Investigating the effect of synthetic nitrogen application rate on the milk production, intake, digestion, and nitrogen excretion of dairy cows fed perennial ryegrass or perennial ryegrass-white clover herbage

The objective of this experiment was to investigate the effect of synthetic N application rate on milk production, intake, digestion, and N excretion of dairy cows fed perennial ryegrass (PRG) or PRG-white clover (WC) herbage. Twelve ruminally cannulated multiparous Holstein Friesian dairy cows averaging (mean ± SD) 66 ± 12 DIM and 492 ± 41 kg BW were initially enrolled during spring in a 14-d acclimation period. Cows were then randomly assigned to 1 of 4 dietary treatments (DT) in an incomplete crossover design with two 22-d experimental periods. This experimental schedule was repeated during autumn when the cows were averaging 216 ± 12 DIM and 512 ± 37 kg of BW. The 4 DT were as follows: (1) PRG receiving 25 kg of synthetic N/ha per cut (G-L); (2) PRG receiving 50 kg of synthetic N/ha per cut (G-H); (3) PRG-WC receiving 0 kg of synthetic N/ha per cut (GC-L); and (4) PRG-WC receiving 25 kg of synthetic N/ha per cut (GC-H). We observed a DT by season interaction effect on milk yield and milk solids yield. In spring, cows fed GC-L had lower milk yield when compared with cows fed G-H (23.4 and 26.8 kg/d, respectively), and lower milk solids yield when compared with cows fed G-L and G-H (1.53, 1.92, and 1.99 kg/d, respectively). In autumn, cows fed GC-H had greater fat yield when compared with cows fed GC-L (0.89 and 0.78 kg/d, respectively) and had greater milk solids yield when compared with cows fed GC-L and G-L, but were similar to cows fed G-H (1.70, 1.53, 1.53, and 1.58 kg/d, respectively). We observed a DT by season interaction effect on DM and OM intake and on OM digestibility (OMD). In spring, DM and OM intake was similar among DT, whereas in autumn cows fed GC-H had greater DM and OM intake when compared with cows fed other DT. Cows fed GC-L had lower DM digestibility when compared with cows fed other DT in both spring and autumn. In spring, cows fed GC-L had lower OMD when compared with cows fed other DT, whereas in autumn, cows fed G-L had lower OMD when compared with cows fed G-H and GC-H. We observed a DT by season interaction effect on N intake, urinary N concentration, and urinary N output. In spring, cows fed G-H had greater N intake, urinary N concentration, and urinary N output when compared with cows fed other DT. In autumn, cows fed GC-L and GC-H had greater N intake and urinary N output when compared with cows fed G-L and G-H (800, 818, 608, and 626 g/d, respectively, and 447, 439, 265, and 247 g/d, respectively). Urine output did not differ among DT in spring, whereas in autumn, cows fed GC-L and GC-H had greater urine output when compared with cows fed G-L and G-H (66.0, 64.6, 47.3, and 46.8 kg/d, respectively). Overall, reducing synthetic N application rate on PRG-only swards by 25 kg of N/ha per cut generally had no effect on animal performance, intake, or digestibility, and it improved N excretion profiles during the spring. However, these N excretion benefits were not evident during the autumn period. Similarly, cows fed PRG-WC herbage receiving reduced application rates of synthetic N exhibited more favorable N excretion profiles during spring, whereas due to excessively high WC proportions during autumn, cows fed PRG-WC herbage had greater N intake, urinary N output, and urine output when compared with cows fed PRG-only herbage. These results indicate that additional strategies will be required to effectively mitigate urinary N excretion during periods of elevated nitrate-leaching risk.