Lactation and weaning weight relationships were studied in 37 cows grazing Southern Utah irrigated pastures during the summer of 1979. The herd included 23 Hereford cows ranging in age from 2 to 12 years and 14 Simmental-Hereford half-blood cows aged 3 to 9 years. The suckling calves were sired by Hereford bulls. The average daily milk yield of 6.16 kg was affected by the cow breed. Simmental-Hereford cows produced 7.11 kg of milk per day, while the Herefords produced 5.21 kg per day. Milk production declined with increased days in lactation. The decline was linear in the Simmental- Hereford cows and quadratic in the Herefords. Milk production was also affected by the sex of the suckling calf. Heifer calves received 6.44 kg per day compared t o 5.80 kg per day for bull calves. Calf birth weight and test day weight influenced milk yield as cows nursing heavier calves produced more milk. Cow breed affected milk composition. Simmental-Hereford cows had higher percent solids-not-fat, while Herefords were higher for percent protein. Calf weights were affected by sex of calf. Bull calves weighed 33 .89 kg at birth, while heifers weighed 32.14 kg. At weaning, bulls weighed 188.63 kg, while heifers weighed 177.14 kg. While no breed difference was evident for birth weight, calves from Simmental-Hereford dams weighed more over the duration of the study than calves from Hereford dams. The difference between the mean 205-day adjusted weights for the calves from the Simmental-Hereford and Hereford cows was 24.96 kg (230.50 kg and 205.54 kg, respectively). Milk yield was highly correlated to calf gain (r 0. 71). The breed of dam influence upon weaning weight was largely a result of differences .in quantity of milk produced.