Physiological stress is one of many concerns facing the modern broiler producer. Research has yet to reveal mechanisms that would allow the producer to efficiently minimize the detrimental impacts of physiological stress on broiler performance. One possible approach for improvement in this area is to elucidate the effects of different nutrient regimens on stressed broilers. Sufficient prior knowledge of the effects of stress on metabolism and nutrient digestibility, as well as effective stress induction models, is essential to any researcher interested in delineating the effects of nutrients on stressed broilers. This review examines the physiological mechanisms that activate and control stress, as well as the effects of stress on immunity, metabolism, and broiler performance. Validation procedures for stress induction models are also examined. Additionally, this review examines available research focused on amino acid digestibility in the presence of stress, digestibility assays necessary for this research, and nutrients that have been shown to have the potential for stress reduction or amelioration.

Two experiments (Exp.) were conducted to establish amino acid (AA) digestibility coefficients (DC) for broilers given corticosterone (CS)-induced stress using the apparent ileal digestibility assay. For Exp. 1, 192 Ross x Ross 708 male broilers were placed into 16 floor pens (12 birds/pen). For Exp. 2, 120 Ross x Ross 308 male broilers were placed into 10 floor pens (12 birds/pen). Pens contained nipple drinkers, pan feeders, and soft-wood shavings. Both experiments were completely randomized designs using pen as the experimental unit. In both experiments, chicks were given a common starter diet from d 1 to 20. From d 21 to 30, broilers were provided a control diet or the control + 15 mg of CS/kg of diet dissolved in soybean oil (8 and 5 replications/treatment in Exp. 1 and 2, respectively). Diets were based on corn (65.07%) and soybean meal (26.36%) and contained an indigestible marker (chromic oxide 0.3%). Diets were formulated to contain 3,175 kcal of ME, 18.5% CP, 0.79% digestible TSAA, and 1.00% digestible Lys. Stress validation was accomplished by measuring BW gain, feed intake, and liver weight on d 30. Evidence that stress occurred was apparent due to the fact that broilers fed CS had lower BW gain and higher liver weight than those fed control. On d 30, the ileal contents were removed from 3 birds/pen, pooled, dried, and analyzed for AA content. Amino acid DC were calculated using the following equation: DC = 100 -(dietary marker % x ileal AA %) / (ileal marker % x dietary AA %) x 100. Amino acid digestibility did not differ (P > 0.05) between treatments in either experiment. Based on this research, future research should be directed at establishing DC for other nutrients in stressed broilers or the effect of different nutrients on the stress response.

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This research was conducted to evaluate immunity (experiments 1 to 3), cardiac function, and ascities resistance (experiment 4) of progeny chicks from broiler breeders fed diets differing in trace metal level and source. Broiler breeders received a control diet (75 mg of Zn and 83 mg of Mn added/kg of diet), the control diet supplemented with inorganic Zn (75 mg/kg of diet) and Mn (80 mg/kg of diet), the control diet supplemented with organic Zn (75 mg/kg of diet) and inorganic Mn (80 mg/kg of diet), or the control diet supplemented with organic Zn (75 mg/kg of diet) and Mn (80 mg/kg of diet) in experiments 1, 2, and 3. In experiment 4, the control diet and diet supplemented with organic sources of Zn and Mn were fed to broiler breeders. Immune organ weights, circulating granulocytes vs. agranulocytes, CD4 and CD8 positive T cells, cutaneous basophil hypersensitivity, and antibody titers to SRBC and breeder vaccinations were measured in progeny. Some supplemental mineral treatments increased (P < or = 0.05) cutaneous basophil hypersensitivity and relative bursa weight. All supplemental mineral treatments increased (P < or = 0.05) relative thymus weight. In experiment 4, electrocardiograph, pulse oximetry, heart rate, hematocrits, ventricle weights, and ascites incidence were measured in progeny reared in a cold-stress environment. The supplemental organic minerals increased (P < or = 0.05) left ventricle plus septum and total ventricular weights. Although progeny ascites incidence did not differ between breeder mineral treatments, breeders fed supplemental Zn and Mn sired progeny with improved cardiac functional capacity and some improvements in immunity.

