Photoperiod is known to be important in regulating the pattern of wool growth in sheep. The aim of this study
was to investigate the effect of natural changes in photoperiod on live weight, skin follicle activity and fiber
characteristics of grazing Kermani sheep in the south of Iran, Jiroft (28° 40′ N and 57° 44′ E, elevation 650 m).
Ten male and 10 female Kermani sheep with initial live weights of 31.9 ± 0.8 and 30.7 ± 0.9 kg (mean ±
s.e.) respectively, were used in a 365 day study. Percentage of active primary (PAP) and percentage of active
secondary (PAS) follicles, ratio of secondary to primary follicles (S/P) and follicle density (FD) were measured in
skin samples, taken from the right mid-side of the animals at monthly intervals. Greasy and clean wool growth
rates and fibre diameter were determined from patch samples (10 cm × 10 cm) harvested at the end of every
month. The value for PAP was greatest (p < 0.05) in summer and spring and lowest (p < 0.05) in winter
(98.2 ± 0.8, 84.1 ± 0.9, 75.6 ± 0.8 and 97.0 ± 1.1% for summer, autumn, winter and spring, respectively);
seasonal differences in PAS were similar (99.3 ± 0.8, 88.5 ± 0.7, 82.9 ± 0.8 and 98.9 ± 0.7% for summer,
autumn, winter and spring, respectively). Clean wool growth rate was greatest (p < 0.001) in summer and
spring, and lowest (p < 0.001) in winter (0.7 ± 0.03, 0.4 ± 0.04, 0.3 ± 0.01 and 0.7 ± 0.05 mg/cm2
/day
for summer, autumn, winter and spring, respectively). Fibre diameter was greatest in spring (34.8 ± 0.3 μm)
and lowest in winter (29.1 ± 0.2 μm). The values for wool growth rates and fibre diameter were greater
(p < 0.05) in male rather than those observed in female sheep. Fibre and skin follicle characteristics were not
affected by a season × sex interaction. These results demonstrated that there is a seasonal pattern in fibre
follicles activity and wool growth in grazing Kermani sheep.
