Effect of animal category (prepubertal, pubertal and pregnant) on in vitro embryo production in Holstein heifers

The aim of this study was to evaluate the in vitro embryo production of Holstein heifers (Bos taurus). A total of 179 animals of 3 categories: prepubertal heifers (PP, 8 to 10 months old; n = 60); pubertal heifers (PU, 10 to 12 months old; n= 60) and pregnant heifers (PR, 14 to 18 months old; n = 59) were submitted to ovum pickup (OPU), at random stages of the estrous cycle. Animals were classified as cyclic (pubertal) or non-cyclic (prepubertal) based on the presence or absence of a corpus luteum during ultrasound evaluations (Mindray® DP 2200) performed 14 days prior to and immediately before the OPU procedure. Six replicates with 10 animals of each category were carried out, resulting in 30 donors per replicate. In each replicate, all follicles >2 mm were aspirated and the total of recovered structures, quantity and quality of viable oocytes were recorded. Recovered viable oocytes were submitted to in vitro production and the embryonic development (cleavage rate and blastocyst rate) was evaluated. On day 7 of in vitro production the embryos were vitrified for later transfer into recipients. All oocytes were fertilized by sexed semen from the same sire of Holstein breed (Bos taurus) and semen batch. Data were analyzed with the GLIMMIX procedure using the SAS® software. Pubertal heifers had a greater number of recovered oocytes (PU = 15.6 ± 1.4a vs PP = 9.8 ± 1.3b vs PR = 9.8 ± 1.6b ; P < 0.001) as well as viable oocytes (PU = 9.1 ± 0.95a vs PP = 4.6 ± 0.6b vs PR = 5.6 ± 1.1b; P < 0.001) compared to other categories. In contrast, cleavage rate was similar between pubertal and pregnant heifers (PU = 56%a vs PR = 78%a vsPP = 31%b ; P < 0.001). Interestingly, pregnant heifers had a greater number of embryos produced per OPU (PR: 1.76 ± 0.3a vs PU: 0.9 ± 0.2b vs PP: 0.13 ± 0.1c; P < 0.001) and ultimately greater blastocyst rate (PR = 37%a vs PU = 4%b vsPP = 2%c ; P < 0.001) when compared to other heifercategories. In conclusion, pregnant heifers were more efficient in terms of in vitro embryo production compared to prepubertal and pubertal Holstein heifers (Bos taurus).