Avian tibial dyschondroplasia: The interaction of genetic selection and dietary 1,25 ‐dihydroxycholecalciferol

Dietary supplementation with glycosaminoglycans reduces locomotor problems in broiler chickens

This study aimed to assess the influence of glycosaminoglycan (chondroitin and glucosamine sulfates) supplementation in the diet on the performance and incidence of locomotor problems in broiler chickens. A completely randomized design was carried out in a 3 × 3 factorial scheme (3 levels of chondroitin sulfate -0, 0.05, and 0.10%; and 3 levels of glucosamine sulfate -0, 0.15, and 0.30%). Each treatment was composed of 6 replications of 30 broilers each. The performance of broilers (average weight, weight gain, feed intake, feed conversion, and productive viability) was assessed at 7, 21, 35, and 42 d of age, whereas the gait score, valgus and varus deviations, femoral degeneration, and tibial dyschondroplasia were assessed at 21 and 42 d of age. Increasing levels of glucosamine sulfate inclusion linearly increased the weight gain from 1 to 35 and from 1 to 42 d of age of broilers (P = 0.047 and P = 0.039, respectively), frequency of broilers with no femoral degeneration in the right and left femurs, and the proliferating cartilage area of proximal epiphysis at 42 d of age (P = 0.014, P < 0.0001, and P = 0.028, respectively). The increasing inclusion of chondroitin and glucosamine sulfates led to an increase in the frequency of broilers on the gait score scale 0 (P = 0.007 and P = 0.0001, respectively) and frequency of broilers with no valgus and varus deviations (P = 0.014 and P = 0.0002, respectively) also at 42 d of age. Thus, chondroitin and glucosamine sulfates can be used in the diet of broiler chickens to reduce their locomotor problems.

Bone circulatory disturbances in the development of spontaneous bacterial chondronecrosis with osteomyelitis: a translational model for the pathogenesis of femoral head necrosis

This review provides a comprehensive overview of the vascularization of the avian growth plate and its subsequent role in the pathogenesis of bacterial chondronecrosis with osteomyelitis (BCO, femoral head necrosis). BCO sporadically causes high incidences of lameness in rapidly growing broiler (meat-type) chickens. BCO is believed to be initiated by micro-trauma to poorly mineralized columns of cartilage cells in the proximal growth plates of the leg bones, followed by colonization by hematogenously distributed opportunistic bacteria. Inadequate blood flow to the growth plate, vascular occlusion, and structural limitations of the microvasculature all have been implicated in the pathogenesis of BCO. Treatment strategies have been difficult to investigate because under normal conditions the incidence of BCO typically is low and sporadic. Rearing broilers on wire flooring triggers the spontaneous development of high incidences of lameness attributable to pathognomonic BCO lesions. Wire flooring imposes persistent footing instability and is thought to accelerate the development of BCO by amplifying the torque and shear stress imposed on susceptible leg joints. Wire flooring per se also constitutes a significant chronic stressor that promotes bacterial proliferation attributed to stress-mediated immunosuppression. Indeed, dexamethasone-mediated immunosuppression causes broilers to develop lameness primarily associated with avascular necrosis and BCO. Prophylactic probiotic administration consistently reduces the incidence of lameness in broilers reared on wire flooring, presumably by reducing bacterial translocation from the gastrointestinal tract that likely contributes to hematogenous infection of the leg bones. The pathogenesis of BCO in broilers is directly relevant to osteomyelitis in growing children, as well as to avascular femoral head necrosis in adults. Our new model for reliably triggering spontaneous osteomyelitis in large numbers of animals represents an important opportunity to conduct translational research focused on developing effective prophylactic and therapeutic treatments.

Effect of temperature during the incubation period on tibial growth plate chondrocyte differentiation and the incidence of tibial dyschondroplasia

