Milk production, reproduction, longevity, efﬁciency
The objective of this study was to analyze the relationship between age at first calving with dairy production and reproductive indicators in dairy cows, with different productive potential. Retrospective data of 260 Holstein American-Canadian biotype cows with information from birth to the end of lactation, collected from 1992to 2012 in the dairy farm belonging to the agro-technical school Gral. San Martín, National University of Rosario, were used. The farm is located in the city of Casilda, province of Santa Fe, Argentina. Cows were divided into two categories: pure cows (n = 103) and cows with breeding records (n = 157). Each cow belonging to each of the aforementioned groups was characterized according to the values of two reproductive indicators: age at first calving in days and calving-conception interval in days and two productive indicators: milk production adjusted to 305 days of lactation in liters, and number of calving. There were no significant differences between groups in age at first calving. The pure cows presented higher production (t = 4.493, P <0.0001) while the cows with breeding record were more fertile (t = 2.039, P = 0.043). Groups did not differ in the number of calving. Differences in the behavior of pure cows and cows with breeding records, with different productive potential, puts into discussion the objective of exerting pressure to achieve a first delivery at 24 months as a generalized proposal in the grazing systems.