Monday, April 22, 2024

Genetics, eating behaviour can influence N leaching

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New Lincoln University Pastoral Livestock Production Lab research has shown the role a cow’s genetics and eating behaviour plays in reducing the level of nitrogen it excretes in its urine. The research by PhD student Cameron Marshall shows that what cows with phenotypically lower milk urea N eat – and how they eat – is important in reducing their environmental impact.
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New Lincoln University Pastoral Livestock Production Lab research has shown the role a cow’s genetics and eating behaviour plays in reducing the level of nitrogen it excretes in its urine.

The research by PhD student Cameron Marshall shows that what cows with phenotypically lower milk urea N eat – and how they eat – is important in reducing their environmental impact.

It found that these cows chew and ruminate eaten pasture differently, which alters the N excretion patterns to the environment.

The research has been published by Marshall in two papers in scientific journals as part of his doctoral thesis.

The other research was from another experiment that looked at the differences between cows with a lower and higher milk urea N concentration phenotypes when these animals ate plantain rather than a ryegrass diet.

It found those cows with the low milk urea N phenotype urinated significantly less N excretion per urination event, thus reducing the potential N leaching to waterways.

Cows with low milk N concentrations are a genetic trait. The research used the herd from the Lincoln University Ashley Dene Research and Development Farm herd where this trait has a natural variation.

It took the eight highest and the eight lowest animals with milk urea N concentrations from the herd and placed them indoors in crates so Marshall could collect and analyse everything these animals excreted and defecated when fed the two diets.

It found that those cows who consumed plantain had a 32-47% reduction in nitrate leaching when extracted through urine compared to a ryegrass diet.

The study on grazing differences was undertaken last year. It took 45 animals from the herd that had a mix of high and low milk urea N concentration breeding values and put them outside in a paddock of ryegrass/white clover and a paddock of ryegrass/white clover and plantain.

The research measured the N concentration in the cow’s urine and then using an equation, calculated its N loading rate onto the soil and modelled what the leaching rate is.

The plantain variety used in the indoor research was Agritonic from Agricom.

Marshall fitted the cows with grazing recorders, which fitted on bands around the cow’s neck and sat under its jaw. These recorded every time the cow opened and closed its mouth and using software, it allowed him to differentiate between biting to eat and rumination.

“What we found was that the low (milk urea N concentration) cows masticate more. They have a greater level of oral processing,” Marshall said.

In other words, they chew their food properly.

This is thought to lead to smaller food particles entering the rumen, which changed its fermentation dynamics. The food passes through the rumen faster, meaning less time for microbial fermentation and less urea excreted in urine.

The new research follows on from previous work from Marshall, which showed that cows selected for low milk urea N had a 28% reduction in the urinary urea N loading rate per urine patch than cows with higher milk urea N breeding values.

 Those ‘better cows’ also yielded an increase in milk protein percentage.

“The results of this new research indicate two promising tools that temperate pastoral dairy production systems can use to reduce N losses and ameliorate the negative impact on the environment,” Marshall said.

Marshall would like to see this latest research expanded to further explore the research into the rumen.

“I think based on these results, we have shown that these animals appear to be quite different. The way that they are processing their food, starting with the grazing behaviour, appears to be quite a different digestion process which we think is resulting in the differences in N excretion,” he said.

He says it would be worthwhile exploring that concept on commercial farms.

While the N reductions in breeding for low n cows are not as large as the reductions that can be made in switching feeds, breeding for this trait accumulated over generations.

It was a permanent solution to lowering N leaching in cows, whereas using plantain only worked for as long as the cows were eating this feed.

“It’s time we started looking at these incremental continuous approaches,” he said.

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