Dietary Crude Protein in Pigs

Impact of Dietary Crude Protein on Metabolic Utilization of Energy in Pigs

Author/s :

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Abstract.


Availability of free amino acids allows to reduce the dietary crude protein (CP) level without any marked detrimental effect on performance of piglets, growing pigs or lactating sows, as far as the supply and the balance of essential amino acids meets the requirements. A major consequence of dietary CP reduction is a concomitant decrease of urinary nitrogen excretion and therefore an attenuation of pollution from the manure; the reduction is about 10% per one percent reduction of dietary CP content. With regard to energy utilization, reduction of dietary CP is first associated with less urinary energy (0.040 MJ/kg feed per one percent CP) and secondly with an improved efficiency of utilization of metabolizable energy for net energy and a subsequent lowered heat production. This variation is due to a higher efficiency of starch energy (82 vs 60% for protein). In order to take into account these changes in metabolic utilization of energy related to changes in dietary CP, it is necessary to evaluate feeds on a net energy basis. It is also recommended to use an appropriate protein evaluation system (digestible amino acids). Finally, the reduction of heat production with low CP diets helps the pig to better tolerate a heat stress due to exposure at high ambient temperatures.

The essential AA can be totally provided by proteins provided by the ingredients. However, the balance between essential amino acids in proteins, especially those from plant origin, is not adequate to meet the animals requirements. The situation is the most critical for cereals which represent the predominant protein fraction in most pig diets but with low contents of essential AA such as lysine, sulfur amino acids, threonine or tryptophan in their proteins; these AA are therefore considered as the first limiting ones in most pig diets. According to that situation, proteins are then provided in excess in order to meet the nutritional constraints for these first limiting essential AA. However, these first limiting AA can be industrially synthesized and directly incorporated in the diets as free amino acids. The crude protein (CP) level can then be lowered and a major consequence of using such diets is a reduction of urinary nitrogen excretion (Canh et al., 1998; Le Bellego et al., 2001).

The present paper will consider nutritional aspects related to the utilization of diets with variable CP levels and levels of amino acids supplementation; emphasis will be given to the importance of adequate and efficient protein and energy evaluation systems for implementing such nutritional strategies.

Dietary crude protein level and performance of growing pigs and lactating sows.

Literature indicates that the reduction of dietary CP level from an optimal level (i.e., control diet supplying essential AA at or above requirements) depresses performance of both growing pigs or lactating sows when no free AA are supplemented (Noblet et al., 1985; Castell et al., 1994). This suggests that some essential AA become progressively limiting. Supplementation with free AA improves performance (daily BW gain) which become equivalent to those obtained with the control diet (Homb and Matre, 1989; Kerr et al., 1995; Figueroa et al., 2002). The first studies on this topic concerned mostly the effects of lysine and methionine supplementation. Availabilities of threonine and tryptophan allowed further reductions of dietary CP and performance were maintained at levels similar to those obtained with the control diet (Le Bellego et al., 2002). Data presented in tables 1 and 2 indicate that a further reduction of dietary CP which implies in addition to lysine, methionine, threonine and tryptophan, an isoleucine and valine supplementation in order to meet the AA requirements of the animals, does not affect the response of piglets, growing pigs or lactating sows.

Literature data also suggest that the supply of non essential AA is very seldom limiting at these relatively low dietary CP levels. Overall, these studies indicate that, from a technical point of view, performance of pigs can be maintained when the dietary CP level is reduced as far as the supply of all essential AA is maintained at or above the animals requirements (Lenis et al., 1999). Furthermore, the available literature studies indicate that for wheat or corn and soybean meal diets, the supply of non essential AA would not be limiting when free lysine, methionine, threonine, tryptophan, isoleucine and valine are supplemented. However, it should be mentioned that isoleucine and valine are nowadays far too expensive to be used in pig diets, which means that under practical conditions, supply of non essential AA is never limiting.

