1. Introduction
Lamb per capita consumption in the USA is low due to excess fat on lamb carcasses and consumer perceptions that lamb meat is high in saturated fat [
1,
2]. Consumers are looking for healthier food options including leaner meat products [
3]. The industry is responding to consumer needs by producing leaner lambs through genetics [
4]. This is often achieved through targeting genes that alter specific muscling traits with use of terminal sire breeds [
5]. Texel sheep are known for their increased muscling, which is sometimes referred to as doubling muscling. This increased muscle mass is due to a G to A transition in the 3′ UTR of myostatin (MSTN) which then creates a target site for miRNAs to inhibit translation [
6]. Studies have shown the advantages of using Texel sires to increase muscle and decrease fat on carcasses with feedlot finishing [
7,
8]; however, limited information is available on the impact of terminal sire breeds in pasture-finishing systems.
Production of lean lamb is important for the industry to remain competitive in the marketplace. Current grading systems were developed in 1960s when lambs were fatter and had higher yield grades. For the lamb industry to advance, the ability to accurately and quickly estimate carcass composition is imperative to meet these industry goals. Carcass yield grade and retail cut yield are based solely on fat thickness and no measures of muscle size and mass are included in the current USA grading system [
2,
9,
10]. The use of terminal sire breeds produces leaner, heavier muscled lambs that predominantly fall into yield grade 1 and 2. Imaging technology has been shown to be accurate in carcass composition predictions [
11,
12]. One of these technologies is dual energy X-ray absorptiometry (DXA), which was developed to measure human body composition using X-rays at two different energies. DXA technology was first used in livestock around 1996 and has since shown potential to accurately predict whole carcass composition [
13]. Therefore, the objectives of this study were to explore: (1) the impacts of sire and dam breed on growth, carcass composition and meat quality in a pasture-based system, and (2) the use of DXA to rapidly assess carcass or primal fat percentage, to rank these lambs on leanness.
3. Results
A total of 79 lambs (158% lamb crop) was born in this study, comprising 38 female and 41 male lambs (
Table 1). Lambing rate was greater (
p < 0.05) for Suffolk dams than Southdown dams but did not differ (
p > 0.05) by sire breed. Lamb birth weight was greater (
p < 0.01) for Texel-sired compared to Southdown-sired lambs. Weaning weight tended to be greater (
p < 0.10) for Texel-sired than Southdown-sired lambs. Growth rate was similar between all lambs except for d 14 to 28 when lambs born to Southdown dams had higher (
p < 0.05) average daily gain compared to lambs from Suffolk dams.
Circulating myostatin concentrations were measured by ELISA and values are shown in
Figure 1. Myostatin concentration differed by dam breed (
p = 0.0002) and animal age (
p < 0.0001). Sire breed did not alter myostatin concentrations and all interactions were non-significant (
p > 0.22). Myostatin concentrations were greater (
p < 0.01) on d 45 than d75 or 110, regardless of sire or dam breed. Lambs born to Southdown ewes had lower (
p < 0.01) myostatin concentrations than lambs born to Suffolk ewes.
Carcass characteristics and individual muscle weights of wethers by dam and sire breeds are shown in
Table 2. Hot and chilled carcass weights were greater (
p < 0.01) for Texel-sired than Southdown-sired lambs and for lambs born to Suffolk dams compared to Southdown dams. Dressing percentage and ribeye area were greatest (
p < 0.05) for Suffolk × Texel lambs than other breed combinations. Flank streaking, conformation and quality grade scores were higher (
p < 0.05) for Texel-sired lambs compared to Southdown-sired lambs. Suffolk × Texel carcasses had heavier (
p < 0.05) rack, leg and total primal weight than other breed combinations; however, these changes were related to differences in carcass weight and when expressed on a percentage basis did not differ (
p > 0.05). Lambs born to Southdown dams had a lower (
p < 0.05) percentage of weight in leg primals compared to Suffolk dams. Suffolk × Texel lambs had greater (
p < 0.05) longissimus, semitendinosus and semimembranosus muscle weights than other breed combinations.
Predicted total fat percentage of carcasses, all four primal cuts, rack and shoulder cuts from DXA scans differed (
p < 0.05) by dam and sire breed but no interactions were observed (
p > 0.05). Total carcass and primal fat content as measured by DXA was greater (
p < 0.05) for Southdown-sired than Texel-sired lambs and for lambs from Southdown dams than Suffolk dams (
Table 3). The percentage of fat in the loin, rack and leg was lower (
p < 0.05) for Texel-sired lambs than Southdown-sired lambs. Lambs from Southdown dams had greater (
p < 0.05) fat content in the rack and shoulder than Suffolk dams. Total primal fat percentage and total carcass fat percentage had a high agreement (r = 0.98), indicating that DXA measurements could be taken on vacuum packaged primals instead of carcass or side to facilitate scanning and food safety parameters. The leg and shoulder primals had the highest (r = 0.78 and 0.79, respectively) correlations with total carcass fat percentage; whereas the rack and loin had lower correlations (r = 0.60 and 0.59, respectively) with total carcass fat mass. Ranking carcasses for leanness based on DXA scans for total carcass fat percentage showed that sire breed altered (
p < 0.05) leanness of the carcasses, with Texel-sired having leaner carcasses than Southdown-sired. Ranking on carcass yield grade did not show any differences (
p > 0.05) in sire or dam breeds, which demonstrates its inability to separate carcasses for leanness in pasture-finished lambs. Stepwise equations were developed to predict carcass fat percentage using DXA shoulder fat percentage which explained over 60% of the variation (carcass fat, % = 9.84 + 0.63 × DXA shoulder fat percentage; r
2 = 0.62).
