The influences of nutritional protein and energy during early and mid pregnancy on milk production and postpartum reproductive parameters were determined in 70 beef heifers of two composite breeds (Bos indicus X Bos taurus). At artificial insemination (AI), heifers were divided into four dietary treatment groups identified by the level of protein, and to a lesser extent energy, fed during the first and second trimesters: high/high (HH), high/low (HL), low/high (LH), and low/low (LL). Milk production was lower in the heifers receiving high treatment in first trimester than that in heifers receiving the low treatment (P=0.01). Milk production was negatively associated with dam body condition score (BCS; P=0.01), nonesterified fatty acids (P=0.001), and leptin (P=0.02) and positively associated with urea (P<0.001) concentrations during lactation. Increased dietary protein in the first trimester increased or decreased concentrations of colostral protein dependent upon genotype (P=0.03). Colostral protein was positively associated with bovine pregnancy associated glycoprotein from late gestation (P=0.007). Milk fat was negatively associated with BCS (P=0.007) and influenced by genotype (P=0.003). Dietary treatment did not affect the postpartum reproductive performance of beef heifers. Gestation length (P<0.001) and the postpartum interval to first estrus (PPI; P=0.02) were positively associated with calf size. Placental size was negatively associated with placental expulsion time (P<0.01). Prepartum BCS of the heifers was negatively associated with PPI (P=0.01). Overall, high levels of nutrition during early gestation are detrimental to milk production in beef heifers.