A major consequence or interest of using low protein diets is the reduction of nitrogen excretion, mainly in the urine (Le Bellego et al., 2001; Figueroa et al., 2002). This nutritional strategy then represents an efficient solution to reduce the nitrogen pollution in the environment originating from pig production. Data presented in tables 1 and 2 illustrate reductions of nitrogen waste equivalent to 30-40% of what is measured with conventional diets. These data, in agreement with literature, suggest a 8-10% reduction of nitrogen waste per one percent reduction of the dietary CP level. Other advantages of low CP diets such as improvement of health in early weaned piglets (less digestive disorders) or reduction of water intake have also been shown; they will not be considered in the present paper.


Table 1: Effect of reduction of dietary crude protein level on performance of piglets and growing pigs.

Trial 11
Trial 22
Crude protein, %
20.4
16.9
20.1-17.5
15.6-13.3
Digestible lysine, g/MJ NE
1.01
1.02
0.85-0.70
0.85-0.70
BW range, kg
11.8 to 26.8
27 to 100
Voluntary feed intake, g/d
1039
1048
2750a
2575b
Daily BW gain, g
661
663
1098
1057
MJ NE/kg BW gain
16.3
16.7
25.7
25.7
Carcass lean, %
58.7
59.7
Nitrogen waste, kg/pig
0.36a
0.24b
3.65a
2.23b

1 From Le Bellego and Noblet (2002); corn, barley and soybean meal based diets; the high CP diet was moderately supplemented with HCl-lysine (0.20%), methionine (0.08%) and threonine (0.09%) and the low CP diet was highly supplemented with HCl-lysine (0.60%), methionine (0.20%), threonine (0.27%), tryptophan (0.07%), isoleucine (0.12%) and valine (0.19%).

2 From Le Bellego et al. (2002); wheat, corn and soybean meal based diets; the two values for CP and lysine content correspond to diet characteristics in the growing and the finishing periods, respectively; only the low protein diets were supplemented with HCl-lysine (0.43-0.41%), methionine (0.10-0.07%), threonine (0.17-0.15%), tryptophan (0.04-0.04%), isoleucine (0.03-0.04%) and valine (0.08-0.07%).

a, b Values are significantly different (P<0.05) within each trial if different exponents are indicated.


Table 2: Effect of reduction of dietary crude protein level on performance of lactating sows1

Crude protein, % 17.6 14.2
Digestible lysine, g/MJ NE 0.82 0.82
Voluntary feed intake, g/d 6707 6507
NE intake, MJ/d 69.7 68.8
Water intake, l/d 25.5 25.4
BW after farrowing, kg 260 258
Lactation BW loss, kg 16 15
Lactation P2 loss, mm 3.7 3.5
Litter size 10.8 10.6
Litter weight gain, g/d 2933 2872
Nitrogen waste, kg/sow 4.37a 3.10b

1 From Renaudeau et al. (2001); 28-d lactation; wheat, corn and soybean meal based diets; the high CP diet was moderately supplemented with HCl-lysine (0.17%), methionine (0.04%) and threonine (0.09%) and the low CP diet was highly supplemented with HCl-lysine (0.55%), methionine (0.15%), threonine (0.25%), tryptophan (0.06%), isoleucine (0.13%) and valine (0.22%)

a, b Values are significantly different (P<0.05) if different exponents are indicated.


Dietary crude protein level and metabolic utilization of energy.

The reduction of nitrogen supply to the pig under constant or above requirements essential AA supplies is associated to an equivalent reduction of fecal + urinary nitrogen excretion. It has also been stated that most of the reduction occurs at the urinary level. From a compilation of their studies, Noblet et al. (2002) have shown that urinary energy (Euri) is linearly related to urinary nitrogen, the latter being directly dependent on nitrogen intake. The following equation has been proposed for growing pigs:

Euri, MJ/kg feed DM = 0.19 + 0.0310 x N urines (g/kg feed DM).

