The effect of rider weight and additional weight in Icelandic horses in tölt: part I. Physiological responses

Abstract

This study examined the effect of increasing BW ratio (BWR) between rider and horse, in the BWR range common for Icelandic horses (20% to 35%), on heart rate (HR), plasma lactate concentration (Lac), BWR at Lac 4 mmol/l (W4), breathing frequency (BF), rectal temperature (RT) and hematocrit (Hct) in Icelandic horses. In total, eight experienced school-horses were used in an incremental exercise test performed outdoors on an oval riding track and one rider rode all horses. The exercise test consisted of five phases (each 642 m) in tölt, a four-beat symmetrical gait, at a speed of 5.4±0.1 m/s (mean±SD), where BWR between rider (including saddle) and horse started at 20% (BWR20), was increased to 25% (BWR25), 30% (BWR30), and 35% (BWR35) and finally decreased to 20% (BWR20b). Between phases, the horses were stopped (~5.5 min) to add lead weights to specially adjusted saddle bags and a vest on the rider. Heart rate was measured during warm-up, the exercise test and after 5, 15 and 30 min of recovery and blood samples were taken and BF recorded at rest, and at end of each of these aforementioned occasions. Rectal temperature was measured at rest, at end of the exercise test and after a 30-min recovery period. Body size and body condition score (BCS) were registered and a clinical examination performed on the day before the test and for 2 days after. Heart rate and BF increased linearly (P0.05), but negative correlations (P<0.05) existed between body size measurements and Hct. While HR, Hct and BF recovered to values at rest within 30 min, Lac and RT did not. All horses had no clinical remarks on palpation and at walk 1 and 2 days after the test. In conclusion, increasing BWR from 20% to 35% resulted in increased HR, Lac, RT and BF responses in the test group of experienced adult Icelandic riding horses. The horses mainly worked aerobically until BWR reached 22.7%, but considerable individual differences (17.0% to 27.5%) existed that were not linked to horse size, but to back BCS.

Keywords: Icelandic horse; heart rate; lactate; rider weight; weight carrying capacity.

Figure 1

Body condition score of the back (place of the saddle), based on the Henneke scale (Henneke et al., 1983) but modified to an accuracy of 0.25 of a score (the spot symbolizes the spine).

Figure 2

(a) Heart rate, (b) hematocrit, (c) plasma lactate concentration and (d) breathing frequency (least square means±root mean standard error) after five phases (each 642 m in tölt) in an incremental exercise test in eight Icelandic horses where the BW ratio between rider and horse started at 20% (BWR₂₀), was increased to 25% (BWR₂₅), 30% (BWR₃₀) and 35% (BWR₃₅), and then decreased to 20% (BWR_20b). Blood samples were collected and breathing frequency counted at end of each of the five phases. Heart rate was recorded during the last minute of each phase. ^a,b,c,dSignificantly different mean value (P<0.05).

Figure 3

Relationship between back body condition score of the horses (BCS_back) (Henneke scale 1–9; Henneke et al., 1983) and the BW ratio between rider and horse at which the horses reached 4 mmol/l (W₄) (y=11.706×−29.11, r ²=0.56).