The current study aimed to evaluate the effect of wheat straw (WS) as a forage fiber source vs beet pulp (BP) as a non-forage fiber source supplemented by fat sources soybean oil (SBO) vs palm fatty acid (PLF) on growth performance, protozoal population, and urinary purine derivatives (PD) in dairy calves. Fifty-two new-born Holstein female calves (3 days of age) were assigned randomly to one of four treatments: 1) starters contained WS with SBO (WS-SBO); 2) starters contained WS with PLF (WS-PLF); 3) starters contained BP with SBO (BP-SBO); and 4) starters contained BP with PLF (BP-PLF). An equal amounts of fiber (75 g/kg dry matter) and fat (25 g/kg dry matter) was offered in experimental treatments. Milk feeding schedule was constant among experimental treatments. The starter intake was higher in calves received BP with PLF diet (P = 0.02) compared to calved received SBO diet. This coincided with fiber × fat source interaction (P = 0.05) for average daily gain (ADG) and hence BW was higher in calves fed BP along with PLF (P = 0.04). The heights for withers (P = 0.04) and hips (P = 0.01) were higher at weaning when calves received BP with PLF compared to SBO diets. Moreover, the higher ruminal concentration for short-chain fatty acids (P = 0.05) and the higher digestibility for organic matter (P = 0.03) and neutral detergent fiber (P = 0.05) were achieved when calves fed BP with PLF compared to SBO diets. Supplemental SBO reduced starter intake (P = 0.04), total dry matter intake (P = 0.01) and ADG (P = 0.01) compared to PLF diet. The protozoa population was reduced (P = 0.01) when calves were supplemented with SBO compared to calves fed PLF. Furthermore, supplemental SBO reduced total PD concentrations (P = 0.02) and consequently reduced microbial protein synthesis (P = 0.02) compared to PLF diets. Feeding BP compared to WS reduced acetate to propionate ratio (P = 0.02) during the pre-weaning period. In conclusion, based on the current study conditions, feeding W