Three experiments (Exp) were conducted to delineate a suitable model for inducing conditions mimicking physiological stress with minimal bird handling. For Exp 1, Ross x Ross 308 male chicks were fed a control diet or a diet containing 5 mg of corticosterone (CS)/kg from d 1 to 7. For Exp 2, Ross x Ross 508 broilers received 1 of 4 dietary treatments: control; control + 4 IU/kg of BW of adrenocorticotropin (ACTH)/d i.m. from d 21 to 27; control + 8 IU/kg of BW of ACTH/d i.m. from d 21 to 27; or control + 15 mg of CS/kg of diet for 14 d from 21 to 35 d of age. In Exp 3, Ross x Ross 308 broilers were fed high or low nutrient density (ND) from 1 to 41 d of age, and 0 or 20 mg of CS/kg of diet from 18 to 21 d of age. Performance parameters (BW gain, feed intake, feed conversion, and mortality) were measured in all 3 experiments. In Exp 1, CS administration significantly decreased BW gain and decreased feed intake and mortality. In Exp 2, although ACTH injection resulted in significantly depressed performance responses relative to the control, CS administration yielded significantly stronger results. In Exp 3, ND and CS interacted (P = 0.04) to affect feed intake from d 0 to 34. Broilers fed diets containing high ND and CS had higher feed intake than broilers fed low ND and CS. From 0 to 21 and 0 to 42 d, CS decreased feed intake. Increased dietary ND improved BW gain and feed conversion in Exp 3. Also, CS decreased and increased BW gain and feed conversion, respectively, during all periods in Exp 3. Dietary addition of CS negatively impacted performance of broilers, and increasing dietary amino acid density did not ameliorate these effects.

L-carnitine promotes mitochondrial β-oxidation of long chain fatty acids and their subsequent transport across the inner mitochondrial membrane. Although the role of L-carnitine in fatty acid metabolism has been extensively studied, its role in live performance and carcass responses of commercial broilers is less understood. The objective of this research was to determine if Lcarnitine fed at various levels in diets differing in CP and amino acids impacted on live performance and carcass characteristics of commercial broilers. Two floor pen experiments were conducted to assess the effect of dietary L-carnitine in grower diets. In Exp. 1, Ross×Hubbard Ultra Yield broilers were placed in 48 floor pens (12 birds/pen) and fed common diets to d 14. A two (0 or 50 ppm Lcarnitine) by three (173, 187, and 202 g/kg CP) factorial arrangement of treatments was employed from 15 to 35 d of age (8 replications/treatment). An interaction (p<0.05) in carcass yield indicated that increasing CP (187 g/kg) resulted in improved yield in the presence of L-carnitine. Increasing CP from 173 to 202 g/kg increased (p<0.05) BW gain and decreased (p<0.05) feed conversion and percentage abdominal fat. Feeding dietary L-carnitine increased back-half carcass yield which was attributable to an increase (p<0.05) in thigh, but not drumstick, yield relative to carcass. In Exp. 2, Ross×Ross 708 broilers were fed common diets until 29 d. From 30 to 42 d of age, birds were fed one of seven diets: i) 200 g/kg CP, 0 ppm L-carnitine; ii) 200 g/kg CP, 40 ppm L-carnitine; iii) 180 g/kg CP, 0 ppm L-carnitine; iv) 180 g/kg CP, 10 ppm L-carnitine; v) 180 g/kg CP, 20 ppm L-carnitine; vi) 180 g/kg CP, 30 ppm L-carnitine; and vii) 180 g/kg CP, 40 ppm L-carnitine (6 replications of 12 birds each). BW gain, feed conversion, mortality (30 to 42 d), and carcass traits (42 d) were measured on all birds by pen. There were no treatment differences (p<0.05). However, the addition of 40 ppm L-carnitine in the 200 g CP/kg diet increased (p = 0.06) thigh yields relative to BW in comparison to birds fed diets without L-carnitine, which was further confirmed via a contrast analysis (0 vs. 40 ppm L-carnitine in the 200 and 180 g CP/kg diets; p<0.05). These results indicated that dietary L-carnitine may heighten metabolism in dark meat of commercial broilers resulting in increased relative thigh tissue accretion without compromising breast accretion.

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