Abstract

Considerable research has been published in the last 10 years on the amino acid requirements of growing pigs. Over 80 publications are briefly summarised in this review and undoubtedly numerous publications have inadvertently not been included.

Data indicates the best practical method to express lysine requirements over a wide range of environmental conditions and genetics may be in relation to growth rate with nursery pigs requiring approximately 19g of standardised ileal digestible (SID) lysine per kg of gain and finishing pigs requiring approximately 20g per kg of gain. These requirements have increased over time.

For practical diet formulation, other amino acids are best expressed as a ratio relative to lysine. The research published in recent years greatly improves our understanding and allow us to propose lysine requirement estimates and ratios for other amino acids for each dietary phase of growth.

Introduction

Numerous papers have been written on amino acid requirements of growing pigs and methods to assess and express requirements. In this paper, the authors focus on research conducted from 2000 to 2010 and on empirical studies rather than the factorial approach to determine requirements. In collecting information for this paper, they express surprise at the relatively few publications on the lysine requirement of growing and finishing pigs and the large number of trials that have been conducted to determine the relative ratios of other amino acids relative to lysine.

The requirements for amino acids can be expressed as a percentage of the diet, grams per day, grams per unit of energy, or grams per unit of body weight. For lysine, all of these methods will be used. The requirement for other amino acids will be expressed as ratio relative to lysine. Amino acids can also be expressed on a total basis, apparent digestible basis, true digestible basis, or standardized digestible basis. As has become the standard for diet formulation, standardised ileal digestible (SID) amino acids will be used in this paper.

One area that the authors do not address in this paper is the wide variability in amino acid levels within and between ingredients and between laboratories. These differences exist and must be dealt with when formulating swine diets. For the purposes of this paper and our research at Kansas State University, ingredient SID amino acid and ME levels provided by NRC (1998) were used in diet formulation for experiments and for comparisons to have a common point of reference. To compare any amino acid recommendation from this paper, it is best to formulate a corn-soybean meal diet with your ingredient loading values and NRC (1998) ingredient loading values for the amino acids and energy to adjust the recommendations to your ingredient nutrient loadings. For ease of comparison, the optimal ratios relative to lysine are provided using NRC (1998) ingredient nutrient loadings and converted to Brazilian (Rostagno, 2005) ingredient nutrient values.

Lysine Requirements

Nursery pigs

Numerous research trials have explored the SID lysine requirement of nursery pigs in recent years. Researchers at Kansas State University and the University of Missouri conducted a series of experiments under field and university conditions to determine the lysine requirement from 5 to 10kg and 10 to 25kg. For the lighter weight range, the requirement estimate was found to be between 1.35 and 1.40 per cent SID lysine (4.0 to 4.2g per Mcal ME; Gaines el al., 2003; Nemechek et al., 2011b). This requirement was similar to the estimate found by Dean et al. (2007) of 1.4 per cent SID lysine or 18.9g of lysine per kg of gain for six- to 12-kg pigs.

For 10- to 25-kg pigs, Kendall et al. (2008) conducted five experiments with 3,628 pigs and found the SID lysine requirement to be 1.30 per cent SID lysine (3.80g per Mcal ME). This was equivalent to 19g of SID lysine per kg of gain. Schneider et al. (2010) titrated energy and lysine levels simultaneously in two separate trials with different genotypes. With one genotype, the optimal SID lysine:ME ratio was approximately 3.4 to 3.6g per Mcal ME, while the optimal ratio was 3.9 to 4.2g per Mcal ME for the other genotype. However, when expressed relative to gain, the requirement was approximately 19.0g of SID lysine per kg of gain for both genotypes. In another large field study, Lenehan et al. (2003) found the SID lysine requirement for 10- to 20-kg pigs was 1.40 per cent; however, when calculated on a 'g per kg' of gain basis, the optimal level was again 19g of SID lysine per kg of gain. In a cooperative study involving several universities in the United States, Hill et al. (2007) confirmed that the lysine requirement of nursery pigs of modern genotypes were higher than recommendations of NRC (1998).

