Interaction between feed use efficiency and level of dietary crude protein on enteric methane emission and apparent nitrogen use efficiency with Norwegian Red dairy cows

We assessed the interactive effects of gross feed use efficiency (FUE, milk yield/kg DMI) background (“high” = HEFF vs. “low” = LEFF) and graded levels of dietary CP (130, 145, 160, and 175 g/kg DM) on milk production, enteric methane (CH(4)) emission, and apparent nitrogen use efficiency (NUE, g milk protein nitrogen/g nitrogen intake) with Norwegian Red (NRF) dairy cows. Eight early- to mid-lactation cows were used in a 4 × 4 Latin square design experiment (2 efficiency backgrounds, 4 dietary treatments, and 4 periods each lasting 28 d). The diets were designed to be identical in physical nature and energy density, except for the planned changes in CP, which was a contribution of slight changes in other dietary constituents. We hypothesized that HEFF cows would partition more dietary energy and nitrogen into milk components and, as such, partition less energy in the form of methane and excrete less nitrogen in urine and feces compared with their LEFF contemporaries. We observed no interactions between dietary CP level and efficiency background on DMI, other nutrient intake, NUE, CH(4) emission, and its intensity (g CH(4)/kg milk). Gradually decreasing dietary CP from 175 to 130 g/kg DM did not affect DMI, milk and energy-corrected milk yield, and milk component yields and daily CH(4) emission. However, decreasing dietary CP increased NUE and reduced urinary nitrogen (UN) excretion both in quantitative terms and as proportion of nitrogen intake. The HEFF cows showed improved NUE and decreased CH(4) emission intensity compared with the LEFF cows. In the absence of interaction effects between efficiency background and dietary CP level, our results suggest that CH(4) emission intensity and UN excretions can be reduced by selecting dairy cows with higher FUE and reducing dietary CP level, respectively, independent of one another. Furthermore, UN excretion predictions based on milk urea nitrogen (MUN) and cow BW for NRF cows produced very close estimates to recorded values promising an inexpensive and useful tool for estimating UN excretion under the Nordic conditions where ordinary milk analysis comes with MUN estimates.