Stability of bovine milk progesterone under different storage and thawing conditions

The objectives were to determine the effects of storage temperature (21 or 4°C), a preservative agent (Brotab 10®) (exp. 1), thawing temperature (37, 21, or 4°C), repeated freeze-thaw cycles (exp. 2), and length of storage at -20°C (exp. 3) on the stability of bovine milk progesterone (P4) over an 8-wk period. Whole-milk samples of 19 pregnant dairy cows were analyzed for P4 using an enzymeimmunoassay (Quanticheck®). In exp. 1, mean P4 concentrations declined (P < 0.01) from 0 to 28 d (5.2 ± 0.1 vs. 3.3 ± 0.1 ng mL-1), but not any further at 56 d. However, P4 decline was lower (P < 0.01) at 4°C than at 21°C at 3 and 56 d, respectively (4.3 ± 0.1 and 3.8 ± 0.1 vs. 3.9 ± 0.1 and 3.0 ± 0.1 ng mL-1). Brotab 10® tended (P < 0.08) to reduce P4 decline. In exp. 2, thawing temperature and repeated freeze-thaw cycles, and in exp. 3, the length of storage at -20°C, did not greatly affect P4 stability. Regardless of the temperature, P4 concentrations declined in all experiments by 1.1 ± 0.1 mL-1 in the first 3 to 7 d of storage and remained relatively stable thereafter, except when stored at room temperature in the absence of a preservative agent. In conclusion, P4 in whole-milk samples remained relatively stable for up to 3 d at 21°C and for up to 14 d at 4°C, even in the absence of a preservative agent. For periods longer than 14 d, whole-milk samples are best stored at -20°C for optimum stability of P4. Key words: Milk progesterone, progesterone stability, storage conditions, enzymeimmunoassay