Although lysine requirements of nursery pigs have increased in recent years and vary with environmental conditions and genotype, when expressed relative to growth rate, empirical studies in recent years have consistently found the requirement to be 19g per kg of gain.

Finishing pigs

Interestingly, more experiments have been published in recent years on the lysine requirement of nursery pigs than the lysine requirement of finishing pigs. Numerous trials have been conducted within production systems to assess the lysine requirements of finishing pigs under field conditions. However, relatively few of those experiments have been published as most are proprietary to the production system where the experiments were conducted. Main et al. (2008a) and a few abstracts (Srichana et al., 2004ab; Gaines et al., 2004a; Bergstrom et al., 2010) are the only lysine titration experiments published in recent years. PIC (2008) also conducted a series of lysine titration trials to determine their recommended Lys:ME ratios.

Certainly, the lysine requirements of finishing pigs are even more variable than the requirement of nursery pigs due to the wide range of genetic capability for lean growth, health status, energy intake and environmental conditions. The authors' discussion will focus on the requirements of high lean growth modern genotypes that would be expected to have a protein accretion rate of approximately 150g per day (lean gain of 0.85lb per day or 385g per day). For lysine recommendations for pigs with lower protein deposition rates, the National Swine Nutrition Guide (2010) provides methodology for estimating their lysine requirements.

Main et al. (2008a) conducted a series of seven experiments to determine the lysine requirement of growing-finishing gilts and barrows under commercial conditions. The equations (lysine:calorie ratio, g per Mcal ME = -0.0133 × BW, kg + 3.6944 and = -0.0164 × BW, kg + 4.004, for barrows and gilts, respectively) best described the Lys:calorie ratio that met biological requirements and optimised income over feed cost (IOFC) of the pigs (PIC, L337 × C22; 35 to 120kg) tested in their experiments. On an SID basis, the optimal ratios from their experiment would be: (Lys:ME, g per Mcal = -0.0116 × BW, kg + 3.214 and = -0.01427 × BW, kg + 3.483 and, for barrows and gilts, respectively).

These studies also suggest lower feed cost per kg of gain could be obtained with only marginal reductions in biological performance and IOFC when feeding marginally deficient lysine diets early (35 to 70kg) in the grower-finishing period, as compared to more severe penalties in growth and economic performance of feeding marginally deficient diets in late finishing period (70kg to slaughter). When expressed relative to gain, the optimal SID lysine requirement was 20g per kg of gain. Although De La Llata (2007) found lower SID lysine requirements on a percentage basis, the SID lysine required per kg of gain was also 20g per kg in their studies.

Bergstrom et al. (2010) conducted four 28-day experiments with mixed genders (barrows and gilts) to determine the lysine requirement of growing-finishing pigs (PIC TR4 × 1050) from 37 to 129kg. Their results indicated that for pigs weighing 37 to 65kg, 56 to 86kg, 80 to 107kg and 102 to 129kg, performance and IOFC were optimised with SID lys:cal ratios of 2.69, 2.35, 2.09, and 1.79g per Mcal ME, respectively. These results closely match the ratios suggested by Main et al. (2008a) and those suggested by PIC (2008) for gilts of their genotype (SID lysine, g/Mcal ME = 0.000027 × (BW, kg)2 - 0.015318 × BW, kg +4.114302).

Researchers at the University of Missouri have published SID lysine estimates for 30 to 44kg and 44 to 67kg PIC gilts (Srichana et al., 2004a) and 80 to 100kg barrows and gilts (Srichana et al., 2004b). Similar to other researchers, their data suggests the requirement for optimal feed:gain ratio is higher than the requirement for optimal average daily gain. Although they list the estimates as a percentage of the diet, using their data, the estimated requirement is similar to the 20g per kg of gain estimated by other researchers. In a recent experiment, Shelton et al. (2009) again confirmed that 20g SID lysine per kg of gain resulted in optimal performance of 55- to 80-kg gilts.

