Words by: Anne Lee

Researchers working on unravelling the reasons cows with low milk urea nitrogen breeding values (MUNBV) excrete less nitrogen in their urine despite eating exactly the same diet have found a clue in their grazing behaviour.

Lincoln University PhD student Cameron Marshall has analysed swathes of data from his study on 48 cows at the university’s Ashley Dene Research and Development Station.

He used specialised IGER behaviour recorders – a halter like device that can record the animal’s grazing activity and detect the difference between grazing bites and mastication or chewing.

“Ultimately, cows with a lower MUNBV had more oral processing of their food.

“They had a greater number of mastications, especially when offered their new break. “Additionally, they had more chews per boli, which is the regurgitated food during rumination.

“All this may indicate that they are more efficiently breaking down the forage,” Cameron says.

Total feed intake though was similar between the two breeding value lines, but with the lower MUNBV cows spending more time chewing their food and therefore they had greater grazing times.

The initial part of Cameron’s study found cows with lower MUNBVs excrete less nitrogen in their urine.

The MUNBV of cows ranged from -2 to +3 and Cameron found for every unit decrease in MUNBV urinary urea nitrogen excretion dropped by 0.67g/litre.

That amounts to a 165.3g difference in urinary urea nitrogen excretion per day between the highest and lowest MUNBV animals.

In a win:win milk protein percentages increased as MUNBV declined.

Cameron’s supervisor is Lincoln University professor Pablo Gregorini.

He says Cameron’s data is the first indicator that a behavioural factor could be the initial driver of the difference in the cows’ variation in urinary nitrogen.

“A range of factors are known to affect behaviour, both short and long term, internal and external, genetics and epigenetics.

“They’re eating the same feed under the same conditions, but they’re applying different strategies to grazing,” Pablo says.

Cameron’s studies are also looking at what’s going on at a rumen level to see what the differences are in how pasture that’s “chewed more” initially, is processed along the digestive system and how that may result in less nitrogen in urine.

“We’re finding differences in the data and that’s raising more and more questions,” Cameron says.

But by finding the answers and understanding just what’s going on, it then becomes possible to manage the system, Pablo says.

“It may be that we can manage the grazing of these animals differently,” he says.