Abstract 134

Jones,-RM; McLennan,-MW; Dowsett,-KF

Div. Trop. Crops and Pastures, CSIRO, St. Lucia, Qld 4067, Australia.

Tropical-Grasslands. 1989, 23: 2, 108-114; 12 ref.

A herd of up to 16 cows was grazed in 1979-86 on pastures containing mainly L. leucocephala, Setaria sphacelata and Brachiaria brizantha at Samford in SE Queensland. Cows remained on these pastures for 4 years. Four new heifers, which had grazed a leucaena-grass pasture for the previous year, were introduced into the mating herd each year. During the last 5 years of the experiment, L. leucocephala comprised about 40-55% of the diet during mating and early pregnancy. The overall calving percentage was 66%, with a trend towards lower percentages in wetter years. The calving percentage of the farm or control herd which did not graze L. leucocephala was 90%, with little fluctuation between years. The calving percentage of cows formerly grazing L. leucocephala pastures improved from 54% to 75% when they were returned to the farm herd. Bulls used on the leucaena herd achieved high calving percentages (about 90%) when used on the farm herd in subsequent years. It was concluded that high intakes of L. leucocephala could depress calving percentages and should be avoided in the first 3 months of pregnancy. Reasons for the depression in calving percentage, and the possible impact of DHP (3 hydroxy-4(IH)pyridone) degrading bacteria, are discussed.