It is well accepted that lysine requirement estimates from NRC (1998) are deficient for higher lean growth pigs when expressed on a percentage basis. However, the reason for the lower recommendations of NRC appears to be entirely due to the feed intake curve used to generate the estimates. Feed intake with high lean genotypes is lower than the estimates used in NRC (1998), particularly under field conditions. The latest National Swine Nutrition Guide (2010) used the NRC model to estimate protein deposition and lean growth curves to estimate the lysine requirement on a grams per day basis.

The authors then used feed intake from recent publications of field studies to generate a more current feed intake curve. Using this methodology, the estimated amino acid requirements estimated in the publication can be expressed relative to ME with the equation (lysine:ME, g/Mcal = -0.00000146 × (BW, kg)³ + 0.00041 × (BW, kg)² - 0.051 × (BW, kg) + 4.502) for barrows with a protein accretion rate of 150g per day. This lysine to energy requirements predicted with this equation match the estimates from the empirical studies described above for pigs from weaning to 125kg reasonably well. The requirement for gilts with similar protein accretion is slightly higher (Table 1).

Table 1. SID lysine recommendations as influenced by weight
^aAdapted from van Heugten (2010) and Main et al. (2008a). Assumes protein deposition rate of 150 g/d from 20 to 120 kg (barrows: g/Mcal = 0.000146 × (BW, kg)2 - 0.0377 × (BW, kg) + 4.352; gilts: g/Mcal == -0.00000094 × (BW, kg)3 + 0.000306 × (BW, kg)2 - 0.0435 × (BW, kg) + 4.414).
^aPercentage is for a diet containing 3350 kcal ME/kg (corn-soybean meal diet without added fat using NRC (1998) nutrient values.

Based on the publications and experiments described above, lysine requirements of pigs under field conditions can be estimated using one of the following methods:

1. Published values from empirical studies, such as those of Main et al. (2008a) or PIC (2008), obtained with pigs housed in field conditions can be used as a reasonable first estimate. Using the data described above, the lysine:ME ratio for barrows with 150g of protein deposition from 20 to 120kg can be described by the equation (g per Mcal = 0.000146 × (BW, kg)2 - 0.0377 × (BW, kg) + 4.352; Table 1).
2. Use of 20g of SID lysine per kg of gain appears to be a reasonable estimate of lysine requirements. Thus, once a growth curve and feed intake curve are obtained from pigs within a production system, the lysine requirement curve can be estimated with reasonable accuracy. It is important to know the energy content of the diets fed while obtaining the feed intake curve in order to accurately estimate a lysine:ME ratio, which will be needed if the dietary energy level is changed.
3. Requirements can be estimated using a factorial approach using the lysine requirement for maintenance and weight gain as suggested by Rostagno (2005).
4. The lysine requirement can be modeled by estimating the protein accretion or lean growth curve and feed intake curve, such as the method suggested by NRC (1998) and used in the National Swine Nutrition Guide (van Heugten, 2010). This method can be further enhanced by actually measuring the protein and fat deposition curves using real-time ultrasound as suggested by Schinckel and de Lange (1996) and Smith et al. (1999). These authors have found that actually measuring the shape of the protein and fat deposition curves is particularly helpful when dealing with unfamiliar genotypes, health statuses, or environments.
5. Feeding titration experiments conducted in commercial scale field research barns, such as those available in many North American production systems, provide the best estimates of the pigs’ actual responses to altering the lysine content of the diet.

Because ractopamine hydrochloride increases protein deposition when fed in late finishing, the lysine requirement increases when ractopamine is fed. Numerous trials have indicated that the SID lysine:ME ratio for pigs fed diets containing ractopamine in the late finisher (>100kg) should contain approximately (0.92 per cent to 0.95 per cent SID lysine or 2.75 to 2.85g per Mcal ME (Neill et al., 2006; Frantz et al., 2009; Hinson et al., 2008).

The lysine requirement of entire male pigs immunized against GnRH is discussed in another paper at this conference and will not be addressed here.

