Water and ion losses during the cross-country phase of eventing

Loss of total body water and ions during prolonged exercise can predispose the horse to health and performance problems. This study examined total body water (TBW) losses and extracellular (ECF) ion losses during the cross-country (XC) phase of Preliminary, Intermediate and Advanced Horse Trials and CCI level 3-day-events. Jugular venous blood samples and body mass (BM) were collected on 49 horses at rest, Pre-XC, Post-XC and following 30 min of recovery. Plasma was separated from blood cells within 10 min of collection. Plasma was analysed for [Na+], [K+], [Cl-], ionised [Ca+2], [glucose], [lactate] and packed cell volume (PCV) and plasma [protein] ([PP]). Distances ranged from 3000-6000 m for the XC phase with speeds of 500-570 m/min. In general, losses of TBW, Na+ and Cl- increased with increasing level of difficulty. Loss of TBW Post-XC ranged from 2-6.1% of resting values or 8.9-12.6 litres for the Preliminary level to mean +/- s.e. 20.4 +/- 1.8 litres for CCI. Losses of ECF ions ranged from 0.5-6.4% for Na+, 1.2-7.7% for Cl-, gains of 8% to losses of 23% for K+, and gains of 7% to losses of 11.7% for Ca+2 at 30 min Post-XC. There was little recovery following 30 min and deficits of 5.3 +/- 2.5 litres persisted overnight in horses that were tested. Plasma protein and PCV increased Post-XC and PCV remained elevated above resting values at 30 min recovery. It is concluded that significant water and ion losses occur and, in general, increase with increased level of difficulty. There was little change with 30 min of recovery and for some horses losses persisted overnight.