178 MODELLING THE EFFECT OF ELEVATED TESTICULAR TEMPERATURE OF HOLSTEIN FRIESIAN BULLS IN A MODERATE CLIMATE ON REJECTION RATES OF EJACULATES IN SEMEN PROCESSING

Abstract

Heat stress can cause bulls to start producing sub- or even infertile semen. This is usually not noticed immediately during quality control while processing semen. Sometimes, a heat period is already over before an increase of rejection ejaculates is noticed in the laboratory. This delayed effect, but also because a heat period is often not discrete or clear-cut, makes it difficult to relate heat stress with semen quality. The recent study of Rahman et al. (Theriogenology, in press) is therefore of interest, given that heat stress was mimicked by elevation of testicular temperature (∼2°C, 48 h) of 2 Holstein Friesian bulls in a controlled experiment. Analyses of semen, collected 1 week before until 9 weeks after the experiment, revealed that specific stages in spermatogenesis, meiosis and spermiogenesis are susceptible to heat stress. During these stages, the percentages of morphologically normal, progressive motile and membrane-intact cells reduced by approximately 33, 25 and 20%, respectively. The objective of this study was to model how these reductions would affect rejection rates for these parameters in a moderate climate, such as the Netherlands. The standards of the 3 evaluated quality parameters, which are used by many AI stations, are per ejaculate more than 75% morphologically normal cells, more than 35% progressive motile and 50% membrane intact cells after thawing. Normal rejection rates were determined from original scores of at least 2 years from 2 AI stations (CRV, the Netherlands). That is, 11 453 morphology scores of fresh semen, 10 805 progressive motility and 3741 membrane integrity scores of thawed semen from 2065, 492 and 1488 bulls, respectively. To estimate rejection rates as if ejaculates were affected by heat stress, each morphology, progressive motility and membrane integrity score was reduced by 33, 25 and 20%, respectively. Normal vs estimated heat stress rejection rates, based on original and modeled scores, respectively, were 50 vs 100% for morphology, 2 vs 17% for progressive motility and 9 vs 61% for membrane integrity. Rahman showed that already a short period of heat stress can lead to reduced sperm quality for over a month. This study indicates that this effect can be very important for AI stations. In the case of progressive motility, which in the Netherlands is scored for each ejaculate, the estimated higher rejection rate of 17% could lead to a considerable increase of production costs. Membrane integrity is occasionally scored (when requested for export of semen), but morphology is always scored when bulls start to produce semen until ejaculates are produced in succession with more than 75% normal cells. High rejection rates for morphology are typical when bulls (∼11 months old) start, but this decreases quickly for most bulls to 0% within a few months as they mature. When these young bulls suffer heat stress before production, it is possible that the rejection rate will stay at 100% for a long time. When this is suspected, a prolonged test period should be considered to avoid wrongful culling of bulls.