Date: 18 May 2004 Author: Brad Granzin
It is well recognised that most Australian dairy cows experience heat stress or high heat loads during summer. Experiments conducted by the Queensland Department of Primary Industries and the University of Queensland during the mid-90s showed that warm, humid weather (i.e. maximum temperatures >27°C + high humidity, or >32°C + low humidity) often limits a cow’s milk production.
Studies with heat-stressed, confined cows in southern USA have shown that:
This is because heat-stressed cows have high potassium losses through sweat, and excrete high levels of sodium in urine.
During summer, pasture-fed cows in Australia often consume a diet with a potassium concentration above 1.5% DM. This is because most of our summer pastures (with the exception of perennial ryegrass) have a high potassium content. Hence we would expect little benefit to feeding additional potassium.
In contrast, sodium concentrations are below 0.5% DM in most pastures offered to dairy cows during summer, with the exception of some tropical grasses. Therefore, there may be some benefit to feeding additional sodium to heat-stressed cows in our systems. Salt is one supplement which contains high sodium levels.
Two trials conducted at Wollongbar Agricultural Institute in northern NSW examined the effect on milk production of feeding additional salt during summer/autumn:
Data on milk production and liveweight change from the two studies are given in Table 1.
|Additional salt fed per cow per day (g)|
|Study 1||Milk (L/day)||16.2||16.8||17.6||16.2|
|Study 2||Milk (L/day)||21.1||–||19.9||–|
Although the results of studies 1 and 2 are not strictly comparable, they do indicate a couple of interesting points: