Effects of rumen undegradable protein on intake, digestibility and rumen kinetics and fermentation characteristics of dairy heifers

The aim of this work was to evaluate the influence of increasing amounts of rumen undegradable protein (RUP) on intake, nutrient digestibility, rumen kinetics and fermentation characteristics and N use efficiency of dairy Holstein heifers. Eight rumen-cannulated Holstein heifers (initial body weight 276 ± 8.3 kg) were used in a replicate 4 × 4 Latin square design with four dietary RUP amounts in the total dietary protein, as follows: 38% of RUP (38RUP), 44% of RUP (44RUP), 51% of RUP (51RUP) and 57% of RUP (57RUP). The experimental period was 84 days, subdivided into four periods of 21 days (14 days for adaptation + 7 days for collections). Eight spot collections of faeces, urine, ruminal content and omasal digesta were performed at 9-h intervals, as follows: on 15th day samples were collected at 0600 h and 1500 h; on 16th day samples were collected at 0000 h, 0900 h and 1800 h; on 17th day samples were collected at 0300 h, 1200 h and 2100 h. Two complete rumen evacuations were performed: one on 19th day 4 h after morning feeding and another on 21st day immediately before morning feeding. Total (kg/day) and relative (g/kg of body weight) intake of dry matter, crude protein (CP), neutral detergent fibre corrected for ash and protein were not affected (P >  0.10) by dietary RUP amounts. Ruminal (P =  0.06) and intestinal (P =  0.09) digestibilities of CP presented a tendency to decrease with increasing RUP. There were no effects of dietary RUP amounts (P >  0.10) on the rates of ingestion, passage and digestion (%/h) as well on the total volatile fatty acid concentration in the rumen. Urinary N excretion presented a tendency (P =  0.08) to decrease with increasing RUP. In addition, retained N presented a tendency (P =  0.09) to increase according to dietary RUP amounts and greater values were observed for treatments 51RUP and 57RUP. Microbial protein synthesis, microbial efficiency and the efficiency of use of N for microbial synthesis decreased with increasing RUP (P <  0.05). Metabolisable protein flow presented a tendency to increase (P =  0.091) and greater values were observed for 51RUP and 57RUP. In general, urinary N excretion decreased, while the flow of metabolisable protein and RUP increased with increasing RUP levels. As a consequence of this pattern, the treatments 51RUP and 57RUP presented increases in the retained N.