Effects of the ionophoric feed additives monensin, narasin and salinomycin on some aspects of production and trace element metabolism in sheep

This thesis describes a study undertaken with the aim of investigating and comparing the effects of the ionophoric feed additives, monensin, narasin and salinomycin, on feed intake, live weight gain, feed conversion efficiency, rumen volatile fatty acids, and trace element absorption and retention in sheep fed either pellets or hay and pellets once per day. The effect of salinomycin supplementation on rumen protozoal numbers was also investigated. Mean daily live weight gain and feed conversion efficiency were increased by each of the ionophores. Narasin supplementation produced the greatest response in finishing lambs. Mean daily feed intake was decreased by monensin and salinomycin, but not affected by narasin. The ratio of rumen propionic acid:acetic acid increased with ionophore supplementation with the greatest increase occurring by day three. All the three ionophores increased the retention of 75Se in heart muscle, kidney and vastus lateralis muscle, but not in liver, blood or plasma in sheep given a single oral dose of 75Se-selenite. Narasin produced the greatest increase in muscle and kidney. Ionophore supplementation also increased hepatic Cu concentrations but decreased hepatic Zn concentrations. Salinomycin supplementation significantly decreased the numbers of protozoa in the rumen. These changes in trace element retention and concentrations resulting from ionophore supplementation are explained through indirect effects of ionophores on rumen ecology, particularly leading to decreases in rumen protozoal numbers. Fewer protozoa in the rumen can lead to an increase in the availability of copper from rumen contents, and to changes in the bacterial ecology so that selenium from selenite passes through selenide into organic forms, such as selenocysteine, which are more readily absorbed and retained in the tissues of sheep