Weaning is probably the most stressful time in a pig's life because of the many changes it must undergo. Before weaning, pigs consume approximately 24 equally spaced meals per day. These meals come in the form of a highly digestible liquid that contains about 35% fat, 30% protein, and 25% lactose (on a dry matter basis). All pigs are conditioned to eating at the same time and only when the sow tells them to eat. After weaning, pigs are expected to adjust to a dry diet with a vastly different composition. They are also expected to tell themselves when, how much, and how often to eat. Also, unlike nursing when there was usually ample space for all pigs to have their meal together, feeders do not typically provide enough space for simultaneous feeding.
From birth until the pig is approximately eight weeks of age, there are also many digestive, metabolic, and immunological changes taking place. For example, the enzymes that are needed to break down various dietary ingredients are developing at different rates. As shown in Figure 1, lactase (the enzyme associated with the digestion of milk carbohydrates) is high at birth, reaches a peak at two to three weeks of age, and then declines rapidly. On the other hand, amylase (the enzyme needed to break down the carbohydrates found in cereal grains) is quite low at birth and increases with age. This is also the pattern for the enzymes involved in the digestion of proteins. Those involved in the breakdown of the simple proteins found in milk are high at birth, while those required for the digestion of the more complex proteins found in cereal grains and oilseed meals are low at birth and increase with age. In light of these facts, the ingredients that should be included in diets for weanling pigs are highly dependent on the age at which the pigs are weaned.
Figure 2 shows the immunological changes that are occurring in the young pig. The newborn pig acquires passive immunity by absorbing the antibodies present in colostrum. The levels of these maternally-derived antibodies are highest on day one post-farrowing and then decline to very low levels by the time the pig reaches about three weeks of age. The pig's own immune system begins developing at approximately three weeks of age, but is not able to mount an effective active immune response until the pig is four to five weeks of age. This makes the two- to four-week-old pig very susceptible to disease and pathogenic stressors.
Because of the behavioral, biological, and immunological changes that are simultaneously occurring at the time of weaning, proper attention to nutrition, management, and the environment is essential for getting the newly weaned pig started on the right foot. A breakdown in just one of these areas can cause problems in the nursery and the subsequent growing periods.
Source: Kentucky Ag Extension