Feed restriction and subsequent realimentation in Holstein Friesian bulls: I. Effect on animal performance; muscle, fat, and linear body measurements; and slaughter characteristics.

Holstein Friesian bulls (n = 75) were used to evaluate the effect of restricted and subsequent compensatory growth on muscular and skeletal growth as well as the recovery of carcass and noncarcass components. Fifteen bulls were slaughtered on Day 0 to provide baseline parameters for carcass and noncarcass measurements. Of the remaining 60 bulls, 30 were fed ad libitum (ADLIB) and 30 were fed a restricted (RES) diet to grow at 0.6 kg/d for 125 d, denoted as Period 1. After 125 d of differential feeding, 15 bulls from each group were slaughtered. The remaining bulls in both treatment groups were then offered ad libitum access to feed for a further 55 d (realimentation), denoted as Period 2, after which they were also slaughtered. All animals received the same diet composed of 70% concentrate and 30% grass silage throughout the experimental trial. As planned, feed intake was greater for ADLIB bulls in Period 1 (P < 0.001); however, there was no difference in feed intake during realimentation (P > 0.05). During Period 1, RES bulls gained 0.6 kg/d whereas ADLIB bulls grew at 1.9 kg/d. During realimentation in Period 2, RES bulls displayed accelerated growth, gaining 2.5 kg/d compared with 1.4 kg/d for ADLIB bulls (P < 0.001). This amounted to a live weight difference between treatment groups of 161 kg at the end of Period 1 after restricted feeding, which was then reduced to 84 kg at the end of Period 2 (P < 0.001). Restricted animals achieved a compensatory growth (or recovery) index of 48% within 55 d of realimentation. During Period 2, RES bulls displayed a better feed conversion ratio (P < 0.001) than ADLIB bulls, indicating better feed efficiency. Ultrasonically measured longissmus dorsi growth was greater for ADLIB bulls compared with RES bulls during Period 1; however, this was reversed during Period 2 (P < 0.001). Metabolically active organs such as the liver and components of the gastrointestinal tract were lighter in RES bulls at the end of Period 1, with no difference in the weights of these components after realimentation (P < 0.01). The improved feed efficiency and muscle growth observed during feed restriction induced compensatory growth may be as a consequence of latent effects of reduced requirements of energetically demanding tissues into realimentation.