The central theme of Dr. Leymaster’s research program is the evaluation, development, and use of dam and sire breeds of sheep in terminal crossbreeding systems. Dam breeds are studied by crossing with Romanov to capture the documented superiority of Romanov crossbred ewes for fertility, length of seasonal fertility, prolificacy, lamb survival, and ewe longevity. Wool (Dorset and Rambouillet) and hair (Dorper and Katahdin) breeds are evaluated for their ability to complement Romanov genetics in intensive and low-input production systems and in fall and spring breeding seasons. In the intensive production system (barn lambing), ewes are limited to naturally rearing two lambs with additional lambs reared artificially. Ewes in the low-input production system (pasture lambing) are completely responsible for rearing of all lambs. The long-term goal is to create an easy-care dam line of hair sheep that can rear triplets on pasture with no labor or supplemental feed. Research with dam breeds is complemented by research with terminal sire breeds. A terminal sire composite population was formed by mating Columbia rams to Hampshire-Suffolk crossbred ewes. Two lines of this population were created, one fixed for the callipyge mutation and one fixed for the normal form of callipyge. Callipyge is a mutation that greatly increases lean and decreases fat in sheep. Research is underway to investigate a second mutation (myostatin) that also affects carcass traits. These two mutations will be studied to determine if their effects on carcass and meat quality traits are expressed independently of one another. Results from this research program will address industry needs to increase reproductive rate, to reduce labor requirements, and to improve efficiency of lean lamb production. These outcomes should increase profitability for sheep producers and improve competitiveness in international markets.