The aim of this study was to describe the distribution of age at first calving (AFC) in dairy herds of Córdoba province and to evaluate the effect of AFC on productive indicators during the first lactation. A retrospective study was used to analyze data from 26,614 cows calved for the first time during the calendar year 2016. A total of 15,181 first-lactation cows were included in the analysis. Registers were stratified by age at first calving (AFC) into 5 groups: 1) 18–21 mo, 2) 22–25 mo, 3) 26–29 mo, 4) 30–33 mo, 5) 34–37 mo. Milk yield (MY) for first lactation cows having at least 5 milk test days was used to analyze production. The General Linear Mixed Models was applied to determine differences in start-up, peak and daily mean milk yield. The fixed effects evaluated were AFC category, calving season and herd size category, herd was fitted as a random effect. The effect of AFC on milk production during the 5 milk test days was evaluated including the random effect of cow nested within farm. The overall median and interquartile range (IQR) of AFC was 27 and 25–30 mo, respectively. Median milk yield per heifer was 27.1 kg/d (IQR = 23–31). MY, start-up and peak milk yield increased systematically from the 1st to the 5th AFC category (Table 1). Milk production adjusted by herd size and calving season for the 5 AFC categories showed lower MY for lower AFC categories at the beginning of the lactation but differences between AFC categories 3, 4 and 5 disappeared by the 4th test day. In conclusion, opportunities exist to reduce AFC, not much affecting MY and reducing rearing costs in dairy herds of Córdoba province. Table 1 (Abstr. M139). Estimates (mean ± SE) of productive indicators according to AFC category¹

	1	2	3	4	5
N	202	5569	5579	2389	1442
Milk yield (MY) (kg/d)	23.0 ± 0.47a	24.7 ± 0.33b	25.3 ± 0.33c	25.8 ± 0.33d	26.5 ± 0.34e
Start-up MY (kg)	19.6 ± 0.56a	21.4 ± 0.30b	22.0 ± 0.29c	22.4 ± 0.31d	22.9 ± 0.33d
Peak MY (kg)	26.4 ± 0.51a	28.6 ± 0.36b	29.3 ± 0.36c	29.9 ± 0.37d	30.9 ± 0.38e

^a–eDifferent letters within a row indicate significant difference at P<0.05. ¹AFC (age at first calving) categories: 1) 18-21 mo, 2) 22-25 mo, 3) 26-29 mo, 4) 30-33 mo, 5) 34-37 mo.