This means that a 1% increase of the dietary CP level will increase the urinary N by 1.45 g per kg of feed DM if we assume a 90% digestibility of CP and a complete catabolism of the excess CP supply. This is equivalent to about 0.045 MJ of urinary energy per kg of feed DM (i.e., 0.040 MJ per 1% variation of dietary CP in as fed feed). Similar figures can be calculated from the study of Le Bellego et al. (2001). Values presented in Table 3 when DE and ME values are compared or the change in ME/DE ratio in Table 4 also illustrate that situation. It is also reflected in the following prediction equation of the ME/DE ratio in compound feeds (Le Goff and Noblet, 2001):

ME/DE, % = 99.8 – 0.19 x CP, %

Table 3. Energy value of starch, crude protein and fat according to energy systems (adapted from Noblet et al., 1994)

  Starch
Crude protein1
Crude fat1
Energy values, kJ/g1      
Digestible energy 17.5(100) 20.6(118) 35.3(202)
Metabolizable energy 17.5(100) 18.0(103) 35.3(202)
Net energy 14.4(100) 10.2(71) 31.5(219)
Heat production, kJ/g 3.1 7.8 3.8

1 Between brackets, energy values as % of starch; crude protein and crude fat are assumed to be 90% digestible; starch is 100% digestible.

In their studies for establishment of a net energy (NE) system, Noblet et al. (1994) demonstrated that the efficiency of ME utilization for NE (kg) is higher for starch (82%) and fat (90%) than for CP (60%). The following prediction equation of kg was obtained (Noblet et al., 1994):

kg= 74.7 + 0.36 x Fat, % + 0.09 x Starch – 0.23 x CP – 0.26 x ADF

In which the chemical characteristics of the diet are expressed as % of the feed dry matter. These results have been confirmed by recent data of van Milgen et al. (2001). That is also illustrated in Table 3 that shows that the ME value of starch and CP are equivalent while the NE value is 30% lower for dietary CP than for starch. As a consequence, the heat production is lowered with low CP diets and energy gain is higher for a constant ME intake (Table 4); the same conclusions are obtained when dietary CP is partially replaced by fat (Noblet et al., 2002). The reasons for this lower efficiency of dietary CP have not been totally clarified. However, there is some evidence that body protein turn over in pigs and the associated energy cost are directly dependent on the dietary CP level (Roth et al., 1999). In addition, the synthesis, excretion and more generally metabolism of urea associated with the excretion of N in excess, represents a non negligible energy cost to the animal.

The reduction of dietary CP is rather equivalent to the partial replacement of CP by starch (and/or fat) with a subsequent increase of the overall efficiency of ME utilization and a lowered urinary energy excretion. Consequently, at a given DE or ME intake, net energy supply and therefore energy gain are higher for low CP diets than for conventional diets. On the other hand, if we assume that NE is a closer estimate of the "true" energy content than DE or ME, conventional CP diets are overestimated and/or low CP diets are underestimated for their energy value when estimated on DE or ME bases. These consequences are illustrated when the relative DE, ME and NE values of ingredients commonly used in pig feeds are calculated (Table 5); obviously, high CP ingredients are overestimated and high starch or high fat ingredients are underestimated for their "true" energy value when it is estimated according to their DE or ME value. This also explains that less DE or ME is required with a low CP diet than with a conventional CP diet for getting the same level of performance; when formulating on a NE basis, the energy required per kg of BW gain is independent on diet composition (Table 6). On the other hand, the feeding of similar amounts of DE or ME in low CP diets will result in more NE or "true" energy available for the pigs with subsequent increased carcass fatness, as previously mentioned (Kerr et al., 1995; Cromwell et al., 1996). The description of NE systems and their advantages over DE or ME systems have been previously described (Noblet et al., 1994; Noblet, 2001). These basic concepts for energy evaluation of pig feeds have been used in recently published feeding tables (Sauvant et al., 2002).


Table 4: Energy utilization of low protein diets.

