Discipline: environment; Key words: dietary crude protein, enteric methane, feed use efficiency, urinary nitrogen.
Increasing input costs and new regulations to reduce nutrient loss have created pressures to improve agricultural nutrient use efficiency. In ruminants, and therefore dairy cattle, the greatest potential to reduce greenhouse gas (GHG) emissions and other environmental pollutants involves improving animal and herd efficiency. Measures include changes to dietary composition, feeding techniques to reduce methane (CH4) generated during enteric fermentation, and proper management of manure to reduce CH4 and nitrous oxide (N2O) released during storage. Since enteric CH4 emission is proportional to daily dry matter intake (DMI), selection for low residual feed intake could reduce GHG emissions and improve dietary nitrogen use efficiency (NUE). In a study by Dr A. Kidane and colleagues , it was hypothesized that dairy cows with higher gross feed use efficiency (FUE, milk yield per kg DMI) would partition more dietary energy and nitrogen into milk components and partition less energy in the form of CH4 and excrete less nitrogen in urine and faeces compared with cows with lower FUE. It was also hypothesized that the sensitivity of NUE to increasing levels of dietary crude protein (CP) would differ between these two divergent groups. The results of the study were published in the Journal of Animal Science, Volume 96 of 2018, page 3967 to 3982, using the title: 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.
In the study the interactive effects of FUE (milk yield per kg DMI), background (“high” = HFUE vs. “low” = LFUE) and graded levels of dietary CP (13, 14.5, 16 and 17.5%) on milk production, enteric CH4 emission and apparent nitrogen use efficiency (NUE, g milk protein nitrogen per g nitrogen intake) were assessed with Norwegian Red dairy cows. Eight early to mid-lactation cows were used in a 4 x 4 Latin square design experiment (two efficiency backgrounds, four dietary treatments and four periods, each lasting 28 days). The diets were designed to be identical in physical nature and energy density, except for the planned changes in CP.
The results showed no significant interaction between dietary CP level and efficiency background on DMI, other nutrient intakes, NUE, CH4 emission, and its intensity (g CH4 per kg milk). The gradual decrease of dietary CP from 17.5 to 13% did not affect DMI, milk and energy-corrected milk yield, milk component yields and daily CH4 emission. However, the decrease increased NUE and reduced urinary nitrogen (UN) excretion, both in quantitative terms and as proportion of nitrogen intake. The HFUE cows showed improved NUE and decreased CH4 emission intensity compared with the LFUE cows. A further observation was that if UN excretion was predicted from milk urea nitrogen (MUN) and cow body weight, it showed a high correlation with recorded values in the Norwegian system.
It was concluded that as there were no significant interaction between efficiency background and dietary CP level, the results suggest that CH4 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, since UN excretion predictions based on milk urea nitrogen (MUN) and cow body weight produced very close estimates to recorded values, this may prove to be a promising inexpensive and useful tool for estimating UN excretion under Norwegian condi