Influence of Lysine Fed During One Phase on Subsequent Performance

Although the lysine level fed in one phase does not have a tremendous impact on the response in subsequent phases, some evidence suggests that pigs will partially compensate for feeding of deficient diets early in life when fed diets adequate in lysine later in the nursery (Nemechek et al., 2010) or finisher stage (Gaines et al., 2002; Collins et al., 2006; Main et al., 2008b).

As an example, Main et al. (2008b) fed diets at or below the pigs requirement in the early finisher and at, below or above the requirement in the late finisher. Overall, pigs fed lysine-deficient diets in early finishing, and at the estimated lysine requirement in late finishing, had lower feed cost per kilogram of gain and similar IOMFC compared with pigs fed at the estimated lysine requirement in both early and late finishing. As long as lysine requirements are met in late finishing, feeding slightly less than the lysine requirement in early finishing can reduce costs without sacrificing overall IOFC.

This ability of pigs to somewhat compensate for previous lysine deficiencies can make the determination of lifetime amino acid requirements problematic. Ideally, the optimal amino acid level to maximize profitability would be fed in each dietary phase. However, if the lysine requirement is not known, the nutritionist should error towards the lower end of lysine estimates in the earlier stages of life and the higher end of estimates in the later stages of the finisher period. The cost in reduced growth rate, poorer feed efficiency, and lowered carcass leanness due to under-formulation in the late finisher is much greater than in the nursery or grower period.

Threonine: Lysine Ratio

The large difference between apparent and standardized digestibility values for threonine has caused some confusion by nutritionists with this amino acid over the years. Deficiencies of threonine cause real, but relatively small reductions in growth and efficiency as compared to deficiencies of the other major amino acids. This has led to an underestimation of requirements and under-formulation for threonine by many nutritionists.

Van Milgen and Le Bellego (2003) conducted a meta-analysis of 22 different studies and found the optimal threonine:lysine ratio increased from 58 per cent at 15kg to 65 per cent at 110kg using a linear-plateau model. Use of curvilinear models resulted in higher requirement estimates. In two separate experiments, Lenehan et al. (2003, 2004) found an optimal threonine:lysine level of 64 to 66 per cent for 10- to 20-kg pigs. James et al. (2003) also found the optimal threonine:lysine ratio to be 60 to 65 per cent for pigs weighing 10 to 20kg. Although Wang et al. (2006) did not report a SID threonine:lysine ratio, the growth rate of pigs in their study can be used to estimate the SID lysine requirement (19g per kg of gain) to calculate an SID threonine:lysine ratio. Their data would suggest the ratio is at least 60 per cent of lysine for growth and 67 per cent for immunity. Li et al. (1999) also demonstrated that the threonine requirement for immunity was higher than the requirement for growth.

For finisher pigs, Frank et al. (2001) demonstrated that the SID threonine:lysine ratio was approximately 65 per cent for 34 to 65kg pigs. This ratio is identical to the optimal ratio suggested by the data of Buraczewska et al. (2006). The SID threonine requirement of growing pigs was estimated at 10.3g per day by Ettle et al. (2004). Assuming growing pigs require 20g of SID lysine per kg of gain, a SID threonine:lysine ratio of at least 60 per cent can be calculated for 35 to 65kg pigs from their data. Plitzner et al. (2007) found an optimal SID threonine:lysine ratio of 62 to 64 per cent with 67 to 113kg pigs and also suggested that the ratio rises as pigs move towards the late finishing stage. Pedersen et al. (2003) also found a similar optimal threonine:lysine ratio of 62 to 64 per cent for finishing pigs. However, Frantz et al. (2005) found the threonine requirement increased to 67 per cent of lysine for 75 to 105kg pigs. Wecke and Liebert (2010) conducted a series of nitrogen balance studies and found the optimal SID threonine:lysine ratio of 61 per cent for 30 to 110kg pigs. Research shows that the minimum requirement for threonine relative to lysine is approximately 60 to 62 per cent in the nursery stage and rises to 64 to 67 per cent in the late finishing stage (Table 2). A ratio of 65 per cent using NRC (1998) ingredient nutrient values is equivalent to a ratio of 68 to 69 per cent using Brazilian ingredient nutrient values of Rostagno (2005; Table 3).

