30 September 2012

3 minute read

The volume of urine and quantity of nitrogen excreted in housed dairy cows more than doubles within an hour after feed is offered. Just one of the interesting findings of work carried out by scientists in New Zealand, testing the hypothesis that there is diurnal variation in the concentration and quantity of urine nitrogen (N) excreted by dairy cows.

Previous work reported that dairy cow urination frequency increased during feeding compared with resting. But, in contrast, urine N concentration reached a peak four hours after pasture was offered. This delay in peak urine N concentration may be associated with proteolysis and microbial growth relative to feeding. Another study found that rumen NH₃ concentration peaked three hours, and blood urea five hours, after pasture feeding.

“There is good evidence that the quantity and concentration of excreted urine N is related to the timing of pasture ingestion,” said DairyNZ’s Dave Clark, adding that the diurnal variation has implications for mitigating N loss from grazed dairy farms because urinary N concentration changed from 0.3 to 0.5 per cent within a day, which is contrary to the assumed 100g N/m² used to describe the dairy cow urine patch soil N loading.

“If the high N loading events could be captured and spread evenly across a paddock, the amount of N loss from grazed dairy systems could be substantially reduced,” he added. “Also, the timing of pasture allocation could be altered to prevent peaks and troughs of herbage intake. For instance, more frequent allocations of pasture may decrease peak urine N concentration, high N urine patch events and subsequent N loss to the environment.”

His team’s work took six lactating Holstein-Friesian dairy cows, which were housed in metabolism stalls and offered ryegrass pasture on an ad lib. basis for 10 days in March 2010, with measurements taken on day nine and 10. Half the daily DM requirement of pasture was cut and offered to each cow at 8.40am. Feed was removed at 1.00pm and cows milked at 3.30pm.

Fresh pasture was cut and offered to cows at 4.00pm. Feed was removed at midnight and replaced at 6.00am prior to being removed again at 7.30am. Pasture intake was measured, and samples collected daily and stored for analysis by near infrared spectroscopy.

“We saw no effect of day on the variables analysed, so the data were analysed as a mean of each two-hour period for the two days,” said Dr Clark. “Urine N concentration varied with time with a peak at approximately four hours after pasture allocation.

“Urine volume and N excreted varied with time with a peak less than an hour after feeding began. There was no effect of time on faecal N concentration, faecal N excreted, and faecal weight,” he added.

Full details: Clark C, Waghorn G, Gregorini P, Woodward S and Clark D: “Diurnal pattern of urinary and faecal nitrogen excretion by dairy cows fed ryegrass pasture twice daily indoors.”

To view proceedings, Advances in Animal Biosciences, of all summaries presented at the ISNH8 Conference, please click here.