-This study evaluated yearling growth and reproductive performance of Hereford heifers at 24-26 months of age grazing on natural pastures at two stocking rates during winter and spring, associated or not to improved natural pastures.Heifers were classified according to three weight groups (Light, Medium and Heavy) and randomly distributed into four feeding management practices: 0.6 AU (1 animal unit = 450-kg body weight) -heifers on natural pastures at a stocking rate of 0.6 AU/ha; 0.8 AU -heifers on natural pastures at a stocking rate of 0.8 AU/ha; 0.6 AUI -heifers on natural pastures at a stocking rate of 0.6 AU/ha and on improved natural pastures between Sept 1 st and Nov 1 st ; 0.8 AUI -heifers on natural pasture at a stocking rate of 0.8 AU/ha and on improved natural pastures between Sept 1 st and Nov 1 st . The stocking rate of 0.8 AU resulted in significant weight loss during winter months (-0.138 and -0.356 kg for 0.6 and 0.8 AU, respectively). Heifers grazing during the spring on improved natural pastures presented higher weight gain before the breeding season and higher body weight and body condition score in the beginning and end of the breeding season in relation to those grazing only on natural pastures. Heifers kept at 0.6 AU/ha obtained 69.7% pregnancy rate, which was significantly higher than the 54.2% of those managed at 0.8 UA/ha.Heifers grazing on improved natural pastures during the spring achieved 83.8% pregnancy rate, whereas those maintained on natural pastures achieved only 40.2%. Light and medium heifers lost less weight during the winter as compared with the heavy ones, but weight differences between groups remained until the end of the breeding season. Heavy heifers have higher pregnancy rates. Heifers submitted to lower stocking rate and those grazing on improved natural pastures conceive earlier.

-Aiming at breeding heifers with 14-15 months of age, 100 Hereford calves were subjected to different feeding systems. Heifers grazed on a natural pasture in early winter, fed or not a supplement (14% crude protein; 72% total digestible nutrients), were transferred to a ryegrass pasture for 73 days, and then returned to a natural pasture for 15 days until the beginning of the breeding season. During the last 15 days on ryegrass pasture and the 15 days of the return to the natural pasture, half of the heifers of the two previous systems were offered the supplement, thereby establishing four feeding treatments. Heifers supplemented during the second period, independently of supplementation during the first grazing period, presented higher average daily gain and greater backfat values than the non-supplemented heifers. The breeding season extended for 75 days. At the start of the breeding season (November 18), heifers supplemented during both periods presented higher body condition score and body weight. The highest percentage of puberty was determined in heifers supplemented in the early winter. Heifers supplemented during both periods presented the highest pregnancy rate. Pregnant heifers presented higher average daily gain when fed the supplement during the second period, as well as higher relative pelvic area, body weight, and body condition score at the start and end of the breeding season. To an expressive pregnancy in 14-to 15-month-old heifers, higher weaning weights and pasture allowance are required to get a better answer to supplements offered.