Table 2. Suggested minimum SID amino acid ratios for growing swine^a
aAdapted from Shannon and Allee, 2010 with updates by authors. Ratios are based on NRC (1998) nutrient levels for ingredients. Nutritionists should review their ingredient nutrient values relative to NRC (1998) to apply these ratios to their diets.
^bTryptophan:lysine ratio appears to be increased when the diet contains large excesses of large neutral amino acids (leucine, isoleucine, valine, phenylalanine, and tyrosine)
^cRatio is at least 60 per cent when high levels of blood meal or cells are included in the diet. Ratio may be lower than 52 per cent when blood cells are not included, but more research is required to verify and to determine the optimal ratio of isoleucine to leucine.

Table 3. Suggested minimum SID amino acid ratios for major amino acids for growing swine using Brazilian ingredient nutrient values to calculate ratios^a
aAdapted from Shannon and Allee, 2010 with updates by authors. Ratios were converted to those achieved with ingredient nutrient values of Rostagno (2005).
^bTryptophan:lysine ratio appears to be increased when the diet contains large excesses of large neutral amino acids (leucine, isoleucine, valine, phenylalanine, and tyrosine)
^cRatio is at least 60 per cent when high levels of blood meal or cells are included in the diet. Ratio may be lower than 55 per cent when blood cells are not included, but more research is required to verify and to determine the optimal ratio of isoleucine to leucine.

TSAA: Lysine Ratio

Ever since Hahn and Baker (1995) suggested that the total sulphur amino acid requirement during late finishing was 65 per cent, numerous trials have been conducted with methionine and cystine. It is generally assumed that methionine must constitute at least 50 per cent of the TSAA ratio (NRC = 48 per cent on weight basis). However, recent data (Gillis et al., 2007) suggest that methionine may need to be slightly greater (55 per cent on weight basis; 50 per cent on molar basis) than cystine in the ratio.

For nursery pigs, Dean et al. (2007) suggested that the requirement for total sulphur amino acids was 10.1g per kg gain or 54 per cent of lysine for six- to 12-kg pigs. Gaines et al. (2005) found a slightly higher ratio of 57 to 61 per cent depending on the response criteria and method of assessing the breakpoint with eight- to 26-kg pigs. Yi et al. (2006) found a similar TSAA:lysine ratio of 58 per cent for optimal ADG with 12- to 24-kg pigs. In a series of experiments, Kansas State University researchers found a similar range of SID TSAA:lysine ratios of 57 to 60 per cent for 10- to 20-kg pigs with Genetiporc (Schneider et al., 2004) and PIC (Schneider et al., 2006) pigs.

In two separate experiments with growing pigs, Gaines et al. (2004b,c) found the optimal TSAA:lysine ratio was 60 per cent for 29- to 45-kg and 45- to 68-kg pigs. Lawrence et al. (2005) found a similar optimal level of 60 per cent for 30- to 60-kg pigs. For late finishing pigs, the data is not as clear. Although Hahn and Baker (1995) suggested a ratio of 65 per cent, Knowles et al. (1998) found a much lower optimal ratio (<60 per cent) for 77- to 110-kg gilts for all response criteria except to minimise fat accretion, which required 65 per cent. Frantz et al. (2009) found the optimal TSAA:lysine level was 58 per cent for late finishing pigs fed ractopamine HCl. It appears that the TSAA:lysine ratio for growing pigs is between 55 and 60 per cent and may increase slightly as pigs reach market weight, unless ractopamine is fed.

Tryptophan: Lysine Ratio

Research on the optimal tryptophan to lysine ratio is difficult to conduct. Because of the relatively small inclusion rates and small differences in range of tryptophan levels tested (ex. 14 to 22 per cent of lysine), diet manufacturing is a challenge to ensure the very low additions are thoroughly mixed. Also, tryptophan is a difficult amino acid to analyse and different analytical techniques yield different results adding to the confusion.

