Effect of forage source and dietary rumen-undegradable protein on nutrient use and growth in dairy heifers

•This study evaluated the effects of substituting 250 g/kg of dietary DM from pangola grass (Digitaria decumbens) with that from cowpea (Vigna sinensis) hay at both 260 and 360 g RUP/kg CP in growing Holstein heifers under tropical conditions.•Feeding cowpea hay increased daily intakes of DM, OM, CP, ADF, and digestible OM as well as apparent digestibility of DM, OM, NDF, and ADF compared to diets containing pangola grass hay that resulted in higher and live weight gain and lower feed cost per kg of weigh gain.•Increasing RUP from 260 to 360 g/kg CP through the use of fish meal, may decreased microbial protein synthesis but enhances live weigh gain, feed conversion efficiency, and N use efficiency.•Feeding legumes and/or increasing RUP concentrations in diets of growing heifers can enhance performance and thereby contribute to reduced age at first calving in dairy farming in the tropics Thirty-two Holstein heifers with mean (± standard error of the mean) age of 6.5 months (±0.12) and live weight (LW) of 166 kg (±1.6) were divided into four groups of eight animals to evaluate the effect of forage source and the concentrations of rumen-undegradable crude protein (RUP) in the diet with a 2 × 2 factorial design. As forage sources cowpea hay (Vigna sinensis L.) or pangola grass hay (Digitaria decumbens Stend) were added to the diet at 250 g/kg dry matter (DM). Also, 350 g/kg DM of chopped fresh King grass (Pennisetum purpureum Schum.) was included in all diets. The RUP proportions were 260 or 360 g/kg of total crude protein (CP). Heifers were housed in metabolism cages. The trial lasted ten weeks, with the first two weeks used for adaptation and the last week for data collection and sampling. Diets had a forage to concentrate ratio of 60:40 (on DM basis), and were offered as total mixed rations ad libitum. Fish meal or urea were used to produce varying concentrations of RUP at similar dietary CP content. Diet ingredients (i.e., hay, grass, and concentrate) and feces were sampled for nutrient analysis. Total collection of feces was performed to estimate apparent total tract nutrient digestibility (ATTD). Urinary purine derivative excretions were determined from urine spot samples to estimate rumen microbial protein synthesis. The effects of forage source, RUP, and its interactions were determined by general linear model analysis. Replacing dietary pangola grass hay with cowpea hay increased daily intakes of DM, organic matter, CP, acid detergent fiber (ADF), and