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Systems for preserving semen in liquid form for artificial insemination were developed before cryopreserved semen became widely available in the 1960s. Advantages of liquid semen include reduced number of sperm per dose, reduced storage and transportation costs, increased insemination speed and safety in the field. A liquid semen dose requires one tenth the sperm number in a frozen semen dose to achieve equivalent fertility (24 day non-return rate: 67.6% for liquid versus 67.8% for frozen). The main disadvantage of liquid semen is its relatively short shelf life, thus limiting its application mainly to countries, like New Zealand and Ireland, with predominantly seasonal dairy production systems. Nevertheless, successful application of liquid semen technology can improve the rate of genetic gain by increasing the utilization of elite sires. This brief review covers the principles of liquid semen preservation and describes why and how this technology is implemented by Livestock Improvement Corporation in New Zealand.

Romney rams, which had either an early (n = 7) or a late (n = 5) increase in testis size prior to the onset of the breeding season, were selected from a larger population (n = 60). In the following year, the two groups were compared for the pattern of seasonal variation in testis size, circulating gonadotrophin concentrations and pituitary responsiveness to a gonadotrophin releasing hormone (GnRH) challenge during the transitional period from the non-breeding to the breeding season. Rams in the ‘early’ group had an earlier and greater increase in testis size than rams in the ‘late’ group, a pattern which was largely repeatable between years. Differences between the groups in the pattern of seasonal variation in testis size were associated with group differences in endocrine function. Thus, rams in the early group had a significantly (P < 0.01) higher luteinising hormone (LH) pulse frequency in March (4.4 ± 0.4 pulses per 8 h period) than those in the late group (1.7 ± 0.3 pulses per 8 h period) and the seasonal increase in plasma follicle stimulating hormone (FSH) concentrations occurred earlier (P < 0.10) in the early group than in the late group. There were also group differences in the pattern of seasonal variation in total LH response to the GnRH challenge. These results show that there are differences among rams of the same breed, in the pattern of seasonal variation in testis size and gonadotrophin secretion, which could potentially be used as criteria to select rams for date of onset of the breeding season in their daughters.

A study was conducted to compare the variation in testis size, gonadotrophin secretion and pituitary responsiveness to an exogenous gonadotrophin-releasing hormone (GnRh) challenge during the transitional period from the non-breeding to the breeding season in rams of the Romney and Poll Dorset breeds. Testis size of rams of both breeds varied significantly (P < 0.001) during the trial period, with a significant (P < 0.01) breed difference in the timing and magnitude of this variation. In Poll Dorset rams, increases in test's size occurred earlier, but the magnitude of seasonal variation was smaller. Overall, mean luteinizing hormone (LH) concentration was significantly (P < 0.05) higher in Romney rams mainly because of a difference (P < 0.001) in the frequency of LH pulses. There was also a significant (P < 0.01) effect of sampling time and significant (P < 0.05) breed × time interactions in LH pulse frequency. Mean follicle-stimulating hormone (FSH) levels exhibited significant (P < 0.01) variation with sampling time, with the increase in FSH levels occurring earlier (P < 0.06) in Poll Dorset rams. There was a significant effect of sampling time on both the peak (P < 0.01) and the total (P < 0.05) LH response to GnRH challenge, but no significant effects of breed or breed × time interactions were detected. These results show that breed differences in seasonally are associated with differences in the pattern of variation in testis size and gonadotrophin secretion during the transitional period from the non-breeding to the breeding season. Some of these differences could potentially be used to select rams within a breed for date of onset of the breeding season in their daughters.