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Starter Pig Amino Acid Requirements to
Gut Health Concerns
31 May 2013
Formulation of starter diets for weaned piglets to lower-than-usual lysine decreases the requirement for expensive protein sources while having minimal effect on performance but it is crucial to pay attention to the levels of other amino acids relative to lysine, according to the K-State Swine Nutrition team of Mike Tokach, Bob Goodband, Joel deRouchey, Steve Dritz and Jim Nelssen. They were addressing the 2013 Kansas Swine Profitability Conference.
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 3 to 5
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 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 % 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 % SID lysine or 18.9g of lysine per
kg of gain for 6 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 % 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 %; 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 % at 15kg to 65% 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 64-66% for 10 - 20kg pigs. James et al. (2003) also found the optimal
threonine:lysine ratio to be 60 to % for 10-20kg 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% of lysine for growth and 67% 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% of the TSAA ratio (NRC = 48% on weight
basis); however, recent data (Gillis et al., 2007) suggests that methionine may
need to be slightly greater (55% on weight basis; 50% 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% of lysine for 6—12kg pigs. Gaines
et al. (2005) found a slightly higher ratio of 57—61%, depending on the
response criteria and method of assessing the breakpoint with 8—26kg pigs. Yi
et al. (2006) found a similar TSAA:lysine ratio of 58% for optimal ADG with 12—24kg
pigs. In a series of experiments, Kansas State University researchers found a
similar range of SID TSAA:lysine ratios of 57 to 60% for 10—20kg 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).