There is also disagreement in the quantity of tryptophan present in key basal ingredients used in many of the research trials, which can dramatically impact the projected ratios because the basal ingredients make up such a large proportion of the tryptophan in test diets. Finally, the level of other large neutral amino acids in the diet may influence the response to increasing tryptophan levels. The optimal tryptophan:lysine ratio suggested by most researchers ranges from 16 to 20 per cent. Although this range is relatively small, the difference can lead to large changes in diet formulation and cost and inclusion of other crystalline amino acids in the diet.

On the low end of the recommended range for nursery pigs, Ma et al. (2010b) suggested that the SID tryptophan:lysine requirement may be as low as 15 per cent for 11 to 22kg pigs. However, data from Nemechek et al. (2011a) demonstrates that 15 per cent SID tryptophan:lysine results in lower average daily feed intake and average daily gain than a ratio of 20 per cent. Guzik et al. (2002) estimated the SID tryptophan requirement for nursery pigs at 0.21, 0.20, and 0.18 per cent of the diet for 5.2 to 7.3kg, 6.3 to 10.2kg, and 10.3 to 15.7kg pigs, respectively.

Using the SID lysine levels suggested above, these ratios would all be less than 16 per cent of lysine. Jansman et al. (2010) found higher estimates for SID tryptophan for 10- to 20-kg pigs, both as a percentage of the diet (0.22 per cent) and as a ratio to lysine (21.5 per cent). In a review of 33 experiments, Susenbeth (2006) summarised that the SID tryptophan:lysine requirement is below 17.4 per cent and likely near 16.0 per cent. Susenbeth (2006) also concluded that feeding at 17 per cent would include a safety margin to cover most of biological variations and that the tryptophan:lysine ratio seemed to be unaffected by body weight, growth rate, lysine and protein concentration in the diet, or genetic improvement of the animals.

There is conflicting data on the impact of sanitary conditions on the tryptophan requirement of nursery pigs. Le Floc'h et al. (2007) found that the requirement to pigs in low sanitary conditions may have a higher response to tryptophan due to the increased requirement of the immune system. However, Frank et al (2010) found the opposite response with pigs having a greater response to increasing trp:lys in clean environment than in a dirty environment.

The optimal tryptophan:lys requirement for finishing pigs also is a contentious issue. In a first experiment with 25- to 40-kg pigs, Quant et al. (2007) estimated that the SID tryptophan:lysine requirement was 15.6 per cent for 25- to 40-kg pigs. In a second experiment, where they increased the levels of other essential amino acids in the diet, they found a higher estimate of 17 per cent (Quant et al, 2009). Guzik et al. (2003) did not suggest SID tryptophan:lysine ratios but reported SID tryptophan requirements of 0.18, 0.14, 0.11 and 0.11 for 30-, 50-, 70- and 90-kg pigs, respectively.

These levels were confirmed by Ma et al. (2010a). Guzik et al. (2004) suggested a Trp:Lys requirement of 19.5 per cent for pigs fed wheat/barley based diets and found that the ratio was not different whether the threonine:lysine ratio was 60 or 65 per cent. However, there was no response to tryptophan when the Thr:Lys ratio was 55 per cent, which is now known to be deficient. Kendall et al. (2007) found that the SID Try:Lys ratio was not greater than 17 per cent in late finishing (90 to 125kg) barrows. Hinson et al. (2010) conducted three experiments with 27 to 45kg, 67 to 85kg and 96 to 117kg pigs and found an optimal SID Try:Lys ratio of 16 per cent over the entire weight range.

