Aim of the study: The effects of forage source (alfalfa hay; AH vs. wheat straw; WS) with rumen undegradable to degradable protein ratio [low ratio (LR) = 28:72; high ratio (HR) = 36:64] were evaluated in young dairy calves. Area of study: Interaction of forage source and protein degradability in starter diet of dairy calves. Materials and methods: Forty-eight 3-d old female Holstein dairy calves (44.5 kg of BW) were allocated in four treatments: (1) AH with low dietary RUP:RDP ratio (AH-LR); (2) AH with high dietary RUP:RDP ratio (AH-HR); (3) WS with low dietary RUP:RDP ratio (WS-LR); and (4) WS with high dietary RUP:RDP ratio (WS-HR). The calves weaned on d 53 of the experiment and remained in the study until d 73 of age. Main results: The average daily gain and feed efficiency were improved in dairy calves receiving HR diets compared to LR diets during the post-weaning period (p < 0.05). The fecal score (p = 0.03) and neutral detergent fiber digestibility (p = 0.04) were improved when calves fed WS diets compared to AH diets. Feeding HR diets increased allantoin (p = 0.04) and microbial protein yield, and reduced blood urea nitrogen concentration (p = 0.03) compared to LR diets. Assessing the interaction effects of the experimental factors resulted that the greatest BW, wither height, and blood beta-hydroxybutyrate, and the lowest urinary nitrogen concentration were observed in the WS-HR treatment (p < 0.05). Research highlights: Feeding WS with high RUP:RDP ratio is recommendable in dairy calves due to the improvement in gain and nitrogen efficiency.
