Abstract
Twenty-four Holstein heifers were randomly assigned to either a control or test group to investigate effects of an incremental growth pattern on the development and differentiation of the mammary gland, expression of milk protein gene, and improvement of productivity. The test group was fed the restriction diet (RD; 20% below that of the control group for all nutrients) for 3 months, followed by the compensatory diet (CD; 25% above that of the control group for all nutrients) for 2 months, then followed by 4-month RD, 2-month CD, 5-month RD, and finally 2-month CD. The heifers on the dietary test regimen gained more body weight and consumed less diet, resulting in improved growth rate efficiency, compared with heifers in the control group (8.3% vs. 7.8%). The amount of retained protein and secreted protein in mammary acinar cell culture was higher for heifers in the test group than for those in the control group (midpregnancy, 2.5 and 1.9 times; late pregnancy, 1.3 and 1.1 times, respectively). RNA dot blot hybridization analysis revealed that accumulation of α S1-casein and β-casein mRNA in mammary gland tissue from heifers in the test group was higher (α S1-casein, 1.37 times; β-casein, 1.74 times) than those from heifers in the control group. Total activity of RNA polymerases and activity of RNA polymerase III in mammary tissue were higher in heifers in the test group than in those in the control group. Heifers in the test group had lower cyclic adenosine monophosphate concentrations than did heifers in the control group during all periods. The incremental growth pattern decreased concentrations of 5′ methyldeoxycytidine (5 mdc) in the mammary gland. The incremental growth pattern offered not only a simple, practical, and cost-effective method for raising dairy heifers but also may have contributed to an increase in the differentiation and functional activity of the mammary gland.
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