Feeding growing and finishing pigs

Growth rate

Growth rate is genetically determined and most mammals, including humans, have a similarly shaped growth curve. Figure 1 (left panel) shows a weight-for-age graph for two genotypes; the lower black line represents average genetics and the upper red line represents improved genetics. With average genetics, the maximum weight achieved is 200 kg, while with improved genetics, a maximum weight of 250 kg can be achieved.

The graph on the right uses the same data, but shows growth rate at various body weights. With average genetics, pigs can hit a maximum growth rate of 970 g/day at 78 kg body weight, while those with improved genetics can exceed 1120 g/day at 93 kg. At higher weights, growth rate falls as the pigs get closer to maturity.

Figure 1. Weight-for-age and growth-for-weight graphs of pigs with two different genetic profiles

Similar weight-for-age and growth-for-weight graphs can be drawn for boars and gilts, although there is less difference between the ‘average’ and ‘improved’ lines.

Some reasons why genetic potential may not be achieved on a commercial farm include growth rates that are compromised by:

• health
• management
• stress
• nutrition
• environment and other factors

Feeding must be targeted at this compromised level rather than at the level of genetic potential. Giving better feeds to “push them on a bit” is rarely successful – assuming the specification of the current feed is aimed at a realistic farm performance level.

Protein

Much of the growth in modern growing/finishing pigs is lean meat – protein, which is mainly composed of amino acids and water. Lean meat is around 80% water, so it is not surprising that the production of lean meat is extremely efficient: the feed conversion is very low. There are 21 amino acids, some of which are ‘essential’. This means that the pig cannot synthesise them in the body, so they must be provided in the feed in the correct ratios.

Fat

Fat is the next major constituent of the growth. Fat has a very high energy value and, therefore, has a poor feed conversion. Excess fat is penalised in most processor specifications. Energy is required to deposit fat, to drive lean growth and for maintenance and heat production where necessary. Net energy (NE, MJ/kg) is the energy remaining for productive purposes after accounting for energy losses in the faeces, urine and as heat.

Minerals and vitamins

As a framework for the lean and fat tissue, there is a skeleton, which is mainly calcium and phosphorus. Minerals, particularly phosphorus, are also found in the lean meat. Finally, vitamins are required at very low levels. They have many functions, but mainly help to facilitate chemical reactions in the body.

Requirement versus appetite

Young pigs have a very high nutrient requirement, but a relatively low appetite. To maximise nutrient intake and growth rate they are given feeds with high concentrations of energy, amino acids and minerals. As pigs age, their appetites improve and their requirements for amino acids and other nutrients decline as growth slows.

There is a linear relationship between energy and FCR – the higher the NE, the better the FCR (in finishing pigs there is approximately a 0.20–0.25 improvement in FCR with an increase in NE of 1 MJ/kg). Pigs attempt to maintain a constant energy intake as the energy level in the feed changes, but they cannot do this perfectly, so grow slightly quicker and are fatter when they consume high-energy feeds.

The optimum energy level is an economic debate and depends largely upon raw material prices:

• If high-energy ingredients, such as wheat, fat and soya, are relatively cheap, then high-energy feeds can reduce feed costs
• In contrast, if lower energy ingredients such as barley, wheatfeed, rapeseed and sunflower meal are relatively cheap, then feed costs may be reduced, despite a poorer FCR

Amino acid supply is more complex than energy. The level of amino acids required in the feed falls with weight. Further, the balance of essential amino acids also changes. The weight range in the population being fed must be considered.

Table 1 gives typical feed specifications for growing and finishing pigs. These should be adjusted where necessary for individual farms, particularly where FCR is low.

Table 1. Nutrient specifications for growing and finishing pigs (boar/gilt mix)

¹Amino acids are presented as ratios to standardised ileal digestible lysine

²Digestible phosphorus level is based on diet formulation without the use of phytase. If phytase is being used, then phosphorus levels must be readjusted. Addition of phytase to the diet is recommended for improved digestibility

³The maximum permitted level of copper in feed for pigs is: 150 mg/kg for pigs up to 4 weeks post-weaning, 100 mg/kg for pigs up to 8 weeks post weaning and 25 mg/kg over 8 weeks post-weaning

In addition to the above nutrient recommendations, the following additives may also be considered:

• Phytase

Phytase enzymes effectively increase the digestibility of minerals and other nutrients and reduce phosphorus excretion

• Cereal enzymes

Cereal enzymes improve the digestibility of plant cell walls

• Organic acids

Acids are generally effective in the 13–35 kg feed and some heavier weights. However, acids are a diverse group with very different properties so should be assessed on an individual basis

Many producers use three feeds from 13 kg to slaughter, but, as weights increase and nitrogen and phosphorus excretion become of greater importance, then four feeds are recommended. This is easy to implement on a batch system.

As pigs grow, their feed intake will increase accordingly. In the early grower phase, a 13-kg pig will eat approximately 0.5 kg/day, up to 2.5 kg/day or more by the time a finisher pig reaches 110 kg.

Pigs reared on indoor or outdoor systems are usually fed similarly and do not have different nutrient requirements. However, there can be a seasonal influence on feed intake, which is more pronounced in pigs kept outdoor or in straw yards.

In cold weather, pigs will increase feed intake to maintain their body temperature. This can cause an unfavourable increase in P2 probe measurements; however, adjusting the ratio of protein to energy in the diets can help to reduce this. For this reason, it can be beneficial to have a summer and winter regime for pigs kept outdoors or in straw-based systems.

While there are pros and cons to each feed form, ultimately, the decision is unit-specific and may be dictated by the feeding equipment available on farm.

Pellets

Pellets are the most commonly used feed form and reportedly increase feeding efficiency and feed intake. This may be explained by the more finely ground particles improving the availability of nutrients, which causes intake to increase because of faster digestion. Additionally, if meal is used, there can be increased feed wastage, so pellets appear to give a better feed conversion ratio. A high proportion of fine particles (40%) in pelleted feed can make it less favourable, thus less efficient. It also has the potential to block feed equipment. Meal is cheaper to produce than pellets.

Meal

In the UK, meal is generally a coarse grist product, which can be beneficial for digestion as well as giving better flowability and reduced dust. Compared with pellets, feed conversion tends to be poorer, but mortality can be lower when meal is fed. Meal has been shown to be beneficial in on-farm health challenge situations by reducing the incidence of diarrhoea.

This was written in conjunction with ABN, Primary Diets and Premier Nutrition.