Digestion and nitrogen utilization in dairy heifers limit-fed a low or high forage ration at four levels of nitrogen intake

The hypothesis of this experiment is that a low-forage (LF) ration will be utilized with greater efficiency than a high-forage ration (HF) by dairy heifers and that the response will be affected by level of N intake. To test this hypothesis, 8 Holstein heifers (beginning at 362 ± 7 kg and 12.3 ± 0.4 mo) were fed 8 rations according to a split-plot, 4 x 4 Latin square design. Treatments were formulated to contain 25 or 75% forage (corn silage and chopped wheat straw) and fed at 4 levels of N intake [0.94 (Low), 1.62 (MLow), 2.30 (MHigh), 2.96 (High) g of N/kg of metabolic body weight per day]. Diets were limit-fed to maintain equal intake of metabolizable energy. Blood samples were collected over d 19 to 20, and feces and urine were collected for 8 d per 28-d period. Organic matter (OM) intake was greater for heifers fed HF, but, due to increased OM digestibility of LF (74.0 vs. 67.6% ± 0.9), digestible OMI was unaffected by forage level. Organic matter digestibility was affected by an interaction between forage level and N intake, increasing to a plateau of 78.01% at 18.43% crude protein for LF-fed and 68.78% at 13.90% crude protein for HF-fed heifers. Apparent N digestibility was greater for heifers fed LF and increased from 47.7 to 80.8% between Low and High N intake. Less N appeared in the feces of heifers fed LF than HF (45.56 vs. 52.60 g/d). Urea-N excretion was not different between forage levels, but increased linearly with N intake. Concentration of plasma urea-N was significantly higher for LF and with increasing N intake. Urea clearance rate (L/h) did not differ between forage levels and increased, but at a decreasing rate, as N intake increased. A significant interaction resulted from urea clearance increasing at a greater rate and resulting in higher values for HF, whereas clearance of urea for heifers fed LF resulted in significantly lower maximal values. Like urea-N excretion, daily urinary N excretion was affected only by N intake. Retained N responded linearly to increased levels of N intake. The significant reduction observed in fecal N excretion for LF was counterbalanced by numerical increases in urinary N excretion so that total N excretion and retention were not different between forage levels. The percentage of N intake that was retained only tended to be affected by an interaction and was not significantly affected by forage level. It is concluded that increasing N intake increases the digestibility of OM, the magnitude of which depends on the level of dietary forage provided. Furthermore, differences in N utilization between LF and HF in this trial were small and were not evident until N intake increased to impractical levels.