Decreased milk production of beef cattle grazing endophyte-infected (EI) tall fescue (TF), an important cool season grass in the southeastern United States, can affect calf growth. The objective was to determine whether a thermal or heat-tolerant Bos taurus breed of cattle, Romosinuano (RO), would tolerate EI TF toxins relative to Angus (AN) cattle. Milk production and quality were examined on AN and RO cows grazing endophyte-free (EF; variety K-31; n = 36) or EI TF (variety K-31; n = 37) along with growth of their Charolais-sired calves in 2007 and 2008 in west central Arkansas. Cows calved between early February and late March (spring). Milk yield and quality, BW, rectal temperature, and serum prolactin were determined at 57.1 +/- 2.6 d postpartum and every 28 d thereafter between April and August. Milk yield was greater in AN compared with RO cows (P < 0.001), but not influenced by forage type (P > 0.88). Percent milk fat was greater in cows grazing EF TF in April, but greater for cows grazing EI TF in July (forage x month, P < 0.001). Percent milk fat was greater for RO than AN cows (P < 0.001). Percent milk protein (P < 0.001) was greater and somatic cell counts (log-transformed; P < 0.001) were less in RO than AN cows. Milk lactose was greater for RO compared with AN cows in June through August (breed x month, P = 0.004). Adjusted weaning BW of calves was similar between EF and EI TF in 2007, but greater for calves from EF than EI TF in 2008 (forage x year, P = 0.03). Rectal temperature was similar between RO cows grazing EF and EI TF, but greater in AN cows grazing EI compared with EF TF in most months (forage x breed x month x year, P < 0.001). Serum prolactin was reduced in both breeds of cows grazing EI TF between April and July of both years and greatest in RO cows grazing EF TF (breed, P < 0.001; forage x month, P < 0.001). These data suggest that RO cows were more thermal-tolerant, but still susceptible to toxins in EI TF as shown by a reduction in serum concentrations of prolactin. However, milk production was not influenced by EI TF as previously observed, but milk fat percent was decreased in early lactation in this group of cows. Milk yield and quality were different between AN and RO cows during the period of lactation observed.