However, other growth-promoting metabolic processes may
also be involved in the improved performance for the biphasic-fed
pigs observed in the current studies. The exogenous
administration of porcine somatotropin (pST) enhances
production responses that result in a greater weight gain,
improved FCR and a preferential deposition of protein at the
expense of lipid accretion (Campbell et al. 1988, 1989; Klindt
et al. 1995). The effectiveness of pST appears to be influenced
by the method of its delivery. Daily injections of pST that result
in large daily peaks in circulating concentrations improve
carcass leanness and FCR, a result of both a decrease in feed
consumption and an increase in the rate of gain (Yen et al. 1990;
Klindt et al. 1992). Whereas the delivery of pST via a sustainedrelease
implant also improves both carcass leanness and FCR,
but the improvement in FCR is a consequence of a decrease in
feed consumption without a change in the rate of gain (Yen et al.
2005). Interestingly, the production responses obtained from the
use of pST implants are similar to those observed for the biphasic-fed
pigs in the current study. This improved productivity
may also be a response to an alteration in the plasma GH profile
for these pigs. Although not measured for these studies, we have
previously shown a doubling in the number of GH peaks from
two to four over a 12-h period for pigs fed bi-phasically,
compared with those fed ad libitum (Scrimgeour et al. 2008).
Therefore, it is conceivable that the improved production
responses we have observed for the bi-phasic-fed group may
be the result of heightened metabolic growth processes, in which
both insulin and GH play a decisive role.