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Effects of in utero heat stress on muscle development of barrows
Muscle development occurs between d 25 to 90 of gestation and can be influenced by environmental factors such as in utero heat stress during fetal development and postnatal growth. Changes in muscle development can alter the quality of the meat product. Therefore the objectives of this work were to determine the effects of in utero heat stress, excess dietary lysine and Paylean on muscle development of barrows. At 25 kg, barrows of control (TN; n=40) or heat stressed (HS; n=40) dams were individually housed and fed a corn soybean meal diet. At this time, barrows were equally and randomly assigned to receive a diet that met (100% NRC; n=20) or exceeded (110% NRC; n=20) NRC lysine requirements. In the last 30 days of finishing, barrows were again equally and randomly assigned to a diet containing 0 (CTL; n=10) or 7.4 ppm Paylean (PAY; n=10). Diets were fed until 121 kg of weight was attained. At slaughter, muscle samples were collected from the longissimus dorsi (LD) and semitendonosus (ST) and used for the ATPase assay. In the LD, the number of primary muscle fibers (PCLD), the number of secondary fibers (SCLD), the size of primary fibers (PDLD) and the size of secondary fibers (SDLD) were determined. The same variables (PCST, SCST, SPST, PDST, SDST) were determined for ST. The ratio of secondary to primary fibers (SPLD, SPST) was also determined for each LD and ST. HS treatment increased (58.17±1.17 vs 60.90±1.06 µm; P=0.09) SDLD. HS by PAY interactions increased SDLD (P = 0.02) and decreased SPLD (P = 0.04). There was a decrease for an interaction between lysine by HS for SCLD (P = 0.08). HS by lysine by PAY interactions increased SCLD (P = 0.03) and decreased SPLD (P = 0.009). In ST muscle, HS treatment increased PDST (57.74±1.09 vs 63.78±1.13 µm; P=0.0003) and SDST (67.70±1.36 vs 72.74±1.40 µm; P=0.01). A lysine by PAY interaction increased PDST (P=0.04), trends for this interactions reduced PCST (P=0.07) and SCST (P=0.10). There were also positive correlations between shear force and PDLD (r=0.31, P=0.01), SDLD (r=0.27, P=0.04) and PDST (r=0.30, P=0.03) and a tendency for SDST (r=0.25, P=0.06). Heat stress during gestation alters muscle development, resulting in increased muscle fiber size at harvest and ultimately a tougher product.
Keywords: heat stress, muscle development, ractopamine