Heat stress is a problem in large parts of the world, including many parts of Iran. Summertime temperatures often cause laying hens to produce thin and soft-shelled eggs. Due to this phenomenon, extreme losses in income from marketable eggs can occur (Koelkebeck et al., 1992). Both heat exposure per se and reduced appetite affect the laying performance and eggshell quality of hens exposed to high ambient temperatures (Balnave and Muheereza, 1997). Whereas egg production and egg weight are influenced to a major extent by the reduction in feed consumption, eggshell quality is influenced primarily by high temperature (Balnave and Muheereza, 1997; Sauveur and Picard, 1987). Birds are heat stressed if they have difficulty achieving a balance between body heat production and body heat loss. This can occur at all ages and in all types of poultry. Poor egg shell quality from hens housed at high temperatures continues to be a major source of economic loss to egg producers (Balnave and Muheereza, 1997). Efforts to improve laying performance at high temperatures have been relatively successful. Methods include the feeding of high nutrient density diets and the use of self-selection feeding (Balnave and Abdoellah, 1990). However, problems associated with eggshell quality have proved difficult to resolve.
The formation of the eggshell in poultry is affected by the blood acid-base
balance (Nobakht et al., 2006). Ca is the first restrictive factor of
the egg shell formation and the carbonate ions is second factor. In hot weathers,
the respiratory speed was increased and respiratory alkalosis that occurs. In
this time, carbon dioxide from the blood and bicarbonate from the blood and
body fluids were losses (Mongin, 1968; Nobakht et al., 2006; Balnave
and Muheereza, 1997). This loss of carbon dioxide is accentuated by the need
for blood bicarbonate to buffer the hydrogen ions produced during eggshell formation
(Lorcher and Hodges, 1969; Makled and Charles, 1987). Poultry are very sensitive
animals to blood acid-base disorders and thus, the blood pH should be close
to physiological limits of 7.35-7.45 (Dibartola, 1992; Carlson, 1997). A reduced
bicarbonate concentration in the lumen of the shell gland adversely affects
eggshell quality (Balnave et al., 1989). Therefore, it is possible that
at high temperatures, hens have a nutritional requirement for bicarbonate. Attempts
to improve eggshell quality through supplementation of the diet or drinking
water with sodium bicarbonate have been equivocal. Although, positive responses
have been observed by some (Makled and Charles, 1987) others have reported no
benefits (Ernst et al., 1975; Grizzle et al., 1992).
This study was therefore, designed to evaluate the effect of dietary sodium bicarbonate supplementation on many layer factors include laying performance, egg quality, egg shell quality and main plasma cations in south west of Iran during the very hot period (42°C) of the year.
MATERIALS AND METHODS
Diets, birds and their management: The experiment was conducted at the
poultry station of Ramin Agricultural and Natural Resources University in southwest
of Iran. Ninety six leghorns (HyLine-W36) layers in 50 weeks of age were used
for the study, which was conducted during the hot months (August-September)
of the year. Four isoenergetic and isonitrogenous diets were formulated (Table
1). Feed and water were ad libitum and the hens in this Experiment
were acclimatized a 16 h daily photoperiod. The temperature is reached to 36-42°C
in experiment period. Half of the layers were housed on the floor and the other
in battery cages located in a closed poultry house.
of experimental diet
|1Supplied/kg of diet: vitamin A, 10000 IU; vitamin
D3, 9790 IU; vitamin E, 121 IU; B12 , 20 μg; riboflavin, 4.4 mg; calcium
pantothenate, 40 mg; niacin, 22 mg; choline, 840 mg; biotin, 30 μg;
thiamin, 4 mg; zinc sulfate, 60 mg; manganese oxide, 60 mg; 2Di
Experimental design and data collection: The experimental Design Was
Completely Randomized (CRD) using a 2x4 factorial arrangement with 2 rearing
system (cage and floor) and 4 level of sodium bicarbonate supplementation (0,
0.5, 1 and 1.5%). Each treatment had 4 replicates and each replicate was allotted
3 birds. Daily egg collection was in the morning (11:00 am). The number of eggs
laid by birds in each replicate was recorded daily. Feed Intake (FI), Egg Production
(EP), Egg Weight (EW), Shell Thickness (ST), Shell Weight (SW), Shell Strength
(SS), Albumen Index (AI), Yolk Index (YI) and Haugh Unit (HU) were measured
Yolk index was calculated as yolk diameter/yolk height and Haugh unit was determined using following equation:
||Albumin height in mm
||The weight of whole egg in grams
For plasma cations assays, 10 mL of blood were collected by venipuncture from
the brachial vein into vacuum tubes and held on ice until all were centrifuged.