Tibial dyschondroplasia (TD) is one of the most prevalent skeletal abnormalities in avian species, causing enormous economic losses and major animal welfare problems. Irregular cell differentiation of the chondrocytes that populate the growth plate has been hypothesized to be involved in the etiology of the disease. We evaluated the effect of incubation temperature at various stages of embryo development and bone formation on growth plate chondrocyte differentiation and the incidence of TD. Eggs were incubated either at a control temperature of 37.8 degrees C, or at 36.9 or 39 degrees C, each for 6 h/ d, during early (0 to 8 d) or late (10 to 18 d) embryo development. At 14 d of incubation and at hatch, tibias were collected and weighed, and their ash and calcium contents were determined. Growth plate chondrocyte differentiation was evaluated by alkaline phosphatase activity and collagen type II and osteopontin gene expression. In addition, the level of the heat-shock protein 90 (Hsp90) was evaluated by immunohistochemistry. The rest of the chicks were raised to 49 d and the incidence of TD was recorded. The incidence of TD increased only when the temperature was altered at the early stages of embryo development, and it was correlated with an increase in tibia ash but not with tibia weight or calcium content. Moreover, increased TD incidence was correlated with delayed chondrocyte differentiation. Early changes in incubation temperature caused an increase in the level of Hsp90 in articular and differentiated chondrocytes of the hypertrophic zone and in the numbers of distinct undifferentiated chondrocytes arranged in columns in the proliferative zone of the growth plate. In summary, the early stages of embryo development and bone formation are of utmost importantance for appropriate growth plate chondrocyte differentiation, and any temperature deviation will increase the subsequent incidence of TD. The increase in TD incidence is probably the result of delayed Hsp90-driven chondrocyte differentiation, supporting the hypothesis that TD is the result of abnormal chondrocyte differentiation.

The Effect of Maternal Dietary Vitamin D3 Supplementation on Performance and Tibial Dyschondroplasia of Broiler Chicks

A series of experiments was conducted to investigate the effects of maternal dietary vitamin D3 supplementation at 4 different times during the laying cycle, on the performance and bone quality of broiler chicks fed a diet that induced tibial dyschondroplasia (TD) or an adequate diet. Ross x Ross broiler breeder hens were fed a corn-soy diet with various levels of vitamin D3 from 24 to 66 wk of age. Eggs were collected at 39, 44, 53, and 64 wk of age and hatched. Chicks from hens fed 250 IU of D3/kg (low maternal D3 or LMD3) and 2,000 IU of D3/ kg (high maternal D3 or HMD3) levels were placed in battery brooders and fed the diets from 0 to 16 d. At 16 d, the chicks were weighed and killed; the left tibias were used for bone ash determinations, and the right tibias were used to score the incidence and severity of TD (0, 1, 2, or 3, where 3 is the most severe). Body weight gain and feed intake were significantly lower for the LMD3 chicks at wk 44 and 64, although there was no difference in weight at hatch. For the first 2 hatches (wk 39 and 44), the LMD3 and HMD3 chicks demonstrated high average TD scores (2.03 and 1.57 vs. 2.05 and 1.75 for the LMD3 vs. HMD3 chicks, respectively) and high average incidences of severe TD (50 and 35% vs. 45 and 34% for LMD3 vs. HMD3 levels, respectively). However, results from the last 2 hatches (wk 53 and 64) showed that HMD3 chicks, compared with LMD3 chicks, had reduced average TD scores (1.39 and 1.47 vs. 1.01 and 0.44 for LMD3 vs. HMD3 levels, respectively) and severe TD incidence (36 and 40% vs. 17 and 8% for the LMD3 vs. HMD3 levels, respectively). In this experiment, as egg production declined toward the end of the laying cycle, hens fed the HMD3 might have been able to deposit sufficient quantities of vitamin D3 in the egg to maintain excellent body weight gain at 16 d of age and reduce the incidence and severity of TD. Hens fed the LMD3 diet were unable to produce similar improvements.

Production and growth related disorders and other metabolic diseases of poultry – A review

In humans, metabolic complaints may be associated with a failure in one of the body hormone or enzyme systems, a storage disease related to lack of metabolism of secretory products because of the lack of production of a specific enzyme, or the breakdown or reduced activity of some metabolic function. Some of these disorders also occur in poultry, as do other important conditions such as those associated with increased metabolism, rapid growth or high egg production that result in the failure of a body system because of the increased work-load on an organ or system. These make up the largest group of poultry diseases classified as metabolic disorders and cause more economic loss than infectious agents. Poultry metabolic diseases occur primarily in two body systems: (1) cardiovascular ailments, which in broiler chickens and turkeys are responsible for a major portion of the flock mortality; (2) musculoskeletal disorders, which account for less mortality, but in broilers and turkeys slow down growth (thereby reducing profit), and cause lameness, which remains a major welfare concern. In addition, conditions such as osteoporosis and hypocalcaemia in table-egg chickens reduce egg production and can kill.

