The aim of this work was to evaluate the effect of herbage allowance of grasslands (Campo biome) during the prepartum and postpartum on productive and reproductive responses of primiparous beef cows. Fifty-four Hereford cows (5.9 ± 0.5 BCS in a 1–8 scale and 472 ± 35 kg BW) were used in a randomized block design with 2 spatial replications and a factorial arrangement of prepartum (fall;high vs. low, PREH vs. PREL) and postpartum (spring-summer; high vs. low, POSTH vs. POSTL) herbage allowance (4 vs. 2.5 kgDM/kgBW of annual mean for high vs. low). Cows were in a continuous grazing system. Cow BW, BCS, and calf BW were determined monthly and at calving and milk production and composition were determined using a milking machine at 130 d postpartum. Calving to conception interval (CCI) and early (during first month of mating period) and total pregnancy were registered using ultrasound. Means from a mixed model were considered to differ when P ≤ 0.05. Cow BW and BCS were greater from mid-winter to the end of summer (last third of gestation to weaning) for PREH than PREL cows and during the postpartum for POSTH than POSTL cows. Milk energy output was between 1.2 and 1.6 ± 0.8 Mcal NEL/d greater for PREH than PREL cows and for POSTH than POSTL cows. Calf BW did not differed at birth but at weaning was 30 ± 5 kg greater for POSTH than POSTL cows. The CCI tended (P = 0.10) to be shorter for PREH than PREL cows (129 vs. 139 ± 6 d) and was shorter for POSTH than POSTL cows (126 vs. 141 ± 6 d). Early pregnancy was greater for POSTH than POSTL cows (54 vs. 39%) while total pregnancy did not differed among treatments (averaged 77%). Cow BCS at calving affected CCI, early and total pregnancy (1 unit of BCS at calving decreased CCI by 21 d and increased early and total pregnancy by 50%). Effects of PRE and POST herbage allowances had an additive effect on productive and/or reproductive responses of primiparous beef cows grazing native pastures.