Supplementary salt for heat-stressed cows

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:

  • feeding above the recommended dietary concentration of potassium (raised from 0.9% to1.5% dry matter (DM)) can increase milk yield by 3–9%;
  • feeding above the recommended dietary concentration of sodium (raised from 0.18% to 0.5% DM) can increase milk yield by 7–18%.

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.

Research at Wollongbar

Two trials conducted at Wollongbar Agricultural Institute in northern NSW examined the effect on milk production of feeding additional salt during summer/autumn:

  • In Study 1, cows were fed an additional 0, 60, 120 or 180 g of salt per cow per day above normal requirements (i.e. a target dietary sodium content of 0.18% DM). Weather conditions were abnormally hot and humid, with little rainfall or cloud cover.
  • In Study 2, cows either were fed 120 g of salt per day above normal requirements, or were not fed any additional salt. During these experiments, cows grazed kikuyu pastures and had access to shade. Conditions during Study 2 were much milder, with cooler temperatures, high rainfall and cloud cover.

Data on milk production and liveweight change from the two studies are given in Table 1.

Table 1. Effects of supplementary salt on milk production and liveweight change (LWC)
  Additional salt fed per cow per day (g)
060120180
Study 1 Milk (L/day) 16.2 16.8 17.6 16.2
  LWC (kg/day) 0.100 0.133 0.124 0.209
Study 2 Milk (L/day) 21.1 19.9
  LWC (kg/day) 0.100 0.368
  • During the hotter conditions of Study 1, cows that were fed an extra 120 g of salt per day produced an extra 1.4 litres of milk per day. The actual concentration of sodium in the diet of the cows in this treatment was 0.54% DM, which is similar to the US findings discussed above. However, those that were fed 180 g of extra salt per day produced less milk than those which were given either 60 g or 120 g of extra salt, and they tended to gain more weight.
  • In the cooler conditions of Study 2, cows that were fed an extra 120 g of salt per day produced less milk (–1.2 litres per cow per day) but gained an extra 0.25 kg of liveweight per day compared with cows that were fed no additional salt.

Although the results of studies 1 and 2 are not strictly comparable, they do indicate a couple of interesting points:

  • Heat-stressed, pasture-fed cows will respond, in terms of milk production, to additional salt supplementation.
  • Heat-stressed, pasture-fed cows supplemented with additional salt above a dietary concentration of approximately 0.5% DM, or which are fed excess salt during cooler conditions, gain more liveweight at the expense of milk production. The exact metabolic reasons for this are unclear.