R.H. Miller1, H.D. Norman, J.R. Wright
Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350
2008 J. Dairy Sci. (?)
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Frequency of abortions among lactating cows during 1995-2005 was studied. Only lactations with breeding dates were used. Abortion frequency was 1.51% among 2,907,561 lactations (>151d pregnant by end of lactation). Analyses were to determine effects of herd-year, month, and stage of pregnancy at end of lactation, breed (Holstein, Jersey, other), and milk yield. Supplemental analyses examined effects of herd size and geographical region. A preliminary analysis of a sample of 483 herds compared the results of PROC GENMOD and PROC GLM. Results were similar and the entire national data set was analyzed using GLM. Abortions were most frequent in February-August, least in November-December. Frequency of abortion decreased from 1st to >8th parity (difference of 0.57%). Frequency of abortion was highest during 152-175d gestation, lowest during 224-250d gestation (difference of 4.61%). Compared to other breeds, Holsteins aborted 0.19% more frequently; frequency in Jerseys was 0.09% lower than other breeds. Frequency of abortion was 0.95% higher in herds with 700-799 lactations than in herds with 50-99 lactations. California herds had highest abortion frequency (2.2% higher than the state with lowest frequency (North Dakota)).
(Key words: abortion, gestation, pregnancy, season, parity)