Diet complexity refers to the use of highly-digestible specialty feed ingredients in nursery diets. Complex diets are typically fed to weanling pigs to provide high-quality feed ingredients and improve intake in the early post-weaning period. As complex diets are more expensive, diet complexity should be rapidly reduced during the course of the nursery.
Nutritional strategies in the nursery have been of great interest because it is generally assumed that pigs that grow faster in the nursery also grow faster in the finisher. However, not all dietary efforts to improve performance in the nursery are rewarded with improvements in growth rate in the finisher. An important distinction to make is whether the dietary effort is able to induce a fundamental or a transitory change in the nursery pig.
Weaning age is able to induce a fundamental change in the pig. The enhancement of nursery performance by increasing weaning age is typically maintained into the finisher as a consequence of increasing weaning weight (Main et al., 2004), but most importantly as a consequence of a physiological change in the pig, like improvements in digestive and immune functions (Moser et al., 2007; Smith et al., 2010; McLamb et al., 2013).
Most nutritional strategies induce a transitory change in the pig, with improvements in performance while being fed in the nursery, but not necessarily in the subsequent finisher period. Diet complexity typically generates this type of response, with significant improvements in feed intake and growth rate while the complex diet is being fed, but no performance advantages thereafter (Whang et al., 2000; Wolter et al., 2003; Skinner et al., 2014; Collins et al., 2017). This is also the case of amino acid concentration (Main et al., 2008), fat (Tokach et al., 1995), antibiotics (Skinner et al., 2014), or milk replacers (Wolter and Ellis, 2001) in nursery diets. In contrast, lactose is able to improve nursery performance with further improvements in finisher performance (Tokach et al., 1995).
Therefore, the value of diet complexity should consider the benefit gained during the feeding period but not projected additional benefit in the subsequent nursery or finisher periods.