Discondroplasia tibial: mecanismos de lesão e controle

A discondroplasia tibial (DT) é atribuída a uma assincronia no processo de diferenciação dos condrócitos, levando à formação de uma camada de condrócitos pré-hipertróficos e de uma cartilagem na tíbia proximal que não é calcificada, mas é resistente à invasão vascular. Além disso, tem sido proposto que, na discondroplasia tíbial, a etapa final do processo de calcificação não ocorre devido ao fato de que os efetores de alguns genes, relacionados com o mecanismo de calcificação do disco de crescimento podem apresentar algumas de suas propriedades químicas ou biológicas alteradas e/ou não serem expressos. Nesse sentido, a compreensão do mecanismo de ação e o papel das biomoléculas e dos minerais relacionados com a discondroplasia tibial poderão contribuir para o conhecimento de doenças do tecido ósseo e estabelecer estratégias de prevenção e tratamento.

Sodium and chloride requirements of young broiler chickens fed corn-soybean diets (one to twenty-one days of age)

Sodium (Na+) and chloride (Cl-) nutritional requirements, dietary electrolyte balance (DEB), and their effects on acid-base balance, litter moisture, and tibial dyschondroplasia (TD) incidence for young broiler chickens were evaluated in two trials. One-day-old Cobb broilers were distributed in a completely randomized design with six treatments, five replicates, and 50 birds per experimental unit. Treatments used in both experiments were a basal diet with 0.10% Na+ (Experiment 1) or Cl- (Experiment 2) supplemented to result in diets with Na+ or Cl- levels of 0.10, 0.15, 0.20, 0.25, 0.30, or 0.35%, respectively. In Experiment 1, results indicated an optimum Na+ requirement of 0.26%. Sodium levels caused a linear increase in arterial blood gas parameters, indicating an alkalogenic effect of Na+. The hypertrophic area of growth plate in the proximal tibiotarsi decreased with Na+ levels. The TD incidence decreased with increases in dietary Na+. Litter moisture increased linearly with sodium levels. In Experiment 2, the Cl- requirement was estimated as 0.25%. Chloride levels caused a quadratic effect (P < or = 0.01) on blood gas parameters, with an estimated equilibrium [blood base excess (BE) = 0] at 0.30% of dietary Cl-. No Cl- treatment effects (P > or = 0.05) were observed on litter moisture or TD incidence. The best DEB for maximum performance was 298 to 315 mEq/kg in Experiment 1 and 246 to 264 mEq/kg in Experiment 2. We concluded that the Na+ and Cl- requirements for optimum performance of young broiler chickens were 0.28 and 0.25%, respectively.

Sodium and chloride requirements of growing broiler chickens (twenty-one to forty-two days of age) fed corn-soybean diets

Two trials were conducted to determine Na+ and Cl- nutritional requirements and dietary electrolyte balance (DEB) and its effects on acid-base balance, litter moisture, and incidence of tibial dyschondroplasia (TD) in broiler chickens during the growing period. Cobb broilers were distributed in a completely randomized design (30 pens) with six treatments, five replicates, and 50 birds per experimental unit at 21 d of age. Treatments used in both trials were a basal diet with 0.10% Na+ (Trial 1) or Cl- (Trial 2) supplemented to result in diets with Na+ or Cl- levels of 0.10, 0.15, 0.20, 0.25, 0.30, and 0.35%. In the first trial, the results indicated an optimum Na+ requirement of 0.15%. The Na+ levels, obtained with supplemental NaHCO3, did not affect blood gas parameters and TD incidence. Litter moisture increased linearly with Na+ levels. In the second trial, the Cl- requirement was estimated at 0.23%. Increasing Cl- levels, provided by NaCl with NaHCO3 to balance Na+, caused a linear effect (P < or = 0.01) on blood gas parameters, with an estimated equilibrium at 0.19% dietary Cl-. No effect (P > or = 0.05) of Cl- levels on litter moisture was observed. The hypertrophic area of growth plate in the proximal tibiotarsus increased with Cl- levels (P < or = 0.001). A nonlinear model describes this response. The best dietary electrolyte balance (DEB) was between 250 to 261 mEq/kg in Trial 1 and 249 to 257 mEq/kg in Trial 2. We concluded that the Na+ requirement was 0.15%, and the Cl- requirement was 0.23% for maximum performance of growing chickens between 21 and 42 d of age, and the best DEB was between 249 and 261 mEq/kg.