The authors believe that feeding less than 16.5 per cent SID Try:Lys greatly increases the risk for poorer average daily feed intake and growth rate. However, more research is clearly needed to document the value of increasing the SID Try:Lys ratio from 16.5 to 20 per cent or greater. We also need better understanding of the potential interaction between health statuses and tryptophan and the role of other large neutral amino acids on the requirement for tryptophan. We also need a clearer understanding of the actual tryptophan levels in feed ingredients and how they are influenced by laboratory method used for the analysis. A ratio of 16.5 per cent using NRC (1998) ingredient nutrient values is equivalent to a ratio of 17 to 17.5 per cent using Brazilian ingredient nutrient values of Rostagno (2005).

Valine:Lysine Ratio

Although there are some differences in the estimates for the optimal Val:Lys ratio, the authors believe that much of the difference may be in the basal valine and lysine levels used in diet formulation. If you formulate the same corn-soybean meal diets with crystalline amino acids using NRC (1998) and INRA or Brazilian (Rostagno, 2005) amino acid values for the corn and soybean meal, a diet containing 65 per cent SID Val:Lys with NRC values will contain 68 per cent SID Val:Lys with INRA values and 69 per cent with values from Rostagno (2005). These differences are minor but may explain much of the difference between the Val:Lys estimates of 70 per cent from Europe (Barea et al., 2009a) compared with 65 per cent from the United States (Gaines et al., 2010)

Numerous valine trials have been published in the last 10 years. Mavromichalis et al. (2001) was one of the first publications to suggest that the valine requirement of nursery pigs was greater than the level suggested by NRC (1998). Their data suggested that 10- to 20-kg pigs required 12.5g of SID lysine per kg of gain. Gaines et al. (2010) found a similar requirement of 12.3g of SID lysine per kg of gain for 13- to 32-kg pigs. Using the requirement of 19g of SID lysine per kg of gain for nursery pigs found by several researchers and discussed earlier in this paper, a SID Lys:ME ratio of 66 per cent can be calculated, which is similar to the 65 per cent reported by Gaines et al. (2010) for 13- to 32-kg pigs and 65 to 67 per cent reported by Wiltafsky et al. (2009b) for eight- to 25-kg pigs. The 65 per cent SID valine:lysine ratio was recently confirmed by Nemechek et al. (2011a) using seven- to 12-kg pigs. A ratio of 65 per cent using NRC (1998) ingredient nutrient values is equivalent to a ratio of 69 per cent using Brazilian ingredient nutrient values of Rostagno (2005).

Isoleucine:Lysine Ratio

Similar to other amino acids, our understanding of the optimal ratios of isoleucine to lysine has increased greatly in the last 10 years. The main confusion in understanding the optimal isoleucine to lysine ratio is the interaction between isoleucine and other branch chain amino acids, in particular leucine.

Spray-dried blood cells have been used in several isoleucine studies to create a basal diet with a low isoleucine:lysine ratio (Parr et al., 2003, 2004; Kerr et al., 2004). The problem is that blood cells contain high leucine levels, which later were determined to increase the isoleucine:lysine recommendation. Subsequently, Fu et al. (2005a,b), Fu et al. (2006a,b,c), Dean et al. (2005) and Wiltafsky et al. (2009a) demonstrated that the SID isoleucine:lysine requirement was 60 per cent or greater in diets containing blood meal or blood cells and closer to 50 per cent for diets without high levels of blood cells. The requirement of 50 per cent or less for SID isoleucine:lysine when blood cells are not included in the diet was confirmed by Barea et al. (2009b) for 11- to 23-kg pigs. Lindemann et al. (2010) also found the SID isoleucine:lysine requirement to be between 48 and 52 per cent for average daily gain. Norgaard and Fernandez (2009) found that increasing the isoleucine:lysine ratio from 53 to 62 per cent did not influence performance of nine- to 22-kg pigs. Dean et al. (2005) also suggested that 50 per cent isoleucine:lysine ratio was adequate for 80 to 120 kg barrows fed corn-soybean meal diets. It appears that the SID isoleucine:lysine is less than 52 per cent for diets do not contain a protein source that provides excess leucine in relation to the isoleucine level, such as blood products. Caution is advised with all branch chain amino acids; however, as feeding as little as five per cent below the minimum ratio (ex. 45 versus 50 per cent of lysine) will greatly reduce feed intake and daily gain.

