Giving a completely randomize design experiment, nine closed houses (125 m [L]×12.5 m [W]) were randomly allocated into two treatment groups (Mizuho; SYnegy Bioproducts, Nawala, Sri Lanka and Rydall OE; Apex Engineering Products Corporation, Aurora, IL,USA) and control. On the cross wall of one end of the each closed house, there were seven exhaust fans (1 m diameter), each having 535 m³/min of air exchange rate. On the opposite end of the each side wall, there were two cooling pads.

The initial thickness of the Paddy husk litter was 6 cm. On day 1, 32,500 male broiler chicks (Cobb 500) were introduced to each house. The initial per bird floor area of 190 cm² increased gradually on day 3, 5, 7, and 10 so that each bird got at least 480 cm² of floor space from day 10 onward. Each house had 1,650 auto drinker nipples and 160 feeder buckets of an auto feeding system. Brooding was done for 10 days. The temperature of the house on day 1 (34°C) was reduced by 0.5°C daily and from day 18 onward the temperature was maintained at 26°C using programmed exhaust fan operating system in which each 0.5°C increase above the target temperature, one fan turned on. Continuous lighting was provided.

Birds were fed commercial broiler booster feed upto day 21 and broiler finisher feed (New Hope Feeds, Sri Lanka), ad libitum (from day 22 onward). Catching started on day 28, but spacing was not reduced with the reduction of the number of birds.

According to the respective manufacturers, Mizuho is a mixture of bacterial culture which suppresses the urease producing bacteria and Rydall OE is a unique enzymatic biocatalyst containing a complex mixture of natural nutrients, vitamins, and trace elements.

According to the manufacturer’s recommendation, fifteen mL of Mizuho solution (11% v/v) was sprayed per m² litter area on day 8, 16, and 25 and also given with drinking water (2 L with 1,000 L of drinking water) on day 16, 25, and 35. Ninety mL of Rydall (1% v/v solution) was sprayed per 1 m² of litter surface on day 8, 16, and 25.

NH₃ levels were measured at 30, 90, and 150 cm heights from the litter surface, at 0600, 1200, and 1800 h of the day. At each height plane, measurements were taken at 20 imaginary points in zig-zag arrangement. A portable gas detector (Crowcon Gas-Pro; Crowcon Detection Instruments, Oxfordshire, UK) was used to determine the NH₃ levels. On an imaginary zig-zag line drawn across the litter, twenty litter sample collecting points were randomly selected. Care was taken to maintain a constant distance between sampling points and feeder/drinker lines. Twenty litter samples taken from each closed house on day 12, 24, and 40, were bulked and five subsamples were analyzed for total N content using Kjeldahl procedure. Daily feed/water intakes were determined by dividing the total house feed/water intake by the total number of birds. Live weights of 320 (on day 7, 14) and 90 (on day 21, 28, 35, 40) randomly selected birds from each closed house were used to determine the growth performance parameters.

Data were statistically analyzed using SPSS. NH₃ data were analyzed as a completely randomize design in 3×3 factorial arrangement. Treatment factors were three litter amendments (control, Rydall, and Mizuho), three heights (30, 90, and 150 cm) and three time points (0600, 1200, and 1800 h). Tukey test was used for the mean separation when a main effect was significant at p<0.05. Growth performance and litter N contents were analyzed as a completely randomize design with three treatments.

NH₃ was first detected as early as day 8 in control group whereas NH₃ was not detected until day 10 in litter amended groups (Figure 1). The NH₃ level in Mizuho amended closed-houses reached maximum level of 85.6 ppm as early as day 17 whereas control and Rydall recorded their maximum levels (89.4 ppm and 85.9 ppm) on day 21 and 22, respectively. The significant quadratic relationships of air NH₃ levels over the production period, in control (p = 0.001, R² = 0.83), Rydall (p = 0.001, R² = 0.73) and Mizuho amended groups (p = 0.001, R² = 0.74) showed that irrespective of the liter amendment, the NH₃ level reached its maximum level when birds were around 25 days old and then declined. Our results are in agreement with those of Redwine (15) who found an increase in ammonia levels up to day 35 and subsequent reduction in fan ventilated broiler closed houses in which the targeted temperature being maintained (20°C) was lower than that of the present study (26°C). Therefore, as shown by other studies [12–14,20,21], high temperature conditions under which this study was conducted may be the reasons for higher ammonia level and its earlier peak observed in the present study.

None of the two way interactions between amendment, time of the day and the height of the measurements and the three way interaction among them was significantly different. Rydall amendment significantly reduced the NH₃ level compared to control but there were no significant difference between the NH₃ levels of control and Mizuho groups (Table 1). Rydall resulted in 12.5% and 14.7% reductions in NH₃ level compared to control and Mizuho, respectively. The per bird cost of Rydall application (0.48Rs) was also calculated to be two times lower than that of Mizuho (0.97Rs). Effectiveness of Rydall was higher than that of Ca(OH)₂ (6% reduction) and comparable with Ferrous sulfate at 100 g/kg litter [22]. Meanwhile effectiveness of Rydall was found to be lower than some commonly used litter amendments such as alum at 100 g/kg litter (36% reduction), sodium bisulfate (21% reduction) [22] and PLT at 20 kg/100 m₂ litter (upto 50% reduction) [23]. Our results with Mizuho agree with others (19) who also used microbial amendments

Taking animal performance and health aspects into consideration, it has been recommended [24] that, NH₃ level of a poultry house should not exceed 25 ppm. However, broilers in control, Rydall and Mizuho groups were exposed to more than above limit from day 11, 12, and 11 onward, respectively, up to the day 41. Though air NH₃ levels showed a quadratic relationship over the growing cycle, even at the end of the production period, the NH₃ level was around 40 ppm.

