What is a weaned pig?

A weaned pig in North Carolina in 1997 is a very different animal from a weaned pig of 25 years ago, and also from a weaned pig in many other parts of the world today. In most of Europe, weaning pigs at less than 21 days of age is discouraged, or even illegal, based on animal welfare concerns. And the shift to weaning younger and younger has never been driven by piglets’ needs, but by the desire to increase reproductive output from sow herds. Rather the needs of the weaned piglet, and particularly nutrition, have been a constraint on reducing weaning age with the aim of improving sow productivity. That is, we have designed systems in which piglet needs are pushed near the limit.

Also, a weaned pig is not just a weaned pig. When we have 16 day weaning from a farm managing farrowing rooms all-in/all-out, what do we really have? Either pigs of something like 12 to 20 days of age, with weights that might range from around 7 to 15lb (and you do hear lower!), undergoing an abrupt dietary change that under natural condition would be gradual over a period of months. On the other hand, we may have a bit more uniformity in weight with the compromise that all-in/all-out isn’t exactly what it sounds like. And those heavier 12 day old pigs that are weaned early do not have the gut maturity of a smaller 18 day old pig. In the days of 28 day weaning, when age in a group of weaned pigs typically ranged from 24 to 32 days, variability was less important with respect to the physiologic needs of the pigs. However, the earlier we wean pigs, the more significant this variability in piglet size and maturity becomes.

Knowing how to feed these little guys is a science in its own right, and is being discussed by Eric van Heugten at this meeting. Here we will review some other factors that can be critical to health and survival of the 16 day old, 7 to 15 lb, weaned pig. The key thing to remember is that these are generalizations, and we are dealing with variable populations. All the pigs coming off the truck do not have the same requirements. Compromises have to be made, and typically we should consider the needs of the poorest (smallest) piglets. The key is probably identification and separation of the poorer pigs for TLC (diet, location in barn, supplementary heat).

Temperature

The pig is a homeotherm (maintains constant body temperature), and stable body temperature is important to the maintenance of general body functions. There is a normal temperature gradient such that the body ‘core’ is maintained at a higher temperature than the outer layers. Body heat comes from either metabolic activity or the environment, and can be lost to the environment. In the stable situation, heat gained must equal heat lost for body temperature to be constant. Obviously, under field circumstances some fluctuations in heat production and loss, and in body temperature, do occur.

Key points

• The main heat source for pigs is feed, and most metabolic heat is generated in the core of the animals body. Heat will then flow from warmer to cooler areas of the body, and from the body surface to its surroundings.
• Pigs lose heat into the environment by radiation, convection, conduction and evaporation. Normally, stable body temperature can only be maintained when the body is warmer than the environment most of the time.
• The exchange of heat between the pig and its environment is affected by the amount of body insulation and the surface area of the animal relative to its body weight (Fig. 1)

Younger animals have less body insulation and have a much greater surface area/volume ratio of their bodies. This means that they are will suffer much greater thermal stresses than larger animals, at both cold and hot temperatures. Also, the fact that feed and its metabolism are the primary heat source for pigs points to added fragility in the post weaning period when feed intake may be minimal. Getting weaned pigs on feed early is vital to preventing temperature stress. Some guidelines of appropriate temperatures and how they change with feed intake are indicated in Figure 2.

Ventilation

Adequate air quality is important to the health and welfare of pigs as well as staff. Common gases associated with pig manure that have potentially deleterious effects include ammonia (NH₃), carbon dioxide (CO₂), carbon monoxide (CO), hydrogen sulfide (H₂S), and methane (CH₄). Failure to maintain adequate air quality may depress feed intake and predispose to respiratory disease. For humans, recommended maximum levels based on 8 hours daily exposure to these gases have been established for people, and maximum recommended levels for pigs have been suggested (Table 1).

Typically some compromise is made between air quality (i.e. ventilation rates) and temperature maintenance, at least in cold weather. For 10lb pigs maintained at 85-90 F, required air flow rates per pig can vary widely from around 1.5 cfm in very cold weather to 30 cfm in very hot weather, but again required rates vary greatly with building design and insulation. As we all know, air movement affects the effective environmental temperature, as shown below:

As expected, the smaller the animal, the more susceptible it will be to the effects of air movement, and smaller pigs should be located where they will be least exposed to drafts.

Humidity is the other factor that many people consider critical in the nursery environment, and 55% to 65% relative humidity should be the goal.

Space and group size

This is more of an issue towards the end of the nursery period, particularly if sow farms are performing better than expected and overstocking results. However, it is not really an issue for the recently weaned pig. A greater problem in cold weather can be whether total animal mass in the building is adequate to maintain temperature. Groups sizes of 10 to 30 are common in the industry. Group sizes of less than 10 give less opportunities for huddling and heat conservation. Experimental studies suggest that feed intake and growth tend to decrease as group size increases at constant stocking density. Comparisons of groups of 10 pigs all from the same litter with groups made up of 5 pigs from 2 litters or 1 pig from 10 litters showed the best performance came from groups made up of five pigs from 2 litters. Subsequently it has been suggested to form groups from the least possible numbers of litters. However, identity is usually lost in our systems.

Water

Suggested guidelines are:

• No more than 10 pigs per nipple
• More than 1 nipple per pen
• Nipple diameter 3/8"
• Flow rate of 1 to 2 cups per minute
• Pressure of 20 psi
• Functional nipples that pigs can reach, and drink from with ease

Neighbors!

With regard to infectious disease, the health of pen-mates and barn mates is probably the biggest factor. The temptation is often there to ‘roll back’ poor doing pigs between barns designed to be managed all-in/all-out. We need to recognize that this practice is designed to promote transmission between barns and perpetuate infectious disease problems. Separation of sick and poor doing pigs into sick pens where they have more space and less competition is advisable and also facilitates individual treatment when necessary.

Stockmanship

When pigs were weaned at 5 weeks of age, the level of stockmanship required to manage them was modest compared with 16 day weaning. While it is usual for us to look at feed and the physical environment when evaluating problems in weaned pigs, it is difficult to overrate the importance of the stockperson. Attention to details and early recognition of problems are vital. Remember that a poor system used well can be better than a good system used poorly. And as we continue to push towards earlier weaning, we are putting greater demands and responsibilities on staff.

Take-Home Message

• Individual pigs within groups weaned at 16-18 days, on average, are at varying stages of development and have varying needs
• Adequate temperature control and getting pigs on feed early are critical
• Make special provisions, environmentally and nutritionally, for low weight piglets