Non-essential Amino Acid Requirement

Although the order can vary with different dietary ingredient mixtures, typically the first five limiting amino acids for most practical diets are lysine, threonine, methionine, tryptophan and valine. However, formulating diets with high levels of synthetic amino acids to the optimal ratio for the first five limiting amino acids often has resulted in poorer performance than diets with higher levels of intact protein sources. Kendall et al. (2004) found that certain non-essential amino acids (e.g. glycine) were required in corn-soybean meal diets with high levels of synthetic lysine and that the nitrogen could not be provided by non-protein nitrogen. In a series of experiments, Powell et al. (2009a,b) and Southern et al. (2010) found that glycine and another amino acid to provide nitrogen were required in diets formulated to the fifth or sixth limiting amino acid in order to maintain feed efficiency at similar levels to control diets.

Another method to ensure that the diet contains enough nonessential amino acids is to place a maximum on the total lysine to total crude protein ratio in diet formulation. The biological basis for a lysine:CP ratio originates from the level of total lysine as a percentage of crude protein in muscle, which ranges from 6.5 to 7.5 per cent (NRC, 1998). Although an average lysine:CP ratios of 6.8 per cent is often cited, a higher lysine:CP ratio can be used in the diet because the lysine released during normal muscle protein breakdown is conserved and recycled with greater efficiency than other amino amino acids. Ratliff et al. (2005) suggested that the total Lys:CP ratio should not exceed 7.1 per cent. Nemechek et al (2011b) found that feed efficiency was only poorer when the total Lys:CP ratio exceeded 7.35 per cent. More research is clearly needed to continue to improve our understanding of non-essential amino acid needs of the pig.

Research Needs

The increased production and economic competitiveness of crystalline amino acids has greatly enhanced research efforts and our understanding of amino acid requirements in the past 10 years. Commercial additions of L-valine and L-tryptophan to diets are now a reality. As these amino acids continue to become more cost competitive relative to soybean meal, more research is needed to increase our understanding in a few key areas:

• Methods to ensure non-essential amino acids are met when formulating to the fifth or sixth limiting amino acid.
• Increased understanding of amino acid interactions such as interactions between branched chain or large neutral amino acids when one or more are fed in excess.
• Methods to evaluate these potential interactions in least cost diet formulation packages. Should each large neutral amino acid be formulated in a ratio relative to the other large neutral amino acids?
  • Because the isoleucine:lysine ratio increases when blood cells or blood meal are included in the diet, how can the cost of increasing isoleucine be factored into the least cost formulation to properly evaluate the cost of blood products.
  • Diets that contain high levels of corn protein (through corn gluten products or dried distillers grains with soluble (DDGS)) contain very high branch chain amino acid levels and low tryptophan levels. Some data indicates the requirement to tryptophan will be greatly increased in these situations; however, more research is required.
• Rapid and low cost methods to verify crystalline amino acid inclusion rates and distribution in diets. Reliance on small inclusion rate ingredients such as crystalline amino acids increases the importance of proper mixing and distribution.
• Validation and increased availability of in vitro methods to assess amino acid digestibility. Ex lysine in DDGS or other heat treated ingredients.

Conclusion

Over the past 10 years, our understanding of amino acid requirements has dramatically improved. Data suggest that the lysine requirement of modern lean genotypes increases over time with continued genetic improvement for lean gain.

Numerous studies have evaluated ratios of other amino acids to lysine, which has provided a framework for diet formulation.

As production of additional crystalline amino acids becomes economically feasible, we will not only see greater use of low-protein amino acid fortified diets, but diets fortified with greater numbers of crystalline amino acids.

Our challenges for the next 10 years will be to understand conditions when and why amino requirements or ratios change and to preserve this information in the public domain.

References

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Further Reading You can view other papers presented at the 2012 Kansas State University Swine Profitability Conference by clicking here.

January 2013