The number of days that broilers were exposed to levels above the maximum recommended NH₃ level of 25 ppm was more or less similar across all the treatments; 30, 29, and 30 days in control, Rydall and Mizuho groups, respectively. In contrast, Moore et al [22] reported that when no amendment was used, air NH₃ levels of broiler houses were above the maximum recommended level throughout the 42 day growing period and alum reduced the birds’ exposure to above the critical limit for 12 days (29 to 40 d). Results of the present experiment clearly suggest that under hot humid conditions NH₃ level of broiler closed houses were higher than the maximum recommended level for broilers for around two thirds of the growing period.

Taking the animal welfare aspects into consideration, some studies [6,25], recommend that broilers should not be exposed to more than 10 ppm of atmospheric ammonia. However, birds in Mizuho, control, and Rydoll group were exposed to more than the said limit from as early as day 9, 10, and 11, respectively.

Occupational Safety and Health Administration (OSHA) sets 50 ppm as the permissible exposure limit for human. The NH₃ level exceeded the threshold of 50 ppm on day 14, 15, and 16 in Mizuho, control, and Rydall groups, respectively. Moreover, the unsafe time windows for human were shorter for Rydall (13 days; from 15 to 27 day) than for control (16 days) and Mizuho (17 days) groups, in two time windows (14 to 28 d and 32 to 35 d). Highlighting the grave concerns on human health hazards, the NH₃ level was higher than 50 ppm for about 25 days. However, analysis on the level of NH₃ at three heights indicated that the possible adverse impacts of high NH₃ level are more serious on birds than on human.

The NH₃ levels were significantly different at 30, 90, and 150 cm heights, giving the highest value at 30 cm level (Table 1). Since the main entry site of NH₃ is the nostrils, NH₃ level at 30 cm height was compared with the maximum exposure level for poultry. The NH₃ levels at 30 cm height exceeded 25 ppm level on day 9, 11, and 13 in Mizuho, control, and Rydall groups, respectively (Figure 2). Irrespective of the amendment used, throughout the production cycle NH₃ levels at 30 cm height were higher than 25 ppm level. Probably the presence of birds might have reduced the air circulation below 30 cm height thereby preventing NH₃ being removed by exhaust fans. NH₃ level recorded at 90 and 150 cm levels were 11.6% and 18.6% lower than that at 30 cm level. In contrast, Lahav et al [26] have reported a sharp increase in NH₃ level upto 20 cm height from the manure surface and no change thereafter, in a layer housing facility. Results of this study highlight the importance of taking NH₃ level measurements at the birds’ height in order to predict the possible impacts. Furthermore, strategies are needed to maintain a safer NH₃ level at the birds’ height. Since the presence of birds and other obstructions such as feeders and drinkers may restrict the horizontal air movement close to the floor, it may be worthy of studying the effectiveness of the means of better vertical air circulation (for example, a fan arrangement on the roof) alone or in combination with side wall-fixed fan arrangement. Alternatively, Lahav et al [26] suggested to separate NH₃ withdrawal from the ventilation system and, devising a separate low flow-rate air capturing system to collect NH₃ rich air in the litter.

Mean height of an adult Sri Lankan was 157 cm [27]. Therefore, NH₃ level at 150 cm was compared with the maximum exposure levels for human. NH₃ levels at 150 cm height exceeded the maximum threshold limit of 50 ppm for human exposure on day 12, 14, and 15 in Mizuho, control and Rydall groups, respectively (Figure 3). In general, after about day 33 NH₃ levels were lower than the recommended level, in all three groups and thus the unsafe periods were 21, 19, and 18 for Mizuho, control, and Rydall group, respectively.

Due to high ambient temperature conditions, higher NH₃ levels were expected during mid day. Contrary, being significantly different among each other, the NH₃ level was highest and lowest at 0600 and 1800 h, respectively and intermediate at midday (Table 1). Meanwhile Zhu et al [28] found no significant variation in NH₃ emission in animal facilities within a day. The NH₃ level at 1200 and 1800 h were 15% and 50% lower than that at early morning. Therefore, results of this study suggest that workers could be exposed to high NH₃ levels when they enter the poultry houses in morning. Since cooling fans had been programmed to operate to control the in house temperature build up, more fans were in operation during daytime, than night in which temperature was low. Consequently, NH₃ build up might have happened during night, giving a higher NH₃ level in the morning. Due to poor ventilation higher NH₃ levels was reported in winter season than in summer during which more fans are operated to reduce the temperature [29]. Though the programming of fans to operate according to the NH₃ level could reduce the NH₃ level, a number of studies [30,31] showed that improvements in ventilation incurred a significant additional cost.

Several studies [7–9], have reported better growth performance when air NH₃ levels were low in broilers houses. Contrary to those studies, in the present experiment, none of the growth performance parameters was significantly different among the treatments (Table 1), probably due to two reasons. Firstly, as discussed earlier, the duration of the exposure to higher NH₃ levels were more or less similar across all three groups. Secondly, the magnitude of reduction of NH₃ level in Rydall group was just 12.5%, compared to control and, thus might not have been strong enough to evoke a positive response. Given the higher NH₃ levels experienced in the present experiment, more effective litter amendments are suggested under hot humid conditions.

As suggested by others [19,22], higher litter N content was expected in Rydall treated litter. Behaviour of the air NH₃ level showed that the effects of Rydall over Mizuho and control was stronger up to day 22 and became weaker thereafter. Probably due to the above reason, Rydall amendment resulted in a higher litter N content on day 24, but not on day 12 or at the end of the growing period; on day 42. The final N contents of the litters were more or less similar to that reported by Moore et al [22] for 42 days old paddy husk based broiler litter (Table 2, 3).