DairyNZ: Genetic's role in N leaching studied

An AgResearch urine sensor cow, a key component to this protentially ground-breaking research. Photo / Brendon Welten

A key commitment of the Dairy Tomorrow strategy is to "protect and nurture the environment for future generations".

Right now, researchers are making good on that by investigating the role of genetics in reducing nitrogen (N) in cows' urine. If we can breed dairy cows that excrete less N in their urine, we can reduce the amount of N reaching our waterways. That's good for farming and good for our environment.

The research involves thousands of cows on farms around New Zealand. Scientists are developing breeding strategies and estimating the expected reduction in N leaching — potentially up to 20 per cent.

Previous research has shown that, in dairy cows fed N-rich diets, milk urea levels and urinary N rise together. But this environmental correlation doesn't necessarily mean that cows with genetically-low milk urea that are fed the same diet as typical cows will have low urinary N.

And if this 'genetic correlation' doesn't hold up, selecting for low milk urea won't reduce urinary N. In addition, we don't know if reducing urinary N would compromise other breeding worth (BW) traits through other, unfavourable, genetic correlations or improve them through favourable ones.

Verifying these genetic relationships isn't easy. It takes many measurements of urinary N, milk urea and other BW traits on related cows to separate the environmental and genetic contributions to these relationships.

To take these measurements, a high-tech gadget developed by AgResearch is attached to free-ranging cows. It channels their urine past a sensor and records the time, volume, and nitrogen concentration of every urination. Pairing direct urinary nitrogen data with indirect measurements such as milk urea and applying complex statistical models can then estimate the genetic correlation between milk urea and urinary nitrogen. If it's high enough, we can use milk urea as a predictor trait.

For farmers to get credit from regulatory bodies for reducing urinary urea through breeding, we'll need to quantify the environmental impacts. Therefore, this programme will also upgrade the Overseer model used by regional councils to monitor compliance. This upgrade will be based on data from studies that monitor nitrogen leaching from farm-scale trials to determine how low-N genetics interacts with alternative pasture plants and crops that also impact urinary nitrogen levels.

KEY POINTS

This programme will help farmers meet environmental targets by:

Developing genetically low nitrogen-excreting animals.
Offering breeding and management strategies to reduce nitrogen leaching.
Reducing sector-wide nitrate leaching by 20per cent.

First published in DairyNZ's Inside Dairy