Increasing sodium in herbage for grazing dairy cows can reduce the milk somatic cell count (Phillips et al., 1996), but the effects of providing salt supplements for cows fed conserved feeds indoors are less clear. Supplementary salt can also reduce oral stereotypic behaviour in individually-housed calves (Phillips and Youssef, 1994). Tethered dairy cows are also behaviourally restricted, demonstrate stereotyped behaviours and might similarly benefit from supplementary salt.

In experiment 1, 36 tethered Estonian Red cows were allocated for nine weeks to a randomised block experiment in which they received a daily restricted allowance of 7.5 kg DM grass silage, 6.6 kg DM grass hay and 2.2 kg DM barley/cow, with the addition of 0, 200 or 400 g NaCl. The salt supplements increased the Na content of the diet from 1 (control treatment) to 6 and 11 g/kg DM respectively. Milk yield was recorded every ten days and somatic cell count (SCC) by fluorescence microscopy. A blood sample was taken from the milk vein of each cow at the end of the experiment for mineral analysis.