 
Trial 11
Trial 22
Crude protein, %
17.4
13.9
21.9-17.4
17.2-12.7
Digestible lysine, g/MJ NE
0.76
0.76
1.05-0.72
1.05-0.72
Energy balance, MJ/kg BW0.60
ME intake
2.46
2.46
2.57
2.57
Heat production
1.42a
1.37b
1.40a
1.34b
Energy retained
1.05a
1.09b
1.17a
1.23b
ME/DE, %
95.5a
96.7b
95.7a
96.7b
NE/ME, %
73.2a
75.3b
73.9a
75.9b


1 From Le Bellego et al. (2001b) and Noblet et al. (2001); 65-kg pigs; wheat, corn and soybean meal based diets; the low protein diet was supplemented with HCl-lysine (0.43%), methionine (0.11%), threonine (0.16%), tryptophan (0.05%), isoleucine (0.04%) and valine (0.09%); indirect calorimetry method was used for measuring heat production;

2 From Noblet et al. (2003); in 25, 55 and 85 kg pigs; wheat, corn and soybean meal based diets; indirect calorimetry method was used for measuring heat production. Values for CP and lysine levels are given for the 25 and 85 kg pigs; values at 55 kg were intermediary.

a, b Values are significantly different (P<0.05) if different exponents are indicated (within trial)



Table 5: Digestible, metabolizable and net energy value of some ingredients for pig feeds1

 
DE
ME
NE
ME/DE, %
NE/ME, %
Fat
243
252
300
99.4
90
Corn
103
105
112
97.6
80
Wheat
101
102
106
97.0
78
Diet
100
100
100
95.9
75
Pea
101
100
98
95.2
73
Wheat bran
68
67
63
94.6
71
Soybean meal
107
102
82
91.2
60
Rapeseed meal
84
80
64
91.7
60
Amino acids mixture
148
142
146
92.2
77

1 From Noblet et al. (1994) and Sauvant et al. (2002); expressed as percentages of the energy value of a diet containing 67.4% wheat, 16% soybean meal, 2.5% fat, 5% wheat bran, 5% peas, 4% minerals and vitamins and 0.10% of HCl-lysine; the so-called amino acids mixture contains 50% HCl-lysine, 25% threonine and 25% methionine.



Table 6: Performance of growing-finishing pigs; effect of energy system1


Crude protein, % 20.1 - 17.5 15.6 - 13.3
Digestible lysine, g/MJ NE 0.85 - 0.70 0.85 - 0.70
DE intake, MJ/d 38.9a 37.3b
ME intake, MJ/d 37.1a 36.1b
NE intake, MJ/d 27.6 27.5

1 From Le Bellego et al. (2002); see table 1 (trial 2) for information on diets and animals; energy intakes were adjusted for similar daily BW gain (1078 g/d) and similar carcass composition at slaughter (24.8% adipose tissues)

a, b Values are significantly different (P<0.05) if different exponents are indicated



Dietary crude protein level and protein evaluation of feeds.

Similar to energy, the relative protein value of ingredients is dependent on the evaluation system. Over the last decade, the evaluation based on crude AA contents has been progressively replaced by an evaluation taking into account the AA losses occurring during digestion. One widely accepted concept is the ileal digestibility of AA corrected for endogenous AA losses or standardized ileal digestibility (SID; Sève, 1994; AmiPig, 2000, Noblet et al., 2002). The SID varies between AA and, for one given AA, between ingredients; it is the highest (#100%) for free AA. If we assume that SID AA content provides a closer estimate of the "true" protein than crude AA content, the value of free AA is clearly underestimated when expressed on a crude basis while the value of most ingredients (except soybean meal) is overestimated (table 7).

Table 7: Total and digestible amino acid content of some ingredients for pig feeds1


DE
Total
Digestible
Lysine Threonine Lysine Threonine
Corn
28
53
26
52
Wheat
36
56
33
55
Diet
100
100
100
100
Pea
176
137
165
123
Wheat bran
68
81
53
63
Soybean meal
340
320
353
335
Rapeseed meal
212
253
180
223
Amino acids mixture
4581
4363
5186
5158

1 From AmiPig (2000) and Sauvant et et al. (2002); expressed as percentages of the lysine or threonine contents of a diet containing 67.4% wheat, 16% soybean meal, 2.5% fat, 5% wheat bran, 5% peas, 4% minerals and vitamins and 0.10% of HCl-lysine; the so-called amino acids mixture contains 50% HCl-lysine, 25% threonine and 25% methionine;


Dietary crude protein level and heat stress in pigs.

