Can genetics reduce dairy farming’s nitrogen footprint?
The animal article of the month for October is “Genetic variation in milk urea nitrogen concentration of dairy cattle and its implications for reducing urinary nitrogen excretion”
Agricultural industries are addressing the challenges of reducing their environmental footprint while maintaining economic viability for farming families and their communities. One challenge acutely affects farms where cattle graze protein-rich forages. This results in high nitrogen (N) intake and high loading of N in urine which is excreted on to relatively small areas – urine patches. The high concentration of N in the urine patch poses two significant environmental issues: some of the N in urine is leached into groundwater; and some is volatilised as nitrous oxide (a potent greenhouse gas). It makes sense to reduce N loading in urine. The question became; can genetics contribute to reducing dairy farming’s N footprint?
Urinary N is difficult to measure in grazing cattle. By using the knowledge obtained from international studies using housed cattle fed total rations of varying N content we know that concentration of milk urea nitrogen (MUN) is linearly related to urinary N excretion. Typically, the relationship is in the order of 15g urinary N per cow per day per unit of MUN (mg/dL). Therefore, MUN is potentially: a worthwhile proxy from which to estimate urinary N loading for monitoring purposes; and a useful measure for farmers to optimise protein content in cow diets.
To understand whether genetics can contribute to reducing the nitrogen footprint, our paper investigates whether selective breeding could be employed to reduce MUN in pasture-fed New Zealand dairy cattle. One advantage of a genetic solution is that it can be easily utilised on-farm simply by selecting semen from desirable bulls with minimal impact on other farming practises. Previous studies involving Holstein cattle have reported that MUN is heritable. However, for cattle genetically selected for low MUN to have lower urinary N excretion than unselected cattle they must partition dietary N differently – dietary N has to go somewhere.
We reported that across New Zealand Holstein-Friesian (HF), Jersey (J) and HF-J crossbred and interbred cattle the heritability of MUN was 0.22. Importantly, in our study cows with lower MUN tended to have higher % milk protein which suggests that animals genetically different for MUN may partition dietary N differently.
Average MUN was 14.0 mg/dL and the range in Breeding Value MUN of sires of the cows was -2.8 to +3.2 indicating there is significant potential to change MUN through breeding.
From our finding we propose that there is an opportunity for genetics to contribute to a reduced nitrogen footprint. On the strength of the results reported, we recommend that further work is required to verify MUN-urinary N relationships in groups of cows genetically extreme for MUN and fed the same diets. This phase could also investigate whether partitioning of dietary N differs between these groups.
The animal article of the month for October is ‘Genetic variation in milk urea nitrogen concentration of dairy cattle and its implications for reducing urinary nitrogen excretion‘ and is freely available for one month.
Author: P. R. Beatson, S. Meier, N. G. Cullen and H. Eding