Formulating Diets for the Grow-Finish Pig

Nursery pig research requires less feed, provides easier replication and needs fewer days to complete a turn. Conversely, finishing pig research takes considerable time and large numbers of pigs are required to find small, but important, differences in performance. ( PORK )

Historically, the grow-finish phase of production received less attention in swine nutrition research. Nursery pig research requires less feed, provides easier replication and needs fewer days to complete a turn. Conversely, finishing pig research takes considerable time and large numbers of pigs are 
required to find small, but important, differences in performance. 

It is much easier to create a 5% to 10% change in nursery performance than finisher performance. However, 75% to 80% of the feed used to produce a pig is consumed in the grow-finish phase of growth. Therefore, the finishing pig phase has huge financial implications for the swine producer.

In reviewing the important areas of concern for finishing pigs, we approach them in terms of how we formulate diets. We first determine the most economical energy level. Then, we determine the lysine:calorie ratio for that energy level. The third step is to set the ratio for other amino acids relative to lysine. Fourth, we set the standardized total tract digestible (STTD) or available protein:calorie ratio. The last step is to set the level of calcium, vitamins, trace minerals, salt and other ingredients, including feed additives. 

Here’s a review of each of these areas.

Energy response, fiber, and carcass dressing percentage

The first and most important step in diet formulation is to set the energy concentration. Even though energy is the most expensive component of the diet, the level used in formulation is often based on history or impact on cost rather than an in-depth analysis to determine the most economical level. 

To set the optimal diet energy level, we must know how an incremental change in dietary energy affects diet cost, pig performance—average daily gain (ADG), F/G—and carcass criteria (dressing percentage, lean percentage).

Additionally, the economic value of these incremental changes are dependent on the market price. Our team has built a model (Soto et al., 2017a) using this information to help calculate the optimal dietary energy level in each phase. The key components of that model are discussed here briefly.

Impact of dietary energy on pig performance and carcass criteria

Ideally, a production system will know how pigs respond to dietary energy changes in their system; however, this is not always possible. In these cases, information on historically expected responses is valuable. For example, Nitikanchana et al. (2015) developed regression equations from a meta-analysis to predict the change in growth rate for each incremental change in nutrient energy (NE) intake. 

The equation from their work predicts ADG, g = 0.1135 x NE, kcal/kg + 8.8142 x Avg BW, kg - 0.05068 x (average body weight, kg)2 + 275.99, provided lysine or other amino acids are not limiting performance. Thus, for every 100 kcal/kg increase in NE, ADG would increase by 113.5 g/d. For feed efficiency, the assumption is efficiency is linearly related to net energy content of the diet. Thus, a 1% change in net energy will result in a 1% change in feed efficiency. 

Changes in dietary NE level do not fully account for this negative impact of fiber on carcass weight. Feeding diets with high levels of fiber increase digesta content in the colon and cecum at processing and reduce dressing percentage (Turlington, 1984). 

The increase in gut fill is a result of the type of fiber in the ingredient. Neutral detergent fiber (NDF) has been shown to result in the digestive contents to swell by absorbing water, thus increasing fecal volume in the large intestine (Coble et al., 2015). 
Soto et al. (2017b) conducted a meta-analysis to determine the impact of NDF in diets fed before slaughter on dressing percentage. With the variables being the number of days in the withdrawal period (WP), NDF level in the dietary phase before the final phase (NDF1), NDF level in the withdrawal period before marketing (NDF2), and the interaction between NDF2 and WP, the regression analysis yielded the equation: Dressing percentage, % = 0.03492 × WP (d) – 0.05092 × NDF1 (%) – 0.06897 × NDF2 (%) – 0.00289 × (NDF2 (%) × WP (d)) + 76.0769. 

This equation was derived from U.S. -raised pigs where dressing percentage is calculated with the head removed. As the equation indicates, high levels of NDF have a negative impact on carcass yield. Increasing the length of the withdrawal period improved carcass yield; however, the effect of withdrawal period was dependent on the level of NDF in the last diet fed (NDF2), as indicated by the interaction term. 

Thus, the optimal energy density for the diets fed immediately before market must be determined on a carcass weight basis to include the dressing percentage component.

Lysine, amino acid ratios and crude protein

Once the dietary energy level is determined, the lysine:energy ratio and amino acid concentrations or ratios relative to lysine must be set. Although the lysine requirement is best determined within the production system or modeled from protein accretion data, it can be estimated using growth rate and feed intake data. 

A simple rule of thumb is grow-finish pigs require approximately 20g of standardized ileal digestible (SID) lysine per kilogram of gain. The requirement is lower in the early grower stage and higher during the peak protein deposition phase; however, 20g/d is a good average of the requirement because growth rate is correlated with protein deposition rate. 

Once SID lysine is determined, the ratios of other amino acids are set relative to lysine. Most amino acid ratios are well established. Ratio estimates used in diet formulation by our research group are summarized in Table 1. 

Much of the disagreement between nutritionists in optimum ratios is a result of different amino acid loadings in diet formulation software. For example, a 68% valine to lysine ratio using amino acid data from one lab might be similar to a 70% or 72% ratio using data from another lab. 

A second disagreement in ideal ratios relative to lysine relates to differences in interpretation of the data. 

Optimal amino acid ratios are often depicted as a point using breakpoint analysis. In reality, a quadratic shape or diminishing returns model more accurately depicts the response to changing amino acid levels in most group feeding situations. 

Thus, a model can be used to determine whether the ratio will provide 99% to 95% of the maximum response. 

This technique allows modeling of the ratio that provides the most economical response for a production system rather than simply the maximum response. An example of this type of modelling is established for the optimal tryptophan:lysine ratio for finishing pigs. Selecting the optimum amino acid ratio is dependent on the value of weight gain and incremental cost to increase the ratio. 

An economic calculator to determine the optimal tryptophan:lysine ratio can be downloaded from:

In many parts of Europe, dietary crude protein maximums are required by law. We do not have these requirements in the U.S. and, thus, diets are often much higher in crude protein than in Europe. We are concerned with minimum crude protein levels, particularly in the diets for heavyweight finishing pigs. 

Kansas State University research continues to indicate diets for pigs over 220 lb. should contain at least 13% crude protein to not limit performance. Although considerable research has been conducted, the reason dietary crude protein cannot be lowered below 13% is not known. 

The optimal number of dietary phases for finishing pigs also has received attention. Increasing dietary phases to more closely match the pigs’ amino acid requirement will reduce nitrogen excretion; however, pig performance is rarely improved by increasing the number of dietary phases. 

As long as the diets fed prior to pigs being marketed are at or above the pigs’ requirement to allow compensatory gain to occur, pigs will compensate for performance lost due to feeding diets that are slightly below their requirement during earlier phases (Main et al, 2006; Menegat et al., 2017).