All interactions between muscle and dam or sire breed were non-significant for proximate composition (
Table 4). Total lipid content was greater (
p < 0.05) for Texel-sired lambs than Southdown-sired. Moisture content was highest (
p < 0.05) for SDTX and lowest for SDSD. Moisture content of the individual muscles was greater (
p < 0.05) for SM than GM, LM, or ST. Total lipid content was greater (
p < 0.05) for ST than GM, LM or SM.
Fatty acid composition by sire and dam breed across muscle from the loin and leg are shown in
Table 5. All two-way and three-way interactions between muscle and dam or sire breed were non-significant. Stearic acid concentration was highest (
p < 0.05) in muscles from SFSD and lowest (
p < 0.05) for SFTX. For the SDTX lamb muscles, trans-11 vaccenic (C18:1 t11) acid concentration was lowest (
p < 0.05) and ratio of n-6 to n-3 PUFA was highest (
p < 0.05) compared to other dam × sire breed combinations. For SFTX lamb muscles, linolenic acid, EPA, and total n-3 PUFA concentrations were greater (
p < 0.05) than other dam × sire breed combinations. Arachidic (C20:0) acid concentration was greatest (
p < 0.05) and CLA lowest (
p < 0.05) for SDTX and SFSD.
Southdown dams produced lambs with greater (p < 0.05) myristic (C14:0), palmitic (C16:0) and saturated fatty acid concentrations in the four muscles examined than Suffolk. Palmitoleic (C16:1) acid was greater (p < 0.05) and cis-11 vaccenic (C18:1) acid concentration was lower (p < 0.05) in muscles from lambs born to Southdown dams. Linoleic (C18:2) acid and total n-6 polyunsaturated fatty acid (PUFA n-6) concentrations were lower (p < 0.05) in muscles from lambs born to Southdown dams. Docosahexaenoic (C22:6; DHA) acid concentration was greater (p < 0.05) in muscles from lambs born to Southdown dams.
Texel-sired lambs had greater (p < 0.05) concentrations of linoleic acid, arachidonic (C20:4 n-6) acid and total n-6 PUFA in muscles compared to Southdown-sired lambs. Oleic (C18:1 c9) acid, cis-11 vaccenic acid, margaric (C17:0) acid, SFA, MUFA and OCFA concentrations were lower (p < 0.05) in Texel-sired than Southdown-sired lambs. Total n-3 PUFA, eicosatrienoic (C20:3 n-3) acid, EPA, docosapentaenoic (C22:5 n-3, DPA) and DHA concentrations were greater (p < 0.05) for Texel-sired than Southdown-sired lambs. Total fatty acid content of the muscles was lower (p < 0.05) for Texel-sired than Southdown-sired lambs.
Fatty acid composition of individual muscles examined in this study showed that many differences existed (
Table 6). Total fatty acid content was higher (
p < 0.05) for LM and ST than GM and SM. Saturated fatty acid concentration was lowest (
p < 0.05) in SM and MUFA concentration was lowest (
p < 0.05) in GM. Concentrations of n-6 PUFA were greater (
p < 0.05) for GM and SM and n-3 PUFA concentrations were greater (
p < 0.05) for GM than LM and ST. The ratio of n-6 to n-3 PUFA was lowest (
p < 0.05) for ST muscle.
All interactions between dam breed, sire breed, muscle and postmortem aging time were non-significant. Warner–Bratzler shear force values were lower (
p < 0.05) in muscles of lambs born to Suffolk than Southdown breeds (
Figure 2A). Sire breed did not alter Warner–Bratzler shear force values. The GM had the lowest (
p < 0.05) shear force value, and the SM had the highest (
p < 0.05) shear force value (
Figure 2B). Postmortem aging reduced (
p < 0.05) Warner–Bratzler shear force values at each time point (
p < 0.05), with the greatest change between d 1 and d 3 (
Figure 2C).
4. Discussion
Terminal sire breeds are often utilized to improve carcass leanness and muscle mass, but they may alter lambing and production characteristics [
18,
19]. Our results show that sire breed did not impact lambing rate but did increase birth weight by 15% for Texel-sired lambs. Lamb growth rates were similar during preweaning growth, except between d 14 and d 28 when lambs from Southdown dams grew faster than Suffolk dams. Weaning weight tended to be higher for Texel-sired lambs. Others [
19] have shown that lamb number per ewe and litter weights were lower for Texel sires compared to more prolific breeds like the Romanov. Freking and Leymaster (2004) reported that Texel-sired lambs had slower growth from d70 to 140 compared to Dorset, Romanov or Montadale-sired lambs. Suffolk offspring have been shown to have leaner growth than other breeds. including Southdown lambs [
20].