Under heat stress, growing pigs (Quiniou et al., 2000, 2001) and lactating sows (Quiniou and Noblet, 1999; Renaudeau et al., 2001) reduce their feed intake in order to lower their heat production. Exposure to temperatures above 25°C can be considered as a heat stress situation, especially for lactating sows. The effects are accentuated when the high ambient temperature is combined with a high relative humidity (Renaudeau et al., 2003). As illustrated in tables 3 and 4, the partial replacement of dietary CP by starch is associated with a reduced heat production of the pig. Increased dietary fat content has similar effects on heat production and the effects of dietary CP reduction and fat addition are additive (Noblet et al., 2002). It could then be hypothesized that low CP diets (and/or fat supplemented diets) would be better tolerated by heat stressed pigs or the effect of heat stress on their energy intake would be attenuated. These hypotheses have been tested in recent studies conducted in growing pigs (Le Bellego et al., 2002) and lactating sows (Renaudeau et al., 2002) exposed to control or high ambient temperatures. The most important results of their studies are presented in Table 8. The interest of low heat increment diets (i.e., low CP and/or high fat) would be less important in growing – finishing pigs in connection with a direct effect of heat stress on protein deposition rate (Le Bellego et al., 2002). The interaction observed in lactating sows concerns firstly the sow body reserves and not the sow milk production.


Table 8. Effects of dietary crude protein level and high ambient temperature on performance of growing pigs and lactating sows (adapted from Le Bellego et al., 2002 and Renaudeau et al., 2002)

Temperature1
Control
High
Diet1
NP
LP
NP
LP
Growing-finishing pigs (27 to 100 kg)        
NE intake, MJ/d 28.1 27.0 23.2 23.6
BW gain, g/d 1098 1057 930 917
Feed cost, MJ NE/kg gain 25.7 25.7 25.0 25.7
Carcass fat content 25.7 24.1 22.0 23.1
Lactating sows (28-d lactation)        
NE intake, MJ/d 69.7 68.8 36.9 43.5
Lactation BW loss, kg 16 15 41 29
Litter weight gain, kg/d 2.93 2.87 2.15 2.24

1 Control temperature was 22 and 20°C in growing pigs and lactating sows, respectively; the high temperature was 29°C at both stages. The difference in CP level between the normal protein (NP) and low protein (LP) diets was 4.5 and 3.5% for growing pigs and lactating sows, respectively.


Conclusions.

This brief review indicates that low protein diets supplemented with free AA in order to meet the pig requirements allow performance equivalent to those obtained with conventional CP levels (with no or quite moderate AA supplementation). Nitrogen excretion is then markedly reduced. However, the implementation of such strategies requires the utilization of adequate or accurate energy and protein evaluation systems. More generally, refinement of pig nutrition is based on more efficient evaluation systems of both requirements of animals and nutritional value of feeds. From that point of view, the NE and ileal AA digestibility concepts are the most advanced ones for estimating feed value accurately. Recently published feeding tables (Sauvant et al., 2002) should help to implement these concepts.
The effect of dietary CP reduction on feed intake of ad libitum pigs remains unclear. Under some circumstances, pigs overconsume feed or at least net energy with a subsequent increased carcass fatness. Other studies suggest a regulation of feed intake according to its NE content and growth and carcass performance are then totally equivalent for both types of diets. Responses appear to be influenced by the type of ingredients (corn vs barley or wheat). The AA balance or the excess of some AA might also be implicated. In any case, further studies are required to understand the mechanisms involved in feed intake regulation in connection with quantity and quality of AA supplies.

Jean Noblet
INRA – UMR VP
35590 Saint Gilles (France)




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