REPRODUCTION OF MERINO SHEEP SUBJECTED TO DIVERGENT SELECTION ON MATERNAL VALUES FOR LAMBS WEANED PER EWE JOINED

Two Merino lines were established in 1988 and were subjected to diverg ent selection for maternal ranking values on lambs weaned/ewe joined ( Lw/Ej). Replacements with high values were selected in the Reproductio n '+' line, while progeny of low ranking ewes was included in the Repr oduction '-' line. The number of ewes lambed/ewe joined was higher (P less-than-or-equal-to 0.10) in the '+' line than in the '-' line durin g 1989, 1991 and 1992. The proportion of multiple lambs in the '+' lin e was higher (P less-than-or-equal-to 0,05) than in the '-' line when pooled for the period 1990-1992 (0.515 vs. 0.425). This improvement in multiple births was accomplished without a reduction in lamb survival , which was higher (P less-than-or-equal-to 0.10) in '+' line lambs th an in '-' line lambs over the same period (0.776 vs. 0.712). When perc entage deviations for Lw/Ej in '+' line ewes from contemporaries in th e '-' line were regressed on lambing year, an increase of 11.6 (R2 = 0 .87) % per year was found. A large portion of this improvement was att ributed to a change in flock structure through the replacement of unse lected ewes by selected young ewes in the two breeding flocks.