BRIEF COMMUNICATION: Effects of birth rank and yearling lambing on long-term ewe reproductive performance

摘要

It has long being recognised that mating ewes at 7 to 9 months of age could potentially improve their life-time monetary return, as an extra litter is produced (Dyrmundsson, 1973). Baker et al. (1981) showed that joining young sheep to lamb at one year of age was not detrimental to their life time productivity. At that time wool sales were very significant contributors to sheep farm incomes and fears of reduced wool weights and quality were reasons reducing the uptake of the practice. Today, despitepotential effect on wool returns being of far less significance, the percentage of yearlings joined is still only 33% (Anon, 2005), mainly because sheep farmers believe that mating young actively growing sheep will have a negative effect on their futurereproductive performances (Kenyon et al, 2004). Since the early work on yearling lambing, the National lambing percentage have increase to 125.6% (MWNZ, 2009). Recently, Kenyon et al. (2008) reported that two-year-old ewes that lambed as yearlings had less multiple births than those which were not exposed to the ram as lambs (50 % vs. 60%). However, little is known about the effect of mating yearlings in modern sheep genotype on the reproductive performance past two years of age. The data analysis reported here investigates the effect of birth rank (1Y) and age of first lambing, either as a one-year old or two-years old (2Y), on litter-size over the first four parities as well as the total of number of lambs produced at four-years of age.