Naturopaths and nutritionists have been recommending zinc to men who are trying to increase their sperm quality for some time, and have reported their own successful results.
However, the scientific community hasn't yet conclusively confirmed or denied the benefits of zinc in semen production. A 1999 Japanese study, published in International Urology and Nephrology, found there was a positive correlation of zinc concentration with sperm concentration and sperm motility.
Additionally, a 2000 Singapore study, published by the Journal of Andrology, concluded "on the basis of the findings of this study and those of other reports, zinc may contribute to fertility through its positive effect on spermatogenesis." Conversely, a 2005 Californian study, published in Human Reproduction, found benefits for sperm production in Vitamins C, E and beta-carotene, but little effects from zinc and folate.
H Fuse, T Kazama, S Ohta and Y Fujiuchi (1999). "Relationship between Zinc Concentrations in Seminal Plasma and Various Sperm Parameters". International Urology and Nephrology, 31(3):401-8.
SE Chia, CN Ong, LH Chua, LM Ho and SK Tay (2000). "Comparison of zinc concentrations in blood and seminal plasma and the various sperm parameters between fertile and infertile men". Journal of Andrology, 21, 53-57.
B Eskenazi, SA Kidd, AR Marks, E Sloter, G Block, and AJ Wyrobek (2005). "Antioxidant intake is associated with semen quality in healthy men". Human Reproduction, 20: 1006-1012. European Society of Human Reproduction and Embryology.