A long-day light program accelerates seasonal coat changes but is without effect on semen and metabolic parameters in Shetland pony stallions

Horses are seasonal breeders, and robust breeds may exhibit a winter hypometabolism when kept under semiferal conditions. In this study, we analyzed the effects of artificial long days on rectal temperature, heart rate, heart rate variability, hematology, coat changes, semen parameters, and plasma testosterone concentrations in Shetland stallions stabled overnight and assigned to a control group (CON, n = 9) kept under natural photoperiod, and a treatment group exposed to a long-day light program from 15 December to 20 March (AL, n = 9). During the 8-month study, rectal temperature, heart rate, and heart rate variability at no time differed between groups. Plasma total protein (P < 0.05), hematocrit, leukocyte, and lymphocyte counts (P < 0.001) first increased and then decreased during the study period but did not differ between groups. Length of the guard hair decreased over time (P < 0.001) and this decrease occurred earlier in AL than in CON stallions (time × group P < 0.001). Hair regrowth was faster in CON than in AL stallions (over time P < 0.001, time × group P < 0.001). Total sperm count increased from January to April (AL) and May (CON; P < 0.001) but did not differ between groups. Sperm motility and percentage of membrane-intact spermatozoa showed no clear seasonal changes and semen parameters did not differ between groups. In conclusion, Shetland stallions showed seasonal variations in hair coat and total sperm count but only changes in hair coat but not semen parameters were advanced by a long-day light program.