Abstract

Nowadays, X- and Y- sperm can be sorted reliably by flow cytometry/cell sorting. However, this new technology is rarely being adopted by dairy farmers in Mexico and in developing countries, mainly due to the higher price of sexed semen straws and to a lack of information and knowledge of the results that can be obtained with its use under field conditions. In 2004, a field experiment started in a dairy farm in Tizayuca, State of Hidalgo, Mexico, aimed to examine fertility and sex ratios after artificial insemination (AI) in heifers under field conditions. Semen from two Holstein bulls was used for the experiment. The semen was sexed and frozen by XY Mexico (subsidiary of XY, Inc., Fort Collins, CO, USA). Only the X fraction of spermatozoa was sorted and frozen in 0.25 cc plastic straws with 2.0 × 10⁶ spermatozoa/straw. Sixty Holstein heifers averaging 14 months of age were artificially inseminated 12 hours after naturally occurring standing heat, using one straw of sexed semen, from June to October 2004. Fertility data of all inseminations were collected by rectal palpation from Days 45 to 60 after AI. A total of 30 heifers were pregnant, indicating a fertility of 50% froma single service. Sex of the calves was confirmed after calving between March and July 2005. From a total of 30 calves born after normal pregnancy, 27 were females (90%) and 3 were males (10%). In contrast, the pregnancy rate obtained in 60 heifers inseminated with non-sexed semen from the same two bulls was 68% (41 pregnant). From this group of heifers, 20 female (49%) and 21 male (51%) calves were born. Chi-square analysis indicated that pregnancy rates and sex ratios between the two groups were significantly different (P < 0.05). This preliminary information indicates that there is an advantage in the sex ratio obtained when using sexed semen for AI in Holstein heifers. This is especially important at a time when importation of replacement heifers to Mexico from Canada and the USA is not an option. However, more experiments are needed to confirm the results obtained in this preliminary study.