Effects of dietary carotene content on mean values and rankings of heifers for plasma carotene concentrations

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Abstract
An experiment was conducted with six Angus (AH) and six Jersey (JH) heifers to determine the effects of dietary carotene on plasma carotene concentrations (PCC) and the consistency of the ranking of heifers on PCC. The heifers were divided into high, medium, and low liveweight groups and each group of four heifers was placed in a separate pen on a feedpad. The heifers were fed pellets comprising 25% lupin and 75% barley grain and containing either 500,250, or 0 mg added carotene/ kg pellets. The pellets were fed at 1.1% of the mean liveweight of each group. Barley straw was available ad libitum. Pellets were given in a sequence as follows: pellets with 500 mg added carotene/kg were given for 27 days followed by those with 250 mg added carotene/kg for 21 days, and 0 added carotene/kg for 70 days. Blood samples were taken every 2–4 days and analysed for PCC. There were no significant effects of breed on PCC except for a more rapid increase in PCC in JH than AH when returned to pasture after being on the feedpad, and fed pellets containing 0 added carotene/kg. When introduced to the feedpad, nine heifers developed clinical signs of acidosis. These effects may have accounted for a low PCC despite the heifers receiving pellets containing 500 mg added carotene/kg. On changing the carotene content of the pellets from 250 mg/kg to 0 added carotene/kg there was an exponential decrease in PCC. Analysis of the data using iterative curve peeling methods indicated a 2‐component exponential model best fitted the data which suggested there were two pools of carotene affecting PCC. Despite large differences in overall mean PCC there were high correlations between PCC measured on different days, in the heifers on the feedpad. In conclusion, the ranking of heifers for PCC was relatively constant provided the carotene intakes of the heifers were the same. The results also suggest that acidosis may interfere with carotene absorption.

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