Apparent ruminal degradation and rumen escape of lysine, methionine, and threonine administered intraruminally in mixtures to high-yielding cows.

Abstract

We studied the kinetics of lysine, methionine, and threonine in six high-yielding dairy cows at peak lactation (stage 1) and 6 mo later (stage 2). The cows were fitted with cannulas in the rumen and duodenum and were automatically fed every 4th h. The three amino acids (AA) were administered intraruminally in mixtures at dosages of 100, 200, 300, and 400 mmol of each, together with polyethylene glycol (PEG) as a liquid marker. Mean rumen liquid pools at stages 1 and 2 were not significantly different. The mean liquid outflow decreased from 13.6 to 9.5 L/h, and there was a significant linear increase in the liquid outflow with increasing dosages of AA. No significant interaction was found between feeding levels and AA dosages on rates of apparent degradation and rumen escape values. Expressed as percentages of the dosage, all three AA studied showed a significant linear decrease in degradation and a significant increase in rumen escape values with increasing dosage. At the feeding levels in stages 1 and 2, the highest relative degradation rates (percentage of dosage) were observed for threonine. The relative degradation rate of methionine was significantly lower than of lysine at the high feeding level but significantly higher at the low feeding level. The mean rumen escape values of threonine, methionine, and lysine across dosages and feeding levels were 16.7, 22.1, and 20.5%, respectively. The flow of the administered AA into the duodenum during an 8-h period after administration increased with increasing dosages, with peak concentrations after 1 h. Thus, the amounts of rumen escape of the three AA were considerable at all dosages, even when the AA were administered in unprotected form.