Chondrocytes and longitudinal bone growth: the development of tibial dyschondroplasia

Growth plate cartilage is central to the process of bone elongation. Chondrocytes originating within the resting zone of the growth plate proceed through a series of intermediate phenotypes: proliferating, prehypertrophic and hypertrophic, before reaching a terminally differentiated state. Disruption of this chondrocyte maturational sequence causes many skeletal abnormalities in poultry such as tibial dyschondroplasia (TD), which is a common cause of deformity and lameness in the broiler chicken. Cell and matrix components of the growth plate have been studied in order to determine the cause(s) of the premature arrest of chondrocyte differentiation and retention of prehypertrophic chondrocytes observed in TD. Chondrocyte proliferation proceeds normally in TD, but markers of the differentiated phenotype, local growth factors, and the vitamin D receptor are abnormally expressed within the prehypertrophic chondrocytes above, and within, the lesion. Tibial dyschondroplasia is also associated with a reduced incidence of apoptosis, suggesting that the lesion contains an accumulation of immature cells that have outlived their normal life span. Immunolocalization studies of matrix components suggest an abnormal distribution within the TD growth plate that is consistent with a failure of the chondrocytes to fully hypertrophy. In addition, the collagen matrix of the TD lesion is highly crosslinked, which may make the formed lesion more impervious to vascular invasion and osteoclastic resorption. Recent studies have applied the techniques of differential display and semiquantitative reverse transcriptase-polymerase chain reaction to RNA obtained from discrete populations of growth plate chondrocytes of different maturational phenotypes. This strategy has allowed us to compare phenotypically identical cell fractions from normal and TD growth plates in an attempt to identify possible candidate genes for TD.

Nutrition and skeletal problems in poultry

Several excellent reviews regarding nutrition and skeletal disorders have appeared in the last 20 yr. This review will cover several areas of vitamin D research, the area of feed deprivation, and bone abnormalities, because there has been considerable interest in these areas during the past 10 yr. Studies indicate that the quantitative requirement for cholecalciferol (D3) for broiler chickens is much greater than previously thought. Ascorbic acid may play a role in stimulating 1-hydroxylation of 25-hydroxycholecalciferol [25-(OH)D3], but the evidence is not clear under exactly what conditions this relationship is important in practical prevention of tibial dyschondroplasia. Studies indicate that dietary supplementation with 1,25-dihydroxycholecalciferol [1,25(OH)2D3] will reduce the incidence of tibial dyschondroplasia in three different strains of broilers bred to develop a high incidence of the disease. But it did not prevent the disease totally in the strains, unless high enough levels of 1,25-(OH)2D3 were fed to reduce growth rate. These studies indicate that these high tibial dyschondroplasia strains have a defect(s) in vitamin D metabolism. Studies continue to elucidate the role of ultraviolet light in preventing leg abnormalities. Only a few studies have been conducted on the efficacy of various vitamin D3 derivatives to prevent tibial dyschondroplasia. Feed deprivation continues to be an intriguing method of preventing tibial dyschondroplasia, and examination of exactly how this prevents the bone abnormality could open avenues for explaining the disease.

Gene expression and tibial dyschondroplasia

Tibial dyschondroplasia (TD) is a skeletal deformity associated with rapid growth in a number of avian species. The disease is the result of a disruption in the cascade of events that occur in the epiphyseal growth plate. Whereas the incidence of TD is susceptible to genetic selection, no specific genetic defect has been identified. Although there are extensive data describing the morphological and biochemical characteristics of the lesion, the mechanism of lesion formation is unknown. However, naturally occurring or induced genetic mutations in other species can provide important clues to possible mechanisms responsible for lesion development. Disruption of normal chondrocyte differentiation by constitutive activation of the parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) receptor, inactivation of the fibroblast growth factor receptor-3 (FGFR-3) receptor, and blocking vascular endothelial growth factor (VEGF) signaling all result in lesions that resemble TD. Impairment of vascular penetration due to the ablation of matrix metalloproteinase-9 (MMP-9) or tartrate-resistant acid phosphatase (TRAP) activity also results in similar cartilage abnormalities. We have integrated these observations with our current knowledge of TD to describe a hypothesis for the sequence of events responsible for the development of tibial dyschondroplastic lesions.

