21 September 2001


Cow fertility and reducing lamb production costs are

key issues for livestock producers, Simon Wragg reports

on the latest Teagasc research results

DESPITE containing high levels of rumen degradable protein (RDP), dairy cattle grazing fresh spring grass should not suffer poor reproductive performance, acc-ording to recent research.

Researchers at Teagasc have been aware for some time of the association between intensively managed high yielding dairy cows consuming high levels of RDP and poor conception rates. RDP is converted into ammonia in the rumen and used by rumen bacteria in conjunction with rumen fermentable energy to produce protein for the cow.

When the intake of RDP is too high or the level of dietary fermentable energy too low, bacteria cannot use all the ammonia available to them.

Researchers believe this excess ammonia diffuses into the bloodstream, enters the liver and is converted to urea; leading to high blood levels of ammonia and urea, both of which have been implicated in reduced fertility.

To assess whether dairy stock were at risk, post-graduate researcher David Kenny studied 162 heifers fed indoors on either a high or low RDP diet, supplemented with either 0 or 3kg DM/head/day of molassed sugar beet pulp as a source of rumen fermentable energy. Rations were fed for 10 days prior to insemination and for a month after to test the effects of high RDP.

Despite the high RDP diet resulting in a four-fold increase in blood ammonia level, from an average of 20umol/l up to 80umol/l and a two-fold increase in urea, from 4mmol/l to 9mmol/l, heifers achieved a pregnancy rate of 62%. This was equal to results from heifers fed a low RDP diet.

The level of RDP in grass is influenced by the level of nitrogen fertiliser applied to pasture, say Teagasc researchers. It is not uncommon for pasture to supply crude proteins levels of 20% mostly in the form of RDP.

A similar trial to the indoor study using heifers grazing pasture dressed with either no additional nitrogen fertiliser or 85kg of N/ha (68 units/acre) and fed the same amounts of molassed sugar beet pulp was conducted to test effects of high dietary RDP at grass.

The addition of nitrogen fertiliser lifted pasture crude protein from 13.6 to 23%, while also raising blood concentrations of ammonia and urea. However, Mr Kenny again found no detrimental effect on reproduction with both groups achieving a conception rate of 71%.

Researchers now believe most Irish grass-based dairy systems face little risk of having reproduction compromised by high levels of RDP in forage. But they recognise high yielding cows, experiencing large negative energy deficits in early lactation, may be more susceptible and could, potentially, be more at risk. &#42


&#8226 High RDP not to blame.

&#8226 Avoid diet energy deficit.

Spring grass is not the cause of poor conception rates in high yielding cows, according to recent research.

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