen excretion during the preweaning period. Our results suggest that PLF is more suitable than SBO in calf diets, especially when calves fed 24% CP, possibly due to improved nutrient digestibility.
