Abstract

Immediately after weaning, the gut undergoes extensive remodeling in the adaption from a liquid to dry diet. This adaptation period presents significant challenges to the nutritionist in devising diets that assist the pig’s transition to dry feed while meeting cost expectations.

From an amino acid perspective, an important dietary attribute to minimise gut challenge and diet cost is to reduce the crude protein in the diet through the optimal inclusion of crystalline amino acids. Reducing crude protein decreases the quantity of fermentable protein entering the large intestine, which lowers post weaning diarrhoea. It also lowers the requirement for protein sources, such as soybean meal, that present immunological challenges to the gut and decreases inclusion of expensive specialty protein sources. Diets for early weaned pigs are often formulated to 1.65 to 1.7 per cent total lysine or greater. Formulation to lower lysine levels (1.35 per cent standardised ileal digestible (SID) or 1.5 per cent total lysine) decreases the requirement for expensive protein sources while having minimal effect on pig performance.

When using diets with lower SID lysine, levels of other amino acids relative to lysine are crucial. Suggested ratios relative to lysine are 58 per cent for methionine and cysteine, 62 per cent for threonine, 65 per cent for valine and 52 per cent for isoleucine (60 per cent if high levels of blood products are used). The tryptophan to lysine ratio continues to be debated with recommended ratios ranging from 16.5 to 20 per cent, depending on nutrient loadings and the particular experiment. Specific amino acids (ex. glycine and either glutamine or arginine) appear to meet the need for non-essential amino acids and have specific roles for gut development.

Gut Remodelling after Weaning

Heo et al. (2012) provide an excellent review of the gastrointestinal changes that occur in weaned pigs after weaning. As a brief summary of their review, the gastrointestinal changes at weaning include:

• Increased stomach gastric pH because of decreased acid secretion capacity and decreased lactic acid production due to lowered lactose intake. These changes may increase susceptibility of piglets to enteric infections at weaning
• Gastric motility is reduced which reduces stomach emptying. The lower motility may allow for pathogenic bacteria to proliferate in the intestinal tract.
• Decreased villus height (due to villus atrophy) and increased crypt depth (hypertrophy of crypt cells) in the small intestine - partly, but not entirely due to decreased intake at weaning - results in decreased digestive capability.
• Decreased lactase (and other pancreatic enzyme) secretion for first three to five days after weaning.
• Reduction in net absorption of fluid and electrolytes and malabsorption of nutrients in small intestine immediately after weaning. The low ileal digestive capability could lead to osmotic diarrhoea by increasing the quantity of nutrients presented to the hindgut.
• In the large intestine, crypt cell number is decreased, which lowers absorptive capacity of large intestine. This reduced absorptive capability can lead to diarrhoea when there is excessive fluid loss from the small intestine.

Minimising Challenge to the Gut

The changes in the gastrointestinal tract mean that the gut is compromised at weaning and time is required for the pig to fully recover their digestive and absorptive capacities. The goal of nutritionists is to help the pig transition through this phase without incurring excessive diet cost. Some ingredients and diet formulation techniques help the pig counteract some of the normal gut changes that occur at weaning. For example, adding lactose to the diet increases lactic acid production, which lowers gastric pH. Reducing the acid binding capacity of the diet decreases the requirement for acid to buffer the pH. Decreasing the soybean meal level in the diet decreases the challenge that an immature digestive tract has in dealing with legume proteins. High levels of soybean meal can cause transient hypersensitivity when the immune system reacts to an unfamiliar protein source (Engle, 1994).

Another method to decrease the challenge that the diet poses to the gastrointestinal system is to lower the crude protein level in the diet. Reducing the crude protein content lowers the need for soybean meal or other protein sources. Presenting the large intestine with a large quantity of undigested nitrogen appears to be a factor in post-weaning diarrhoea (Heo et al., 2012). Lowering the quantity of protein in the diet decreases the ammonia concentration in the small intestine (Bikker et al., 2006) and urea nitrogen and volatile fatty acids in the ileum (Nyachoti et al., 2006). It is thought that the decreased nitrogen concentrations are due to reduced protein fermentation by the bacteria (De Lange et al., 2010).

Until recently, lowering the crude protein level in the diet usually corresponded with reduced growth performance because the minimum requirement for the fourth, fifth, or sixth amino acids (often tryptophan, valine or isoleucine) or non-essential amino acids that have a role in gut development (arginine, glutamine, or glycine) were not met. Numerous recent research trials have demonstrated that performance can be maintained when the crude protein level in the diet is reduced by using crystalline amino acids to replace intact protein sources (Heo et al., 2009; Lordelo et al., 2008; Nemechek et al., 2011a). In order to lower the crude protein level in the diet, we need first to ensure that we are not formulating the diets above the lysine requirement. The requirements for other essential amino acids in relation to lysine must also be known to allow crude protein to be lowered to minimal levels.

Lysine Requirements for 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 five 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/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 25kg 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/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 lyine:ME ratio was approximately 3.4 to 3.6g/Mcal ME, while the optimal ratio was 3.9 to 4.2g/Mcal ME for the other genotype. However, when expressed relative to gain, the requirement was approximately 19.0g of SID lysine/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/kg of gain basis, the optimal level was again 19g of SID lysine/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.

Threonine:Lysine Ratio

The large difference between apparent and standardised 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 10- to 20-kg pigs. 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/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.

TSAA:Lysine Ratio

Considerable research has been conducted in recent years on the Total Sulphur Amino Acid (TSAA) requirement and individual requirements for 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) suggests 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 TSAA was 10.1g/kg gain or 54 per cent of lysine for 6- 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 8- 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.

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. (2010) suggested that the SID tryptophan:lysine requirement may be as low as 15 per cent for 11- to 22-kg pigs; however, data from Nemechek et al. (2011a) demonstrates that 15 per cent SID tryptophan:lysine results in lower average daily feed intake (ADFI) and average daily gain (ADG) 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 pigs weighing 5.2 to 7.3kg, 6.3 to 10.2kg and 10.3 to 15.7kg, 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.

Valine:Lysine Ratio

Although there are some differences in the estimates for the optimal valine:lysine 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 valine:lysine with NRC values will contain 68 per cent SID valine:lysine 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 valine:lysine 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/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 Val: SID Lys 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 8- 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. Excess leucine in the diet increases branch chain keto-dehydrogenase levels, which leads to catabolism of all branch chain amino acids, leading to increased requirement for isoleucine due to the increased breakdown of circulating levels.

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 ADG. Norgaard and Fernandez (2009) found that increasing the isoleucine:lysine ratio from 53 to 62 per cent did not influence performance of 9- to 22-kg pigs.

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 (Ex. 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 non-essential amino acids is to place a maximum on the total lysine to total crude protein (CP) 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 lysine: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.

Non-essential amino acids appear to play a particularly important role immediately after weaning due to their high requirement for intestinal growth. Glutamine serves as a primary fuel for the intestinal mucosa. Glutamine and glyine stimulate polyamine sysnthesis. Arginine is the precursor for polyamines and nitric oxide which is important for regulation of intestinal blood flow and migration of intestinal epithelial cells. Numerous other roles of the non-essential amino acids are reviewd by Wu (2011).

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Further Reading

You can view other papers presented at the 2013 Kansas Swine Profitability Conference by clicking here.

May 2013