Tall fescue grazing offers least efficient nitrogen utilisation
Tall fescue grasses supported lower milk yield and milk protein production than either perennial ryegrasses or timothy grasses, according to a trial recently completed by research scientists at Aberystwyth’s Institute of Grassland and Environmental Research.
“And this effect was associated with lower intake of tall fescues, but also reduced microbial protein synthesis in the rumen that may, in part, be linked to differences in the extent of release of nutrients during mastication and ingestion,” the institute’s Eun Joong Kim told delegates at this year’s British Society of Animal Science annual conference.
His team set out to compare nitrogen utilisation efficiency by dairy cattle grazing grass species differing in chemical and morphological characteristics.
Rapid breakdown of herbage proteins in the rumen and inefficient capture of nitrogen (N) by the rumen microbial populations are a major source of N loss and pollution in pasture-based ruminant agriculture.
“And the degree of cell damage during mastication and ingestion varies between grass species with consequences for release of cell contents – protein, sugars and lipids – into the rumen. Consequently, grazing cattle on different grass species may provide an opportunity to manipulate N efficiency,” he said, explaining the rational behind the trial.
Twelve multiparous Holstein-Friesian cows in mid-lactation, with similar milk yields, profiles and stage of pregnancy, were grazed on three forage plots – timothy, perennial ryegrass or tall fescue. All animals received 4kg of dairy concentrate daily.
At the end of each measurement period during the trial, chemical composition of the herbage was assessed and herbage intake was estimated. Milk yield was recorded and milk samples and spot urine samples were taken during each milking on the final two days. Milk samples were analysed for fat and protein, and N and purine derivatives were determined in urine. Effects of grass species were examined by analysis of variance with cow pair and period treated as random effects.
“And we found that the N, neutral detergent fibre and water-soluble carbohydrate contents of the three grass species were quite variable between measurement periods,” said Dr Kim. “Herbage intake estimates were consistently lower for timothy swards.”
Yields of milk and milk protein were lower for tall fescue, but there was no difference in milk fat output among the forage treatments. Urinary PD:creatinine ratio – an index of rumen microbial protein synthesis – was also lower for tall fescue. And relative urinary N output, although not significantly different between forages, was numerically higher for tall fescue grass.
“Overall, cows grazing tall fescue pasture showed lower efficiency of N utilisation with approximately 27-40% more urine N per kg of milk protein, despite relatively similar protein content of the different cultivars when averaged across the study,” he added.
Presented to the British Society of Animal Science Annual Meeting, March 31 to April 2, 2008, Scarborough, UK.
Full details: Kim EJ, Lee MRF, Ellis N, Sanderson R, Scollan ND and Dewhurst RJ: “Effect of grass species of efficiency of nitrogen utilisation in Holstein Friesian dairy cows.”
Summary (pdf) Presentation_112kim (pdf)
For further information visit http://www.bsas.org.uk/Publications/Annual_Conference_Proceedings/or contact BSAS on 0131 445 4508 or bsas@sac.ac.uk
