CHANGES IN LEVELS OF BLOOD CONSTITUENTS DURING FATTENING OF HEREFORD AND ANGUS COWS

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Abstract
Blood samples were taken after 36, 94, and 136 days on feed from 20 Angus and 20 Hereford nonpregnant, nonlactating cows being fattened on two diets of similar caloric content. Carcass measurements averaged 31 mm for backfat and 4.3 for marbling score when the animals were slaughtered soon after the third blood sampling. Blood PCV and hemoglobin levels increased (P < 0.05) from 36 to 94 days on feed but did not change significantly from 94 to 136 days. Blood glucose and mean corpuscular hemoglobin concentration increased (P < 0.05) from 36 to 136 days. Total plasma cholesterol levels increased (P < 0.05) from 36 to 94 days on feed and then decreased (P < 0.05) to 136 days; however, the levels at 136 days remained higher (P < 0.05) than at 36 days. Plasma triglyceride levels tended to increase through the trial, but the changes were not significant. Plasma NEFA levels increased (P < 0.05) from 94 to 136 days. Angus cows had higher (P < 0.05) PCV values and blood glucose levels than Hereford cows only at 136 days. Body weight gains in the period from 94 to 136 days on feed were significantly correlated (P < 0.01) with changes in the same period in blood levels of glucose (r = 0.46), total cholesterol (r = 0.67), and triglycerides (r = 0.49). Results of this study suggest that even after long periods of high energy feeding, the metabolism of fattening cows was still adjusting, as indicated by changes in levels of some blood constituents.