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869
Efficacy of dual x-ray absorptiometry as a means to measure mammary gland development in dairy heifer calves
A non-invasive means to assess mammary gland growth in dairy heifers is highly desirable from a research and animal welfare standpoint. We evaluated dual x-ray absorptiometry (DXA) as such a tool. Thirty-six Holstein heifer calves were reared on: 1) a control milk replacer (MR) fed at 454 g powder/d (CTL; 20% crude protein [CP], 20% fat), or 2) an enhanced MR fed at 1135 g powder/d (E; 28% CP, 25% fat). The MR was fed for 8 full weeks with weaning (half milk intake) occurring during the last week. Starter feed was offered after week 4 but was balanced between treatments. At weaning, a subset of calves were sacrificed (n = 6/diet). Remaining calves received estradiol (E2) implants and were sacrificed at week 10. The 4 treatments were: 1) CTL, 2) CTL + E2 (CTL-E2), 3) E, and 4) E + E2(E-E2). After sacrifice, udders were removed and half-udders were snap-frozen. Frozen mammary glands were scanned and lipid and lean tissue contents were determined with and without the skin intact. Correlations were calculated between mammary gland lipid content via DXA, and biochemical extraction using PROC CORR function in SAS. Correlations were also calculated between mammary gland DXA lean tissue values and mammary parenchyma weight and DNA content. Correlations with skin removed were slightly higher. Overall, lipid content of the mammary gland as determined by DXA was highly correlated with lipid content determined biochemically, regardless of skin presence (r = 0.92; P < 0.01 for skin intact; r = 0.94; P < 0.01 for skin removed). With one exception, correlations were similar within treatments (r = 0.59, 0.91, 0.94, 0.91, and 0.96, 0.93, 0.97, 0.92 for treatments CTL, CTL-E2, E, and E-E2 with and without skin, respectively; P < 0.01 for all except for CTL with skin intact [P= 0.16]). Correlations between mammary gland lean tissue content determined by DXA and dissected parenchyma weight (r = -0.01; P = 0.94) or parenchyma DNA content (r = 0.22; P = 0.21) were low and non-significant. Results indicate that DXA analysis can be used to evaluate fat content of the mammary gland in young heifers but not the mass of mammary parenchymal tissue. It remains to be determined if this technology can be used in intact animals.
Keywords: calf, dual x-ray absorptiometry, mammary gland