131 RETROSPECTIVE DATA ANALYSIS OF HOLSTEIN HEIFER GROWTH IN EMBRYO PRODUCTION CENTERS

Abstract

The reduction of the generation gap has become of main interest in the race for the genetic improvement of cattle and in reducing the environmental impact of dairy farms. Sexual precocity of high genetic merit heifers is therefore essential, as it allows the development of their potential as soon as possible thanks to embryo collection. Many factors influence the sexual precocity and embryos production of bovine females. A major effect of the calves’ growth rate has often been observed on puberty precocity in experimental trials. This study was carried out on data recorded from January 2010 to May 2015 on 617 Holstein heifers housed in 2 centers of embryos production with similar breeding conditions in France. The purpose was to find optimal breeding values of Holstein heifers for embryo production in these centers. The heifers entered at the centers between 4 to 15 months old (after a quarantine of 30 days) to produce in vivo embryos during 2 to 4 collections before to come back pregnant to their breeder owner. The first treatment of superovulation was programmed when a first heat was observed visually or via an activity sensor (Heatime R) or when a corpus luteum had been detected (ultrasound scans). The studied variables were the date of the first flushing as a precocity indicator and the collection results for the production of embryos (numbers of total and viable embryos). In this population, the average age at entry was 8.5 ± 2.3 months old (minimum = 3.4; maximum = 15.0), the average age at the first collection 14.3 ± 1.2 months (minimum = 11.0; maximum = 19.0), the average daily gain (ADG) between 0 and 12 months was 792 ± 86 g day^–1 (minimum = 492; maximum = 1027), the average number of total structures by collection 9.3 ± 6.1, the mean number of viable embryos 5.5 ± 3.8. Data analysis by multivariate ANOVA test showed an effect on age at first collection for the center (P = 0.009), age at entry (P < 0.001), and ADG between 0 and 12 months (P < 0.001). Heifers with ADG between 0 and 12 months of <650 g day^–1 had a delay of 70 days for the first collection compared to those with an ADG of >900 g day^–1. The ADG between 0 and 12 months had also the strongest negative correlation found with the age at first collection (r = 0.52). Similarly, ADG between 0 and 6 months of the 40 heifers entered in the centers before the age of 6 months was significant for age at the first collection (P = 0.0001), whereas ADG between 6 and 12 months was not. Regarding the production of embryos, only the weight at the first collection tended (P = 0.07) to have an effect on the total number of collected structures: 10.1 ± 6.5 for heifers that weighted more than 420 kg v. 7.3 ± 4.0 for those that weighted less than 340 kg. No effect on the percentage of grade 1 or 2 embryos could be observed. In conclusion, this study conducted in production conditions confirmed the observations made in experimental trials and refined the recommendations and practices of the two centers to reduce the age at first embryo flushing.