15 September 1995

HIGH YIELDERS

COWS CHOOSE A DIET

ENERGY GAP HITS THE

High-flying dairy cows fail to match energy output from feed with their needs for peak lactation. But does that energy gap mean they risk metabolic stress? Jessica Buss visits the Edinburgh-based Langhill Dairy Research Centre to find out

HIGH genetic merit dairy cows fed a high forage, low concentrate diet fail to replace condition score lost in early lactation as effectively as those fed high levels of concentrates.

Evidence comes from studies at the Scottish Agricultural Colleges Langhill Dairy Research Centre, Edinburgh.

High merit cows fed a concentrate-based diet returned to their initial condition score after 40 weeks of lactation. But those on a high forage diet were half a condition score lower.

Dr John Oldham, head of SAC genetics and behavioural sciences, explains that high yielding cows cannot match energy output from feed during peak lactation and mobilise body fat to bridge the gap.

The gap widens with increased selection for high merit cows. The energy deficit occurs on both high forage and low forage grass silage-based diets, despite an increase in dry matter intake as index rises.

Dr Roel Veerkamp, SAC animal breeding specialist: "It looks as if we are selecting for animals which can, increasingly, milk off their backs."

However, he says a decrease in condition score may not prove detrimental to performance of high merit cows. The key is to look at ways of sustaining the high-flying adequately, with nutrition to prevent too great a loss of body condition and to avoid health and welfare problems.

A three-year, £600,000 project is under way to pin-point the long-term effects of metabolic stress on reproduction. Funded by the Scottish Office Agriculture and Fisheries Department (SOAFD), it will be carried out at Langhill, the Hannah Research Institute, Moredun, and the Roslin Institute.

SAC researchers hope to discover whether reduced condition will affect future lactations or fertility. Dr Oldham cites condition score studies of cows which show that on average those with a lower condition score have lower health status and reduced fertility.

"But there are high yielding cows capable of retaining reproductive performance and health," he says. "This proves some animals can cope. We may be able to select these animals for breeding."

Dr Oldham also stresses that a short calving interval may not be essential for high merit cows.

"When a cow produces well through a large part of lactation, why put her back in calf when her milk yield will drop," he says. "We shouldnt just assume that the logic of a 365-day calving interval devised 20 years ago is right today."

GIVEN the choice, dairy cows opt to eat a balanced ration that supplies adequate nutrients to match their current performance.

This was the conclusion after preliminary choice feeding trials using Hokofarm automatic feeders at the Scottish Agricultural Colleges Langhill Dairy Research Centre.

Mid-lactation cows were allowed to eat two complete diets – one high in protein, the other low in protein.

The cows chose between 70% and 75% of the high protein diet and 25-30% of the low protein diet within four to five days, claims Dr John Oldham, head of genetics and behavioural sciences.

All cows had experience of both types of diet before the trials began.

Further studies should give an understanding of the effects of offering different forage supplements, adds SAC animal scientist Nick Friggens.

"The aim of the studies is to improve systems for predicting intake of certain feeds and therefore prediction of performance," he says. "In the long run this should allow farmers to plan ahead and make best use of resources by targeting animals to meet quota more efficiently," he says. &#42

"It looks as if we are selecting for animals which can milk off their backs" – Dr Roel Veerkamp.

Given the choice dairy cows select rations according to their performance.

"High-yielding cows cannot match energy output from feed during peak lactation and mobilise body fat

to bridge

the gap" –

Dr John Oldham.

There are high-yielding cows in the Langhill herd capable of resisting metabolic stress to maintain reproductive performance and health. Researchers believe we may be able to select these animals for breeding.