Forage addition to starter diets has favorable effects on ruminal development but may reduce starter intake and growth. Due to the crucial role of P for ruminal microbial activity, and insufficient fiber-digesting bacteria activity in early weeks of growth, we hypothesized that shortness of dietary P may contribute in reduced fiber digestibility in young calves. Thus, interaction of dietary fiber and P in young calves need to be more evaluated. Consequently, the current study evaluated the effect of forage feeding level (no alfalfa hay (NAH) vs. 100 g of chopped alfalfa hay (AH) per kg of starter) at either 0.4% (0.4P) or 0.8% P (0.8P) on growth performance, digestibility of nutrients, ruminal fermentation, and microbial protein synthesis (MPS). Forty-eight female Holstein calves (39.2 ± 3.7 kg) were assigned randomly to the four experimental treatments including NAH-0.4P, NAH-0.8P, AH-0.4P, and AH-0.8P (n = 12, each) during the pre-weaning (d 3-53) and post-weaning periods (d 54-73). The P contents were 0.41%, 0.84%, 0.42%, 0.82%, and phytate-P contents were 0.25%, 0.26%, 0.28%, 0.29% for the experimental treatments cited above, respectively. Milk feeding schedule was identical among treatments and calves had ad libitum access to water and starters throughout the experiment. Based on forage addition × P level interactions, the least and greatest starter intakes pre-weaning were observed in AH-0.4P and AH-0.8P, respectively. Compared to all other groups, calves in AH-0.8P had greater average daily gain pre-weaning and post-weaning, greater body weight and higher withers height at weaning and the end of experiment, higher hip height at weaning, and greater urinary excretion of PD, the latter indicating improved microbial protein synthesis. Feeding AH diet to calves increased ruminal acetate concentration, reduced ruminal propionate concentration, and tended to decrease ruminal butyrate concentration and increase ruminal pH when compared to NAH-fed calves pre-weaning. A level of 0.8P vs. 0.4P tended to increase organic matter and neutral detergent fiber digestibility, increased total volatile fatty acid production in the rumen pre- and post-weaning; whereas, ruminal ammonia nitrogen concentration and urinary nitrogen excretion were decreased by 0.8P. It is concluded that forage addition to starter diets has high potential to improve growth performance in young dairy calves. However, currently recommended dietary P levels of ~0.45% may be insufficient to support fiber digestibility, microbial protein synthesis and growth, especially pre-weaning, when forage-containing starters are high in phytate-P.
