Resumen

SCHWALBACH, L.M.J.; BESTER, N.  y  GREYLING, J.P.C.. Effects of high dietary energy level on the cryotolerance of ram semen. S. Afr. j. anim. sci. [online]. 2006, vol.36, n.5, pp.99-102. ISSN 2221-4062.

In general ram semen does not cryopreserve acceptably in terms of post-thawed survival and fertilization. In South Africa it is common to feed high energy diets to breeding rams in performance testing schemes and in preparation for shows and sales, as conditioned (fat) rams often realize better prices. Anecdotal evidence suggests that high energy diets and the over-conditioning of rams reduce sperm cryotolerance. This over-conditioning/obesity of rams is often blamed for the poor quality and freezing ability of their semen. Thus a study to evaluate the effects of high dietary energy levels on the cryotolerance of ram semen was conducted during the natural breeding season. Twenty four, 11 - 12 mo old Dorper rams were randomly allocated to two groups (12 rams each) and fed ad libitum at two energy levels: A low energy (LE; 6.52 MJ ME/kg DM) and a high energy (HE; 9.39 MJ ME/kg DM) diet for a period of 127 days. The rams in the HE group recorded a higher body weight gain, grew faster (average daily gain of 229 vs. 112 g/d, respectively) and were heavier (body weight of 71 vs. 56.5 kg, respectively) at the end of the trial. At the end of the trial period, semen was collected from all rams with the aid of an artificial vagina and cryopreserved using a one-step dilution (1+4) technique. The fresh and frozen (post-thawed) semen samples were evaluated using standard laboratory techniques for quality (overall and progressive sperm motility, percentage live and abnormal sperm) and were compared between the two diet groups. The fresh semen quality results provided the baseline reference or control values. As expected, the cryopreserved semen (post-thawing) recorded a lower percentage live and motile sperm than the fresh semen samples. Overall motility (57 vs. 60%), forward sperm progression (1.7 vs. 1.8) and percentage live sperm (50 vs. 60%) of cryopreserved semen (post thawing) were lower in the HE group compared to the LE group. Results also indicated that, although rams fed HE diet recorded lower semen quality values than those on the LE diet, for the qualitative semen parameters considered (both fresh and post-thawed semen), these differences were not significant. It could be concluded that the conditioning of yearling rams for a period of four months with a diet containing up to 9.39 MJ ME/kg DM does not seem to have any detrimental effect on semen cryotolerance. Further research on the effect of HE diets (at higher concentrations and for longer periods) on the cryotolerance of ram semen is warranted to allow more conclusive recommendations regarding the use of such diets to condition rams intended for semen collection and artificial insemination.

Palabras clave : Feeding; energy level; semen cryopreservation; cryotolerance; sperm; Dorper; rams.

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