1. Supplements of between 2 and 8 g of sodium bicarbonate (NaHCO3)/l in drinking water provided between 21 and 49 d of age to broilers at 31 degrees C increased growth after 35 d of age and food intake after 42 d of age compared to unsupplemented controls. The most consistent responses were observed with 2 g NaHCO3/l. 2. Differences in weekly water intakes with different concentrations of NaHCO3 in the drinking water did not equate with variations in weekly food intake and growth. 3. Unexpected high mortality from dehydration and nephrosis was observed with a 10 g NaHCO3/l supplement. This mortality was associated with major changes in blood acid-base balance and plasma electrolyte concentrations. 4. Dietary supplements of NaHCO3 gave significantly better food conversion and numerically better growth in broilers at 31 degrees C than supplements of potassium bicarbonate (KHCO3) providing similar dietary concentrations of bicarbonate. 5. Supplements of NaHCO3 were more toxic to broiler chickens when supplied in the drinking water than in the diet. This appeared to be related to the intake of sodium and alterations to cellular membrane ionic potential.

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Recent studies have shown that the relative feed intake (FI) and bodyweight gain (BWG) responses of chronically heat-stressed broilers to equimolar additions of DL-methionine (DLM) or 2-hydroxy-4-(methylthio) butanoic acid (HMB; Alimet) can be altered by changing the arginine:lysine (arg:lys) ratio of the diet. However, no information is available concerning responses of broilers during acute heat stress. Broilers were exposed to either 22°C or 30°C during 3 consecutive 5-day experimental periods between 28 and 43 days of age. Responses to equimolar additions of the 2 sources of supplemental methionine activity were similar during exposure to 22°C at 28-33 days and 38-43 days of age. However, between 33 and 38 days of age, when the temperature was 30°C, increasing the dietary arg:lys ratio from 1.04 to 1.35 significantly improved the FI and BWG of broilers fed HMB, but not DLM. At an arg:lys ratio of 1.35, the BWG of broilers fed HMB was significantly greater than that of broilers fed DLM. Therefore, under acute heat stress conditions the choice of a supplemental source of methionine activity should be dependent upon the dietary arg:lys ratio. A R 0 2 1 1 7 R e s p o n s e o f b r o i l e r s t o a r g : l y s r a t i o J . C h e n e t a l .Additional keywords: feed intake, bodyweight gain, DL-methionine, 2-hydroxy-4-(methylthio) butanoic acid, Alimet, acute heat stress. Australian Journal of Agricultural ResearchJ. Chen et al.