Ratio Estimates for Broiler Chicks

Data in Table 13.1 show, with the exception of NRC (1994) estimates, empirical

© CAB International 2003. Amino Acids in Animal Nutrition, 2nd edition (ed. J.P.F. D'Mello)

Table 13.1. Ideal ratios (%) of essential amino acids for broiler chicks as proposed by various

investigators.

Amino acid

Baker (1997)a

NRC (1994)b

Mack etal. (1999)°

Baker etal. (2QQ2)d

Lysine

100

100

100

100

Methionine

36

42

NDe

ND

Cystine

36

33

ND

ND

SAAf

72

75

70

ND

Threonine

67

67

59

56

Valine

77

75

76

78

Isoleucine

67

67

66

61

Leucine

109

100

ND

ND

Tryptophan

16

17

17

17

Arginine

105

104

104

105

Histidine

35

29

ND

ND

Phe + Tyr

105

112

ND

ND

Calculated on a true digestible basis based on numerous requirement studies using crystalline amino acid diets fed to chicks during the second and third week of life (see also Baker and Han, 1994). bRatio of requirements on a total amino acid basis for chicks fed a maize-soybean meal diet from 0 to 3 weeks of age, with the lysine requirement corrected to 12 g kg-1 (instead of 10 g kg-1). Recalculation of data from Mack etal. (1999) using the broken-line true digestible lysine requirement estimate (9.2 g kg-1) for maximal feed efficiency as a reference point. Data were obtained from chicks fed low protein (172 g CP kg 1) maize-soybean meal diets from 20 to 40 days of age. dBased on true digestible requirement ratios presented herein. The broken-line requirement was taken to be the higher of individual estimates for gain and gain/feed ratio. 8ND, not determined. fSAA, sulphur amino acids.

Calculated on a true digestible basis based on numerous requirement studies using crystalline amino acid diets fed to chicks during the second and third week of life (see also Baker and Han, 1994). bRatio of requirements on a total amino acid basis for chicks fed a maize-soybean meal diet from 0 to 3 weeks of age, with the lysine requirement corrected to 12 g kg-1 (instead of 10 g kg-1). Recalculation of data from Mack etal. (1999) using the broken-line true digestible lysine requirement estimate (9.2 g kg-1) for maximal feed efficiency as a reference point. Data were obtained from chicks fed low protein (172 g CP kg 1) maize-soybean meal diets from 20 to 40 days of age. dBased on true digestible requirement ratios presented herein. The broken-line requirement was taken to be the higher of individual estimates for gain and gain/feed ratio. 8ND, not determined. fSAA, sulphur amino acids.

estimates of ideal AA ratios. Our original estimates (Baker and Han, 1994) were modified only slightly (Baker, 1997), and these estimates are compared to estimates of NRC (1994), many of which were based on the Han and Baker (1994) data, and to recent estimates made by Mack et al. (1999), as recalculated, and to new information that is presented in this review. The fact that different stages of broiler growth are represented would seem to present problems in comparing the ratio estimates shown in Table 13.1. However, this is based on the assumption that ideal ratios (relative to lysine) for maintenance of certain amino acids (e.g. threonine, tryptophan, sulphur amino acids (SAA)) may greatly exceed the ideal ratio of these same amino acids for protein accretion per se. Recent evidence, however, showed that the maintenance requirement for lysine was much higher than had been previously assumed (Edwards and Baker, 1999). This, then, suggested that the ideal ratios for tryptophan and SAA may not change at all during the 6-, 8- or 10-week growth period of a broiler chick

(Emmert and Baker, 1997), and also that the threonine :lysine ratio may increase only slightly with advancing age.

Legitimate estimates of ideal AA ratios have several prerequisites: (i) the same basal diet, same sex and strain of chicks, and same assay period should be used in all requirement bioassays, (ii) true digestibility of AA in the basal diet must be known, (iii) clear-cut graded responses should occur to the limiting AA being studied, and (iv) objective and consistent curve-fitting procedures should be used to predict the requirement for each AA being investigated. For the most part, these criteria were met in the study by Mack et al. (1999). However, the low protein maize-soybean meal diet (172 g kg-1 CP) used in their study did not allow good and clear-cut graded responses to some of the AA (e.g. tryptophan and isoleucine) they investigated. Also, they observed as have we (Han and Baker, 1991, 1993, 1994; Baker et al., 2002) that lysine requirement estimates for maximal feed efficiency consistently exceed those for maximal weight gain. For most AA, the requirement for maximal weight gain is very similar to the requirement for maximal feed efficiency (Mack et al., 1999; Baker et al., 2002). However, SAA requirements may be similar to lysine in that gain/feed requirements often exceed weight gain requirements (Schutte and Pack, 1995; Baker et al., 1996; Mack et al., 1999). Because, Mack et al. (1999) provided broken-line requirement estimates for both gain and feed efficiency for only lysine (only gain requirements were given for the other AA), the recalculated ratios for the Mack et al. (1999) data are based on the broken-line requirement estimates for maximal gain/feed for lysine, but for maximal weight gain for the other AA. In their paper, broken-line requirement estimates for maximal gain (for all AA) were used to estimate ratios, and this is likely to have overestimated the ideal ratios. Indeed, using gain/feed requirements for lysine and gain requirements for the other AA, despite questionable responses to tryptophan and isoleucine, resulted in ratio estimates (with the exception of threonine) that were in good agreement with our original estimates (Baker, 1997). The new empirical estimates from our laboratory (Baker et al., 2002) shown in Table 13.1 will form the basis for the discussion that follows.

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