Intensified nursing dramatically accelerates growth performance and the size of the body frame in Japanese black and holstein crossbred steers

Crossbred (Japanese Black male and Holstein female) steers were used to investigate the effects of feeding a large amount of high-protem milk replacer on growth performance, morphological change and myogenic regulation in skeletal muscle. Group HP (n = 7) was fed high-protein milk replacer (Crude Protein (CP): 29%, maximum intake 2.0 kg day^-1) and group C (n = 14) was fed a standard milk replacer (CP: 26%, 0.5 kg day^-1) from 56-147 days of age. Changes in Body Weight (BW), body frame measurements, plasma Insulin-like Growth Factor-1 (IGF-1) and plasma Alkaline Phosphatase (ALP) were investigated. M. Longissimus Thoracis (LT) samples were obtained by biopsy to investigate the myofiber type composition, diameter and the mRNA expression of Myogenic Regulatory related genes by quantitative real-time RT-PCR. A large amount of high protein milk replacer (Intensified Nursing) in group HP improved their synthesis of IGF-1 in the liver and accelerated skeletal development by maintaining high levels of plasma IGF-1 and ALP concentration, which was highly controlled by CP intake from diet. The myofiber type composition and size in the LT was not influenced by nutritional condition and aging during the nursing period. Intensified Nursing extends myofiber length rather than increasing the thickness of the myofiber, according to the increase in the size of the body frame during this phase. The expression of MyoD and IGF-1 receptors in the LT decreased at the early stage of the nursing period without the influence of nutritional condition. This suggested the change would control the level of differentiation from the myoblast to the myocyte.