Allometric growth of protein, amino acids, fat and minerals in slow- and fast-growing young chickens

Allometric growth of body constituents and apparent efficiency of amino acid and nitrogen retention were examined in slow-growing laying-type cockerels (SG) and in fast-growing male broiler hybrids (FG) during the growth period from hatch to Day 22. The respective allometric coefficients for water, protein (N x 6.25), ash and fat in relation to body weight were 0.971, 1.080, 1.096 and 1.284 for SG chickens and 0.977, 1.099, 0.993, and 1.198 for FG chickens. The respective allometric coefficients describing the relationships of water, fat and ash weight with protein weight were 0.894, 1.014, and 1.186 for SG chickens and 0.893, 0.910, and 1.097 for FG chickens. High allometric coefficients for ash in both genotypes likely indicate the rapid growth of skeletal tissues which requires adequate mineral nutrition during this period of growth. The deposition of ash relative to protein was significantly higher (P < 0.05) in SG chickens thus suggesting that the relative growth of ash may be affected by genotype. Allometric coefficients relating amino acids to body protein were less than unity in most cases which indicates that an increasing amount of nonprotein N is deposited in the body with advancing age. Except for cysteine, the apparent efficiency of amino acid retention was lower in SG as compared to FG chickens. The high retention efficiency of cysteine in SG genotype was likely associated with the conversion of surplus methionine to cysteine, required for feather protein synthesis in laying-type birds at an early age.