Physiological response to weight carrying and associations with conformation traits in Icelandic horses used for tour riding

Abstract

Background: Weight carrying capacity is an important trait in riding horses and it may be associated with conformation. This study examined the physiological response to a ridden incremental weight carrying test in 16 adult Icelandic horses used for tour riding. Horses carried 20% (BWR_20%), 25% (BWR_25%), 30% (BWR_30%) and 35% (BWR_35%) of their body weight (BW) in tölt (~ 5.7 m/seconds, 640 m/step), and associations with body measurements and back conformation (score) were examined. Horses were divided into two groups (narrow or broad back) and body measurements were collected. Plasma lactate was analysed in blood samples collected after each step in the exercise test, an exponential equation was fitted, and BW-ratio was calculated for 2, 3 and 4 mmol/L (BWR_La2, BWR_La3 and BWR_La4). Plasma creatine kinase (CK) and aspartate amino transferase (AST) were analysed at rest and 24 h post exercise.

Results: Four out of 15 horses did not reach a plasma lactate concentration of 4 mmol/L, even at BWR_35%. A positive correlation was found between chest width and BWR_La4 and between the difference between height at withers and croup and BWR_La2 (P < 0.05). Hock circumference and the difference between height at croup and back were negatively correlated with BWR_La2 (P < 0.05). The change in CK from rest to 24 h post exercise was negatively correlated with the difference between height at withers and height at back and croup (P < 0.05).

Conclusions: The physiological response to weight carrying was relatively low. A wider chest, "uphill" conformation, straight backline and smaller hock circumference were associated with weight carrying capacity, but group (narrow or broad back) was not.

Keywords: Back; Body measurements; Equine; Rider weight; Weight carrying exercise test.

Fig. 1

Illustration of the thoracolumbar region of the back with different condition scores. Scores are ranging from 4 to 5 with 0.25 increments (a score of 4.25 and 4.75 are not shown in the figure) as described in a previous study [4] and based on the Henneke scale [12]. The backs are illustrated from a caudal view, and the black dots symbolises the spine

Fig. 2

Measurement of the thoracolumbar region at two different sites using a flexible curve ruler. Flexible curve ruler placed at (a) the 18th thoracic vertebra identified by palpation of the last rib (i.e. back angle) and (b) at the height of the caudal part of the last rib (by palpating vertically from the rib up to the dorsal aspect of the lumbar region) (i.e. loin angle)

Fig. 3

Curve from measurements of the thoracolumbar region on Icelandic horses using a flexible curve ruler. The angle (red arrow) of the region was measured from the drawn curve, using a goniometer [10]

Fig. 4

Body measurements collected in 16 Icelandic horses. The measurements were: height at withers (highest thoracic vertebra) (yellow + red arrow) (WH), height at the lowest point of the back (BH), height at the highest point of the croup (tubera sacrale) (TSH), back length (BL), measured from the cranial part of tubera sacrale to the highest point of the withers, body length (BoL), measured from tuberculum majus humeri to the caudal end of the hindquarters (horizontally to tuber ischiadicum), breast depth (DB) (yellow arrow), measured from the withers and straight down to sternum, chest width (CW), measured between the points of the shoulders (tuberculum majus humeri), width of the pelvis between the points of the pelvis (tubera coxae) (WP), width of the hips between the hip joints (WH), maximum circumference of carpus (CC) and hock (tarsus) (TC) and minimum circumference of metacarpus (MCC) and metatarsus (MTC). Notice that the figure only describes the measurements and not the experimental conditions (experimental measurements were performed indoors on concrete floor and with horses in even body position)

Fig. 5

Physiological responses recorded in 14 Icelandic horses during a ridden exercise test. Horses carried 20, 25, 30 and 35% of their body weight (LSM ± SE). Different letters (a–d) indicate differences between body weight ratio values (P < 0.05)

Fig. 6

Relationships between physiological parameters and body measurements in Icelandic horses performing a weight carrying test. The figure shows the relationship between a body weight ratio (BWR) at the lactate threshold (BWR_La4) and chest width (r = 0.77, P < 0.05), and between BWR at a plasma lactate concentration of 2 mmol/L (BWR_La2) and b the difference between height at withers and height at croup (tubera sacrale) (r = 0.68, P < 0.05), c the difference between height at croup (tubera sacrale) and height at back (r = − 0.71, P < 0.05) and d hock circumference (r = − 0.77, P < 0.05). Blue dots represent horses with narrow back (back score < 4.5), red dots horses with broad back (back score > 4.5) and black dots horses with a back score of 4.5 [12]