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

Abstract

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 digestible organic matter, and ATTD of DM, organic matter, neutral detergent fiber, and ADF (P < 0.05). Duodenal flow of RUP also increased when substituting cowpea hay for pangola grass hay (P < 0.01). Similarly, inclusion of cowpea hay in the diet improved live weight gain (LWG; P < 0.05), and decreased feed costs per kg of LWG (P < 0.01). The increase in RUP decreased rumen microbial protein synthesis (P < 0.01), but tended to increase LWG, feed conversion and nitrogen use efficiency, and feed costs per unit of LWG (P < 0.10). The use of cowpea hay in the diet of dairy heifers increases nutrient intake and ATTD, leading to improved LWG and lower feed costs per kilogram of LWG. Increasing RUP proportions may reduce rumen microbial protein synthesis, but may enhance LWG, feed conversion efficiency, and nitrogen use efficiency. There were no interactions between forage source and RUP proportion on intake and ATTD, but some synergistic effects were observed for the performance parameters.