27 October 1995

High Nin grass makes balanced diet check vital

By Jessica Buss

MILK producers with cows still at grass are urged to check the energy and protein balance of their diets to avoid over-feeding protein.

The warning is based on analysis results which show excessive nitrogen levels in grass with crude proteins up to 28%.

"Cows need energy to balance this free nitrogen, or it will be excreted in the urine, or can end up in the milk," says Farmlabs Mark Harrison.

Derbyshire-based Genus consultant Stephen Brook cites cases of higher than average milk urea levels on some units in the Midlands. High milk ureas signal a poor energy to protein balance in the diet, he says, which could be caused by high nitrogen in grass.

Genus nutritionist Diana Allen confirms that nitrogen in the grass breaks down to ammonia in the rumen and, when in excess of the needs of the rumen bugs, can lead to excess urea being excreted in the urine or the milk.

She cites Swedish research showing that urea levels in the milk are related to the amount of rumen degradable protein (RDP) in the ration, and the balance between protein and energy.

Excessive amounts of RDP in the diet, including that from high nitrogen in grass, or a shortage of fermentable metabolisable energy (FME), cause high milk ureas.

Tony Blackburn, Reaseheath College, Cheshire, says adequate starch and sugars should be available to give high levels of FME. Energy is needed to help rumen bacteria use the ammonia released from the protein breakdown. An excess of RDP over and above FME levels in the rumen will cause high levels of ammonia, eventually increasing blood ureas.

Fertility affected

Fertility is also shown to be adversely affected by high blood urea levels (as indicated by high milk ureas), claims Mrs Allen. There is evidence to suggest embryo re-absoption occurs.

To help the rumen bugs make microbial protein from the excess ammonia, she suggests supplementing grass diets with a fermentable energy source, such as molasses or a highly degradable starch such as rolled wheat. This could help reduce excess ammonia in the rumen and therefore milk urea contents.

"Wet autumn grass is low in energy so you must also ensure the energy density of the ration is high enough and the ration is balanced," she adds.