Milk production and lamb growth were characterized in 118 multiparous, 3- to 7-yr-old Rambouillet, Columbia, Polypay, and Suffolk ewes under spring sage range and high mountain meadow grazing from 28 to 98 d postpartum. Daily milk yield as measured by the lamb suckling weight differential technique did not differ (P greater than .05) among breeds, although milk production of Suffolk ewes tended to be higher than that of the other three breeds. Within the Rambouillet, Columbia, and Polypay breeds, total estimated yield of ewes with twins was 13 to 17% higher than that of ewes with singles, whereas in the Suffolk breed, suckling twins increased total milk yield 61% over that of ewes with singles. Twin lambs induced a larger differential in dam milk production in late lactation (70 to 98 d) than in earlier lactation (28 to 70 d). Number of lambs did not influence milk protein, Ca, or P content (P greater than .05). Fat levels in colostrum and 4-d milk were elevated 14 and 20%, respectively, in ewes suckling twins compared with ewes suckling singles. Under range conditions, Suffolk ewes suckling single or twin lambs lost more BW (12 and 21% of 4-d postpartum body weight, respectively) than Rambouillet (4 and 7%), Columbia (5 and 8%), or Polypay (8 and 8%) ewes. Correlation coefficients of milk production and lamb growth rate were positive and significant (P less than .05) up to 56 d of age. Growth rate was less closely associated with milk production for twin than for single lambs.
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