Data analysis: All collected data were subjected to analysis of variance using General Linear Models (GLM) procedure of SAS (ver. 9.1) and Duncans multiple range test were used to determine treatments difference.
RESULTS AND DISCUSSION
The result of this experiment showed that feed intake was increased with NaHCO3 levels (Table 2). Hens diet supplemented with 1.5% NaHCO3consumed greatest amount of feed. It is suggested that increasing of NaHCO3 can overcome, adverse effect of heat stress on hens feed intake. These results agree with other findings. Yörük et al. (2004) reported feed intake affected by addition of dietary NaHCO3. Contrasting with the current study, other studies (Balnave and Muheereza, 1997; Grizzle et al., 1992; Ergin, 1999) reported that 1% NaHCO3 supplementation did not change feed intake in peak producing layers. Hens that rearing in floor system fed more than hens in cage system.
of NaHCO3 supplementation of diet and rearing system on egg
quality characteristics (mean±Standard error of mean)
|a, bMean in the same row for each main factor bearing
different superscript, differ (p<0.05)
of NaHCO3 supplementation of diet and rearing system on plasma
cations (mean±standard error of mean)
|a, b, c Mean in the same row for each main factor
bearing different superscript, differ (p<0.05)
Egg weight and egg production (Table 2) were significantly
increase with NaHCO3 levels in 0-1.5 percentage (p<0.05). Increasing
egg production with inclusion of NaHCO3 agree with finding of El-Gammal
and Makled (1977). They showed that replacing NaCl (0.67%) with NaHCO3
(1%) increased egg production by 6%, whereas increasing NaHCO3 from
1-2% decreased egg production by 9%. Choi and Han (1983) reported that Na provided
as NaHCO3 (1.5%) caused higher egg production in comparing with Na
provided as NaCl (1.4%). However, no change in egg production was also reported
in hens supplemented with 1% NaHCO3 during peak period (Balnave and
Muheereza, 1997; Grizzle et al., 1992). Egg weight was increased with
dietary NaHCO3 that agree with Yörük et al. (2004)
finding. Sauvar and Picard (1987) indicated that in heat stress the egg weight
and egg production affected by decreasing of feed intake and egg shell quality
influenced by above temperature. Egg production and egg weight were significantly
increased in cage system (p<0.05).
Eggshell thickness (Table 2) and plasma potassium (Table 3) were significantly (p<0.05) affected by treatments. Eggshell thickness was decreased from 0.349-0.336 mm by increasing NaHCO3 from 0-1.5% of diets. This is contradict with some researcher which suggest that the use of sodium bicarbonate may aid in promoting better egg shell calcification by alteration of acid-base balance, especially in heat stress situations (Makled and Charles, 1987; Balnave and Muheereza, 1997). Researcher such as Yörük et al. (2004) and Ergin (1999), reported that use of NaHCO3 had no significant effect on egg shell, whereas Emery et al. (1990) reported that use of NaHCO3 increase egg shell thickness. The rearing system had no significant effect on Shell thickness.
The result of this experiment indicated that Haugh unit, albumen index, yolk index shell strength, Shell weight, plasma sodium and calcium were not significantly affected (Table 2 and 3) by addition of dietary NaHCO3 (p>0.05). In this study, the eggshell strength, eggshell weight and thickness were reduced numerically by dietary NaHCO3 supplementation. So may be there is another phenomenon like physiological adaptation to long time heat stress and this adaptation can decrease the effect of dietary NaHCO3 supplementation.
There was positive correlation between adding NaHCO3 supplementation and increased plasma potassium. Eggshell strength and weight were affected by rearing system but not by diet. Caged hen showed greater eggshell weight and more strength eggshell than hen reared on floor. In this experiment, 4 birds placed in a battery cage (60x40 cm) and probably have better ventilation and better nutrient utilization efficiency. Maybe this reason of better cage hen performance. There was significant interactions between rearing system and dietary NaHCO3 on many of the traits (p<0.05).
Inclusion of sodium bicarbonate positively affected laying hens performance and altered inner egg quality, but did not improve shell quality, during the heat stress.
The authors wish to thank Ramin Agricultural and Natural Resource University, Mollasani, Iran for providing financial support.