Previous research has shown that, in dairy cows fed nitrogen-rich diets, non-protein nitrogen levels (indicated by milk urea – not to be confused with urine) and urinary nitrogen rise together.
But this environmental correlation doesn't necessarily mean that cows with genetically-low milk urea, fed the same diet as typical cows, will have genetically low urinary nitrogen.
"In addition, we don't know if reducing urinary nitrogen would compromise other breeding worth (BW) traits through other, unfavourable, genetic correlations or improve them through favourable ones," says Camara.
There is even the possibility of breeding cows with higher milk protein levels if cows with low urinary nitrogen and milk urea levels re-direct the nitrogen reductions to protein production.
To research the amount of nitrogen excreted, a high-tech gadget developed by AgResearch is attached to cows. It channels their urine past a sensor and records the time, volume, and nitrogen concentration each time they urinate.
Pairing direct urinary nitrogen data with other measurements such as milk urea and protein and applying complex statistical models can then estimate the genetic correlation.
"If the milk urea/urinary nitrogen correlation is high enough, we can use milk protein as a predictor trait," says Camara." If it's not, we'll have to find another way."
For farmers to get credit from regulatory bodies for reducing urinary urea through breeding, the environmental impacts will need to be quantified.
Key points
This programme aims to help farmers meet nitrogen targets in three key ways:
• Through selective breeding, cows that produce less urinary nitrogen.
• Offering an additional management strategy for reducing nitrogen leaching.
• Potentially reducing sector-wide nitrate leaching by up to 20 per cent.
