With the current increases in genetic merit and feeding occurring at farm
level, dairy cows are under increasing nutritional stress in early lactation.
Cows obtain their energy at this time from the feeds they eat and from body
reserves. The relationship between body condition at calving and productivity
of dairy cows has been reviewed, with particular emphasis on interactions
between body condition and nutrition in early lactation. Recent research on
the influence of body condition at calving on subsequent milk productivity,
conducted mainly in the United Kingdom with complete diets fed indoors, has
produced results in apparent conflict with the previous results from southern
Australia and New Zealand where cows grazed pasture. In particular, the
overseas research suggests considerably less advantage to improvements in body
condition than had been previously thought. It is concluded that more
information is needed concerning the interaction between body condition at
calving and nutrition in early lactation, with dietary energy and protein both
being important. There is a suggestion that, when complete diets are fed, it
is better to achieve high energy concentrations in post-calving diets by the
use of high-fibre concentrates with a fat supplement, rather than with
high-starch concentrates. This has implications for dairying in Australia,
since cereal grains are the major energy supplement used on many farms in
early lactation and recent research has indicated that immediate marginal milk
production responses to the use of concentrates may be poorer with fat cows
than with thin cows.
Reports from controlled feeding experiments indicate that fat cows need more
dietary protein than thin cows and undegradable dietary protein might be of
more concern than rumen degradable protein. However, in dairy systems where
pasture is a considerable proportion of the diet, benefits of supplying
specific undegradable dietary protein supplements still need to be
established. Recent research has suggested that pasture appears to provide
considerable quantities of undegradable dietary protein, even though the crude
protein in pasture is potentially highly degradable in the rumen.
Body condition at calving may also affect subsequent reproductive performance.
This is due to its association with the degree of negative energy balance
occurring in early lactation and because fat cows may be more susceptible to
metabolic disease(s). While the mechanisms involved are probably quite
complex, increases in animal productivity will generate more stress in cows at
a time of their annual cycle when stress needs to be minimised. Further
understanding is required to link the relevancy of overseas research to
Australian dairy farming conditions where pasture is a key input.
PROLINE SUPPLY FOR EFFICIENT NITROGEN UTILIZATION BY LACTATING RUMINANTS