Dietary 1,25-dihydroxycholecalciferol has variable effects on the incidences of leg abnormalities, plasma vitamin D metabolites, and vitamin D receptors in chickens divergently selected for tibial dyschondroplasia

Three experiments were conducted to examine the efficacy of dietary 1,25-dihydroxycholecalciferol [(1,25-(OH)2D3)] on the development of tibial dyschondroplasia (TD) in chickens divergently selected for high (HTD) and low (LTD) incidences of TD. In Experiment 1, chickens from the two lines were fed two calcium levels (0.75 and 1.0%), with and without 5 micrograms/ kg dietary 1,25-(OH)2D3. In Experiment 2, both lines were fed diets containing 1.0% calcium and 0, 5, 10, or 15 micrograms/kg 1,25-(OH)2D3. The addition of 1,25-(OH)2D3 did not reduce the overall incidence of TD in Experiment 1, but did reduce the incidence of severe TD from 69 to 48% in the chickens receiving the 0.75% calcium diet. In this experiment, LTD chickens had higher plasma phosphorus and bone ash. No line differences were noted between plasma vitamin D metabolites or intestinal vitamin D receptors. In Experiment 2, 5 micrograms/kg of 1,25-(OH)2D3 decreased the incidence of TD from 94 to 76% and number three scores from 69 to 44% (P < or = 0.001). Higher amounts of 1,25-(OH)2D3 further decreased TD, but there was a reduction in body weight above 5 micrograms/kg. Plasma 25-hydroxycholecalciferol [25-(OH)D3] and 1,25-(OH)2D3 were higher and intestinal vitamin D receptors were lower in HTD chickens than in LTD chickens. Plasma 1,25-(OH)2D3 was not affected by dietary treatment, but 25-(OH)D3 was reduced by dietary 1,25-(OH)2D3. Experiment 3 was conducted to examine effects of line and dietary 1,25-(OH)2D3 on plasma vitamin D metabolites and intestinal and growth plate receptors. No effect of genetic line or dietary 1,25-(OH)2D3 was observed for vitamin D receptors concentration or plasma 1,25-(OH)2D3 levels. Plasma 25-(OH)D3 was reduced when 1,25-(OH)2D3 was fed. These results indicate that HTD chickens are somewhat responsive to dietary 1,25-(OH)2D3, but this treatment failed to prevent the lesion in a large portion of the population.

The effects of ultraviolet light and cholecalciferol and its metabolites on the development of leg abnormalities in chickens genetically selected for a high and low incidence of tibial dyschondroplasia

Four experiments were conducted to investigate the effects of ultraviolet (UV) light exposure and several cholecalciferol metabolites on the development of tibial dyschondroplasia (TD) and other parameters associated with vitamin D metabolism in chickens selected for high (HTD) and low (LTD) incidence of TD. In Experiment 1, exposure of chickens to UV light reduced the incidence and severity of TD more in LTD chickens than in HTD chickens, as evident by the significant interactions (P < 0.10 and 0.04). In Experiment 2, the addition of cholecalciferol to diets that were deficient in cholecalciferol linearly decreased the incidence of vitamin D rickets and increased bone ash, but increased the incidence of severe TD. The LTD chickens had a higher maximal bone ash of 40.0 +/- 0.7% than did the HTD chickens, which had a maximal bone ash of 37.0 +/- 0.7%. In Experiment 3, the addition of 5 micrograms/kg of 25-hydroxycholecalciferol [25-(OH)D3], 1-alpha-hydroxycholecalciferol, or 1,25- dihydroxycholecalciferol decreased the incidence and severity of TD in the LTD chickens and had no effect on TD in HTD chickens. In Experiment 4, increasing dietary 25-(OH)D3 increased plasma 25-(OH)D3 levels in both lines, but HTD chickens had higher plasma 25-(OH)D3 levels at 20 and 40 micrograms/kg of dietary 25-(OH)D3. The incidence and severity of TD were reduced in the LTD chickens by dietary 25-(OH)D3, but little effect was noted in HTD chickens. The LTD chickens reached a maximal bone ash at 9.7 +/- 1.9 micrograms/kg and HTD chickens reached the same bone ash at 33.0 +/- 7.0 micrograms/kg. These results indicate that UV light and vitamin D metabolites are not effective in preventing TD in HTD chickens, but that altered vitamin D metabolism does exist between HTD and LTD chickens.