The enhanced muscle phenotype of the Texel is related to a single nucleotide polymorphism (c. *1232 G > A) in the myostatin gene that provides a binding site for miR-1 and miR-206 [
6]. Tellam et al. [
21] reported that lambs sired by an F2 Texel ram had approximately one-third lower circulating MSTN concentrations compared to wild-type sheep. Unfortunately, the timing of the blood sample collection in that study was not described in the paper. We observed that circulating myostatin concentrations differed over time, with younger suckling lambs (d 42) having higher values than weaned (d 75 and 110) lambs; however, there was no difference between sire breeds.
Texel-sired lambs had greater carcass weights, quality and individual muscle weights than Southdown-sired lambs. Dressing percentage, ribeye area, rack weight and leg weight were greater for Suffolk × Texel lambs than other breed combinations. Others have reported that Texel lambs have a similar carcass composition to Suffolk lambs but are considered more compact [
22]. The Texel breed is known for its superior muscling phenotype due to a myostatin mutation [
4]. Because of this mutation, Texel and Texel cross lambs have been shown to have improved carcass lean, with less fat in various locations throughout the carcass [
8]. Overall, the use of the Texel as a terminal sire breed in pasture-finishing systems with limited supplementation did improve carcass weight, muscling and leanness, with similar growth performance measures.
All lamb carcasses in this study had yield grades of 1 and 2 and were graded as Prime and Choice, which is similar to national averages [
23]. Therefore, we examined the use of the DXA technology to rapidly scan the carcass or primal cuts to rank carcasses on leanness. Our results show that percentage of fat could be predicted in the carcass or the four major primals with a high correlation (r = 0.98). Texel-sired lambs had a lower, more desirable, rank for DXA carcass leanness and ribeye area. Others [
24] have reported similar errors in current USDA grading methods for segregating carcass leanness. The use of technology in analyzing carcass composition can help to improve these carcass composition analyses methods [
25,
26,
27]. By utilizing DXA technology, studies have found that other factors such as muscle mass are more useful in predicting carcass composition in lean lambs than fat depth alone [
26,
28]. Connaughton et al. [
29,
30] have reported that DXA technology can be used in the abattoir at chain speed to predict carcass fat percentages with high repeatability.
Limited research is available about changes in fatty acid composition of muscles from the dam and sire breeds used in this study on pasture-based systems. Total fatty acid content of the muscles was lower for Texel-sired than Southdown-sired lambs. Texel-sired lambs had greater concentrations of total n-6 PUFA and n-3 PUFA, and lower concentrations of SFA, MUFA and OCFA. Previous research comparing sire breeds also showed that Texel-sired lambs had higher PUFA and both n-6 and n-3 concentrations than lambs from other sires [
31]. Snowder and Duckett [
32] also reported differences in fatty acid composition of LM by different sire breeds, Dorper and Suffolk, when feedlot finished. Finishing regimes alter fat deposition and fatty acid composition of muscle tissues in sheep [
1,
33], goats [
33] and cattle [
34,
35].
The semitendinosus muscle had the highest total lipids, total fatty acid content, and lowest ratio of n-6 to n-3 PUFA in this study. In contrast, the semitendinosus muscle is considered one of the leanest muscles in beef carcasses finished on pasture [
35]. Saturated fatty acid concentration was lowest in SM and MUFA concentration was lowest in the GM. Relationships between SFA concentration and total fatty acid content are high in beef [
35] and our results in sheep are similar, with leaner muscles having a lower concentration of SFA. Omega-6 PUFA concentrations were greater for GM and SM, whereas, n-3 PUFA concentrations were greater for GM than LM and ST. Overall, the results demonstrate that carcasses of the different sire and dam breeds used in this study were lean (<1.8 g/100 g muscle), with high concentrations of PUFA (6.4 to 8.9% PUFA n-6 and 2.0 to 2.5% PUFA n-3). The ratio of n-6 to n-3 fatty acids was below 4:1, the upper level recommended for human health and reduction in coronary heart disease [
36,
37], for all muscles and breed combinations for lambs finished on pasture with limited grain supplementation.
Tenderness of the lamb muscles as measured by Warner–Bratzler shear force showed that the SM muscle was toughest and GM the most tender. Sire breed did not influence tenderness but dam breed did. Others [
8,
38] have reported that the use of Texel sires did not alter tenderness. In contrast, previous research evaluating different sire breeds found that Texel-sired and Suffolk-sired lamb had higher shear force values in the LM and GM but not the SM or ST, compared to Southdown-sired lambs [
31]. Postmortem aging of all muscles improved tenderness, with the greatest change in shear force value from d 1 to d 3 (72% of total improvement); however, aging to 6 d postmortem further improved tenderness but the magnitude of the change was lower (28% of total improvement) from d 3 to d 6.