The effect of age on fertility was investigated in Thai native chickens. The objective of this study was to determine the effects of age (mature and old) on the morphological characteristics of the reproductive organs and the histological characteristics of the uterovaginal junction (UVJ) tissues, resident sperm in the UVJ, and fertility duration in Thai native hens. We found no differences in the morphological characteristics of the reproductive organs, except for the number of follicles and the sizes of the fifth large yellow follicle in mature hens, which were greater than those in old hens (p < 0.05). The diameter of the sperm storage tubules (SSTs) epithelium was larger in old hens than in mature hens (p < 0.05), whereas the epithelium height was lower in old hens (p < 0.05). The number of sperm in the SSTs was greater in mature hens compared with old hens (p < 0.05). Mature hens showed a higher fertility rate than old hens. Our results suggest that, in old hens, the function of the SSTs is impaired, and sperm cannot be retained. Such a deterioration of the SSTs may be one of the factors involved in the decline in fertility.

Cryopreservation of boar semen is a technique used to extend the shelf life over that of cooled semen, which is useful to preserve genetic material for long periods (Qian, Yu, & Zhou, 2016). Artificial insemination (AI) was the first studied in 1957 (Hess, Teague, Ludwick, & Martig, 1957) and swine industry has been applying this biotechnology for several decades (Rath et al., 2009). On the other hand, this procedure does not exceed 1% of all the swine industry farm worldwide (Yeste, 2015). Frozen semen is commonly used in cattle due to the quality of semen after freezing and the satisfactory conception rate. However, the quality of boar semen after freezing has not been satisfactory. It results in a low conception rate and litter size that is not high enough to meet the requirements of the manufacturer. The issue occurs because of the low semen quality and low survival rate of spermatozoa after the freezing process (Eriksson, Petersson, & Rodriguez-Martinez, 2002). It was found that boar semen are sensitive to exposure to temperatures below 12°C, which induces destabilizing sperm membrane changes called "cold shock" as expressed by decreased sperm motility that appears to be the critical temperature to spermatozoa.

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This study aimed to determine the effects of the frequency of semen collection (once, twice, and thrice weekly) and seasonal variations on the fresh and frozen semen quality of Thai native roosters throughout the year. Data on temperature and humidity were collected and used to calculate the temperature–humidity index (THI). The average temperature and THI were lower in the winter than in the rainy season and the summer (p < 0.05). In contrast, the average relative humidity was not different among the seasons but was higher in the rainy season (p > 0.05). None of the fresh or frozen semen quality parameters were influenced by the frequency of semen collection, but the season did have an effect. The highest sperm concentration was obtained in the winter (p < 0.05). In contrast, the lowest sperm concentration was found during the rainy season, which presented the highest humidity. Regarding the frozen semen quality, the highest malondialdehyde concentration and the lowest motility were found in the summer (p < 0.05). In conclusion, semen collection can be conducted thrice per week for a consecutive year without affecting semen quality while maximizing sperm production. However, the highest sperm production was obtained in the winter, which is also a suitable season for producing semen for cryopreservation.

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