Evidence of increased cholecalciferol requirement in chicks with tibial dyschondroplasia

A series of experiments was conducted to test the hypothesis that vitamin D utilization may not be as efficient in chicks with tibial dyschondroplasia (TD). The basal diet contained 1.0% Ca and 0.45% available P with no supplemental cholecalciferol (D3). Chicks from low TD (LTD) and high TD (HTD) selected lines were fed diets supplemented with various levels of vitamin D compounds and examined for rickets and TD. When chicks were fed a D3-deficient diet containing only 1.25 micrograms/kg added D3, HTD chicks had a greater incidence of severe rickets than LTD chicks (P < 0.05). The LTD chicks did not exhibit TD when fed a diet containing adequate (20 micrograms/kg) D3. The LTD chicks fed a diet supplemented with 5 micrograms/kg D3, however, had 22% incidence of TD. When HTD chicks were fed diets supplemented with 5 micrograms/kg D3 [control diet that meets NRC (1994) requirement for D3], 20 micrograms/kg D3, 5 micrograms/kg 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] or the combination of both D3 (20 micrograms/kg) and 1,25-(OH)2D3 (5 micrograms/ kg), TD incidence was highest in HTD chicks fed the control diet. When HTD chicks were fed diets with an increased dietary level of 1,25-(OH)2D3 (10 micrograms/kg) further reduction of TD incidence (P < 0.05) occurred. A potentially toxic level (Soares et al., 1983) of 1,25-(OH)2D3 (15 micrograms/kg) fed to HTD chicks resulted in still greater suppression of incidence of TD even though growth and feed intake in HTD chicks was greater than those of LTD chicks. It is concluded that the development of TD in HTD chicks is associated with subnormal ability to metabolize vitamin D.

Interaction between dietary 1,25-dihydroxycholecalciferol and calcium and effects of management on the occurrence of tibial dyschondroplasia, leg abnormalities and performance in broiler chickens

1. Two experiments were performed to compare the relative effectiveness of feeding 1,25-dihydroxycholecalciferol (1,25-DHCC) in minimising leg abnormalities in broilers with other methods and to investigate interactions between dietary 1,25-DHCC and calcium. 2. Adding 5 micrograms 1,25-DHCC/kg to a diet containing 12 g calcium/kg was more effective than early food restriction or meal feeding in preventing leg abnormalities but was found to cause a growth depression. 3. The second experiment, which had a factorial design, with diets containing 7.5, 10.0 and 12.5 g calcium and 0, 2.0, 3.5 and 5.0 micrograms 1,25-DHCC/kg, showed linear and quadratic interactions between these dietary factors. Diets with higher concentrations of both 1,25-DHCC and calcium resulted in growth depression associated with hypercalcaemia. 4. The incidence of tibial dyschondroplasia (TD) at 3 weeks of age was highest with the basal diet containing 7.5 g calcium/kg and was markedly reduced by addition of 1,25-DHCC and/or calcium. The incidence was very low or non-existent when 1,25-DHCC was fed at 3.5 micrograms/kg or greater. 5. Feeding 5 micrograms/kg 1,25-DHCC had no effect on plasma 1,25-DHCC concentrations, although at the higher dietary calcium contents plasma concentrations of 25-hydroxy- and 24,25-dihydroxy-cholecalciferol were lower in those birds fed 1,25-DHCC. 6. It is concluded that 1,25-DHCC is most effective in preventing TD without accompanying growth depression when it is fed in conjunction with diets containing less than 10 g calcium/kg.

Plasma concentrations of growth hormone and insulin-like growth factor-I in chickens developing tibial dyschondroplasia

The concentrations of plasma growth hormone (GH) and insulin-like growth factor-I (IGF-I) were measured in broilers during the initial development of tibial dyschondroplasia. Chicks were fed a standard starter ration, or a diet imbalanced in calcium and phosphorus to increase the incidence of dyschondroplasia. At 14 days of age four blood samples were collected and assayed for GH and IGF-I. The chicks were killed at three weeks of age and sections of bone were assessed histologically for evidence of dyschondroplasia. All the chicks displayed a pulsatile pattern of GH secretion. Eight of the group fed the imbalanced diet developed dyschondroplasia which was accompanied by a significant increase in the mean and peak GH concentrations compared with the control group but no increase in basal concentrations. The chicks fed the imbalanced diet which did not develop dyschondroplasia were not different from the control birds. There were no differences in IGF-I concentrations between the groups.