The use of @entity465 ( @entity465 ) for crossbreeding is integral to the UK @entity742 industry where approximately 71% of market @entity742 are sired by @entity465 rams. Early growth of these crossbred @entity742 affects profitability. The objectives of this study were i) to evaluate the effectiveness of index selection among @entity465 on BW and @entity24754 of their crossbred offspring; and ii) to compare the efficacy of that selection within @entity465 breeds. The most widely used @entity465 breeds in the United Kingdom are Charollais, Suffolk, and Texel. These participated in sire referencing schemes in which they were evaluated on a lean growth index designed to increase carcass lean weight at a given age. From 1999 to 2002, approximately 15 high and 15 low lean growth index rams per breed (93 in total, differing in index on average by 4.6 SD) were selected from within their sire referencing schemes and mated to Welsh and Scottish Mule ewes. Their crossbred offspring were reared commercially on 3 experimental farms in England, Scotland, and Wales. A total of 6,515 @entity742 were born between 2000 and 2003. @entity742 were weighed at birth (BWT), 5 wk (5WT), and 10 wk (10WT), and their @entity24754 from birth to 10 wk was calculated. @entity742 sired by high index rams were on average, across breeds, heavier at all ages (P < 0.01) with 0.07    0.03, 0.3    0.1, and 0.4    0.1 kg greater BWT, 5WT, and 10WT, respectively. Their @entity24754 was 5.1    1.9 g/d greater than low-index-sired @entity742 (P < 0.01). Suffolk-sired @entity742 were on average heavier at all ages, with greater @entity24754 , whereas Charollais-sired @entity742 were lightest with smallest @entity24754 . Overall, there was no significant interaction between sire index and sire breed (P > 0.10). Within Suffolk-sired @entity742 , there was little difference between high and low index sires for the traits studied (P > 0.3). High and low index Charollais-sired @entity742 differed in BWT (0.09    0.04 kg) and 5WT (0.3    0.1 kg), and Texel-sired @entity742 differed in 5WT (0.5    0.1 kg), 10WT (0.9    0.2 kg), and @entity24754 (10.2    3.3 g/d; P < 0.01). @entity742 from Scottish Mule dams were heavier, with greater @entity24754 , than @entity742 born to Welsh Mules (P < 0.01). @entity742 reared in Scotland were heavier at all ages (P < 0.01). The results suggest that using index selection in @entity465 can improve the growth of their commercial offspring reared on grass.

Index selection in XXXX improves early lamb growth.

@entity24754 :: ('100532852', 'Gene') :: ['ADG'] @entity465 :: ('MESH:D007153', 'Disease') :: ['terminal sires', 'TS'] @entity742 :: ('9940', 'Species') :: ['Lambs', 'sheep', 'lambs']
@entity465 :: (MESH:D007153,Disease) :: ['terminal sires']
