Impact of heat stress on the growth performance and retail meat quality of 2nd cross (Poll Dorset × (Border Leicester × Merino)) and Dorper lambs
Introduction
Heat stress (HS) is one of the greatest challenges facing the global livestock industry. An increase in global temperature and relative humidity (RH) is likely to compromise animal welfare and production during hot summer months, especially in the warmer parts of the world. HS occurs when an animal is unable to maintain normal core body temperature due to increased ambient temperature which compromises the animal's ability to lose heat from the body. HS is not only detrimental for animal welfare and production, but has been implicated in higher incidence of dark cutting or pale soft and exudative (PSE) meat (Gonzalez et al., 2020; Gregory, 2010; Zhang et al., 2020).
Stress is well known to deplete muscle glycogen stores and lead to lower acidification of postmortem muscle and consequently a higher ultimate pH (pHu) (Scanga, Belk, Tatum, Grandin, & Smith, 1998) and dark cutting. However, the studies reporting the impact of HS on different meat quality traits of ruminants are equivocal, and most of these focused on effects of long-term (≥ 1 month) HS. For example, Kadim, Mahgoub, and Khalaf (2014) reported that seasonal HS (34.3 ± 1.67 °C and 48.8 ± 7.57% RH, 6 m) had a negative effect on fresh colour, tenderness and water holding capacity (WHC) of sheep and goat meat. Conversely, albeit at a much lower ambient temperature, Saha et al. (2013) and Rana et al. (2014) reported that 4 and 8 h (27.8 °C, 81.9% RH; 45 d) heat exposure had no effect on goat slaughter weight and drip loss of the meat. Recently, Archana et al. (2018) showed that seasonal HS significantly increased Longissimus thoracis et lumborum (LTL) pH24, and shear force of meat from Osmanabadi and Salem Black goats, but had no influence on colour and WHC. The majority of researchers agree that high summer temperatures would have negative impact on animal welfare and meat quality (Gregory, 2010; Zhang et al., 2020). However, the extent to which it is affected may vary depending upon the severity of HS which in turn depends upon the daily ambient temperature, RH, and exposure duration (Tang, Yu, Zhang, & Bao, 2013; Zhang et al., 2018, Zhang et al., 2018) as animals may have variable responses to short-term and chronic HS. Ponnampalam et al. (2016) reported that one-week of HS (28–40 °C, 30–40% RH) had no effect on lamb growth performance (slaughter weight, carcass weight and fat depth) or meat quality (pHu and lipid oxidation). Thus, there is a need for further research to elucidate the impacts of HS exposure on growth performance and meat quality of small ruminants.
Hair and wool traits are known to affect heat tolerance in sheep (McManus et al., 2011). Hair sheep breeds such as Pelibuey, Dorper, Katahdin, and their crossbreds have better adaptability to high environmental temperatures which is attributed to improved physiological and metabolic responses (lower thyroid hormone levels and metabolic heat production, and deeper breathing compared with wool sheep breeds (Correa et al., 2012; Romero, Pardo, Montaldo, Rodriguez, & Ceron, 2013; Ross, Goode, & Linnerud, 1985). In Australia, higher carcass yield was reported for Dorper and Damara (African hair sheep) compared to Merino sheep (Almeida et al., 2013). However, it is unknown whether heat tolerance would have any implications for meat quality attributes and growth performance, when heat exposure shorter than 1 month and breeds are compared. The objective of this study was therefore to compare the growth performance and meat quality attributes of hair-type sheep breeds (Dorper) and wool-type sheep breeds [2nd cross; Poll Dorset × (Border Leicester × Merino)] exposed to two weeks of HS during the finishing phase. The choice of breeds for examination was based on the hypothesis that, under HS conditions, Dorper lambs (hair breed) would exhibit a higher degree of thermotolerance, better growth rates and meat quality attributes compared to 2nd cross lambs (wool breed).
Section snippets
Animals and experimental conditions
The experiment was approved by the University of Melbourne Faculty of Veterinary and Animal Sciences Animal Ethics Committee (AEC ID 1714357.1) and the protocols used for the live animal part of this study have been reported elsewhere (Joy et al., 2020). Briefly, forty-eight lambs [Poll Dorset × (Border Leicester × Merino) (n = 24) and Dorper (n = 24)] aged 4–5 months (body weight range: 38–42 kg) were purchased from 5 different breeders across North-East Victoria. All lambs were purchased
THI and growth performance
Temperature–humidity index (THI) is commonly used to measure heat stress which is calculated based on the ambient temperature and relative humidity. An ambient environment with a THI lower than 22.2 is classified as the absence of a heat stress condition. From 22.2 to 23.3 is recognized as moderate heat stress. When THI ranges from 23.3 to 25.6, it is referred to as a severe heat stress condition, and extreme severe heat stress condition when the THI exceeds 25.6 (Marai et al., 2007; St Pierre,
Conclusion
Two weeks cyclic HS had significant negative effect on both Dorper and 2nd cross lambs' physiological responses and growth performance. When exposed to 2 weeks cyclic HS, Dorpers showed higher heat tolerance (less decline of feed intake and body weight and lower RR and RT) than 2nd cross lambs. However, 2nd cross lamb's had higher growth performance compared with Dorpers under the TN conditions. Two weeks of heat exposure caused a small increase in muscle pHu of the two breeds. In terms of
Acknowledgements
Authors would like to acknowledge Faculty of Veterinary and Agricultural Sciences, The University of Melbourne and Lindsay and Alice Gamble trust fund for providing top up scholarship to Minghao Zhang and start up fund to Dr. Surinder Singh Chauhan to conduct this research.
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2022, Small Ruminant ResearchCitation Excerpt :The live animal study was approved by the Faculty of Veterinary and Agricultural Sciences, The University of Melbourne Animal Ethics Committee (Ethics ID: 1714357.1) and was conducted at The University of Melbourne, Dookie Campus, Victoria, Australia (36°23′01.9″S 145°42′52.1″E) over two weeks. The details on animals and experimental design have been previously reported by Joy et al. (2020b) and Zhang et al. (2021). Briefly, 48 lambs of two different breeds, Dorper and 2nd Cross [SC; Poll Dorset X (Border Leicester X Merino)] lambs (24 lambs from each breed; 4−5 months old with live weight = 40.9 ± 0.91 kg, (Mean ± SD) were used in the study.
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