Introduction
The horse is a seasonal polyestrous species associated with increase in day light (Guillaume, 1996; Nagy et al., 2000) temperature and availability of food (Nagy et al., 2000). Long day period increases the mean LH and T2 levels in pony stallions and the onset of the ovulatory season in pony mares (Berndtson et al., 1983).
In the Northern Hemisphere, the natural breeding season of horses usually extends from April to September. Previous studies have shown that seasonal changes in day length have a direct effect on fertility and semen quality in stallions. However, limited information has been available regarding the seasonal variations in testosterone concentration in Arabian stallions of Khuzestan Province (with its specific climatic conditions and different day lengths). This study was conducted to investigate these variations under natural light conditions.
Materials and Methods
Study Area and Climatic Conditions
The study was conducted in Khuzestan Province, Ahvaz City (latitude 48°40′N, longitude 31°20′E, elevation 22.5 m above sea level). The region has an average annual rainfall of 15.48 mm and a temperature range between 1°C and 50°C.
Animals Under Study
In this study, 23 healthy Arabian stallions, aged 3 to 15 years and weighing between 450 and 600 kg, were examined. All horses were fed a standard diet and housed in stables.
Sampling Schedule
To investigate the effect of seasonal changes, blood sampling was carried out during two periods:
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Breeding Season: May to June (Summer Solstice; day length 14 hours and 11 minutes)
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Non-Breeding Season: November to December (Winter Solstice; day length 10 hours and 26 minutes)
Sample Collection and Hormonal Analysis
From each stallion, 10 mL of blood was collected from the jugular vein. After centrifugation, the serum was stored at −20°C. Testosterone concentration was measured using a radioimmunoassay (RIA) kit with a minimum sensitivity of 0.05 ng/mL. All samples were analyzed in duplicate within a single assay run.
Statistical Analysis
Comparison of mean testosterone concentrations between the two seasons was performed using a paired t-test. Environmental temperature, relative humidity, and sunlight duration data were also compared between the two seasons.
Results
Climatic data showed that day length was significantly longer in summer compared to winter < and ambient temperature was also higher in summer (38.4°C vs. 9.7°C).
The mean serum testosterone concentration during the breeding season was 1.08 ± 0.078 ng/mL, whereas in the non-breeding season it was 0.65 ± 0.11 ng/mL, showing a significant difference (<).
Discussion
The results of this study confirm the presence of a seasonal rhythm in the reproductive cycle of Arabian stallions in Khuzestan. This pattern is consistent with previous findings in other horse breeds and ponies in the Northern Hemisphere, which reported the highest testosterone concentrations during the breeding season and the lowest levels in winter.
The role of photoperiod (day length) is central to these changes; light is transmitted through the retina to the pineal gland, leading to a reduction in melatonin secretion. The decrease in melatonin stimulates the release of GnRH, which in turn increases LH secretion and ultimately stimulates Leydig cells in the testes to produce testosterone.
Interestingly, the extremely high summer temperatures in Khuzestan did not have a significant negative effect on testosterone secretion, indicating the physiological adaptability of Arabian stallions in this region to hot climatic conditions.
Conclusion
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Testosterone concentrations in Arabian stallions are significantly higher during the breeding season compared to the non-breeding season.
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Day length is considered the main factor regulating the hormonal cycle in this breed.
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The hot climatic conditions of Khuzestan do not have a significant negative effect on testosterone secretion.
Download Link: Serum testosterone in Arabian stallions during breeding and non-breeding seasons in Iran | October 12, 2011
