Effects of available nutrition and sire breeding values for growth and muscling on the development of crossbred lambs. 1: Growth and carcass characteristics

Abstract

Abstract. The growth and development of 387 crossbred lamb progeny from 9 Poll Dorset sires representing muscle (M), control (C), and growth (G) sire-types was studied. Sires were selected on the basis of their LAMBPLAN estimated breeding values (EBVs) for post-weaning growth (PWWT) and depth of loin muscle (PEMD). Lambs were provided with either LOW or HIGH levels of available grazing nutrition from 10 days of age onwards. Liveweight gain (LWG) throughout the study was less on LOW nutrition than on HIGH nutrition, leading to a 9.5 kg lower weaning liveweight (LW) and a 14.9 kg lower final LW in LOW lambs. After adjustment for final LW, HIGH lambs had significantly greater fat depth at the C-site (approximately 40 mm from the midline over the 12th rib) and tissue depth at the GR site (110 mm from the midline over the 12th rib) than did LOW lambs. This effect was consistent across sire-types. Depth of fat at the C-site was positively associated with the EBV of the sire for fat depth. The improvement in pre-weaning LWG, weaning weight, and final weight of lambs resulting from use of sires with a greater PWWT EBV was dependent upon the level of nutrition. This interaction was identified as different slopes (coefficients) for the regression between PWWT and trait for the 2 nutrition levels, indicating that the expression of the sire's genetic potential for growth at these times was significantly moderated by nutrition. The additional depth of lamb loin muscle resulting from use of sires of higher PEMD EBV was consistent across both LOW and HIGH nutrition treatments, with 1 mm of PEMD leading to a 0.6-mm increase in loin depth. Other consequences of sires having a high genetic capacity for loin muscle depth were reduced carcass C-fat depth with increasing sire PEMD and a tendency for conformation score to improve with the PEMD of the sire. The wool-growth response to improved nutrition was less in M lambs than in lambs of other sire-types, suggesting a difference in priority for protein partitioning between muscle and wool in lambs differing in genetic propensity for muscle growth.

Sire carcass breeding values affect body composition in lambs — 1. Effects on lean weight and its distribution within the carcass as measured by computed tomography

“…This contradicts previous findings from smaller studies where PWWT had no effect , though it does support the general principle that lambs selected for high post weaning weights would have a larger mature size (Huisman & Brown, 2008) and therefore they will be physiologically less mature when compared at the same weight (Berg & Butterfield, 1968;Butterfield et al, 1983;Hegarty, Shands, et al, 2006) …”

“…This appears to align with suggestions made in previous studies, where selection for increased PEMD resulted in an increase in loin depth and weight Hegarty, Shands, et al, 2006;Hopkins, Stanley, Martin, Ponnampalam, et al, 2007), and little effect elsewhere in the carcass. Alternatively, these previous experiments were unable to clearly demonstrate redistribution of lean to the saddle as fore section lean was not recorded and only a small number of indicator muscles (e.g.…”

Progeny of high muscling sires have reduced muscle response to adrenaline in sheep

“…Hegarty et al (2006a and found that the selection of sires with increasing eye muscle depth ASBV resulted in lamb carcases with reduced C-site fat depth, less intramuscular fat within the loin muscle and less total carcase fat, a similar trend to that observed by Hocquette et al (1999) in double-muscled Belgian Blue cattle. In times of psychological or physical stress (such as mustering, transportation or starvation), animals have the ability to mobilise their adipose tissue to form non-esterified fatty acids (NEFA), which then has to be oxidised to form energy in other tissues (Chilliard et al, 2000).…”

“…Thus, the reduced glucose output due to selection for muscling may be linked to any one or a combination of these glucose sources; however, reduced liver gluconeogenesis as a result of lower circulating growth hormone is a possible cause of this effect. Progeny from high muscling sires have shown reduced growth rates to weaning, reduced bone growth and increased muscle-to-bone ratio (Cake et al, 2006;Hegarty et al, 2006a), all suggestive of reduced growth hormone. Furthermore, studies using growth hormone transgenic sheep reported a decrease in eye muscle depth due to increased growth hormone (Adams et al, 2002).…”

The merit of condition score and fat score as alternatives to liveweight for managing the nutrition of ewes

“…Subsequently, the concept of fat scoring live lambs to predict GR tissue depth was developed and used with liveweight to market young sheep for meat (Kirton et al 1991;Hegarty et al 2006). Initially, fat score assessors 'felt lambs by hand mainly over the backbone and tail stump' (Kirton et al 1991) and were as accurate as ultrasonic machines, which measure fat over the eye muscle (Bass et al 1982; as cited in Kirton et al 1991).…”