Abstract
Average levels of thyroxine (T4), triiodothyronine (T3) and reverse-triiodothyronine (rT3) in blood plasma of 159 Estonian Red and Estonian Black and White cows were 55.2 nmol/L, 1.78 nmol/L and 0.25 nmol/L respectively. Animals were grouped according to stage of lactation. The T4 level was significantly lower during the early stage of lactation (45.1 nmol/L), compared with later stages, but increased as the stage of lactation progressed (late stage of lactation - 56.7, dry cows 64.3 nmol/L). The T3 level was significantly higher at the late stage of lactation (1.93 nmol/L) compared with the early stage of lactation (1.71 nmol/L) and level in dry cows (1.71 nmol/L). rT3 showed a trend similar to that found for T4 (lowest plasma concentration in early lactation, 0.19; highest in the dry period, 0.33; late lactation, 0.24 nmol/L). Levels of all thyroid hormones were negatively related to the daily milk yield (T4-r = -.51, rT3-r = -.47, calculated thyroid index as rT3 x T3 x T4-r = -.52, for all p < .0001; T3-r = -.32, p < .01). Plasma thyroid hormone concentrations were affected by energy and dietary protein intake. Differences were found in thyroid hormone levels between the 2 breeds and between summer and winter holding periods for Estonian Red cows, which could all be explained by differences in the feeding level and daily milk yield. It is suggested that maintaining low levels of thyroid hormones in early lactation may be one of the dairy cow's mechanisms of reducing metabolic demand.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.