Modelling the effect of elevated testicular temperature of Holstein Friesian bulls in a moderate climate on rejection rates of ejaculates in semen processing.

Heat stress can cause bulls to start producing sub-, or even infertile semen. This is usually not immediately noticed during quality control while processing semen. Sometimes a heat period is already over before an increase of rejection ejaculates is noticed in the lab. This delayed effect, but also because a heat eriod is often not discrete/ 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 as heat stress was mimicked by elevation of testicular temperature (about 2 °C, 48h) of 2 Holstein Friesian bulls in a controlled experiment. Analyses of semen, collected 1week before until 9 weeks after the experiment, revealed that specific stages in spermatogenesis, meiosis and spermiogenesis, are susceptible for heat stress. During these
stages percentages morphologically normal, progressive motile and membrane intact cells, reduced about 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 as the Netherlands.
The standards of the three evaluated quality parameters, which are used by many AI stations, are: per ejaculate more than 75% morphologically normal cells, and more than 35% progressive motile and 50% membrane intact cells after thawing. Normal rejection rates were determined from original scores of at least two years from two AI stations (CRV, the Netherlands). That is: 11453 morphology scores of fresh semen, and 10805 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 33, 25 and 20% respectively. Normal vs. estimated 'heat stress' rejection rates, based on original and modelled 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 over a month of reduced sperm quality. This study indicates that this effect can be very important for AI stations. In 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 scored occasionally when requested for export of semen, but morphology is scored always 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 about 11 months old start, but this decreases fast for most bulls to 0% within a few months as they mature. When these young bulls suffered heat stress before production, it is possible that for a long time the rejection rate will stay at 100%. When this is suspected a prolonged test period should be considered, to avoid wrongful culling of bulls.