Use of pesticides in agriculture and domestic purposes may have adverse effects on humans and non-target animals. Dichlorvos is widely used as an insecticide to control household pests, in public health, protecting stored product from insects and control of parasites in livestock. As an organophosphate pesticide, dichlorvos works by inhibiting the activity of the enzyme, acetylcholinesterase (Yair et al., 2008). The excessive use of organophosphorus pesticides in agriculture has originated serious problems in the environment (Singh and Walker, 2006). Many environmental pollutants are known to cause reproductive toxicity and have resulted in a decrease in fertility level of human population (Whorton et al., 1972). The world-wide deaths and chronic diseases due to pesticide poisoning is about 1 million per year (Environews Forum, 1999). Environmental contaminants such as agricultural chemicals and industrial wastes have been reported to have adverse effect on the reproduction of birds exposed to them (Fry, 1995). Exposure to pesticides has been reported to cause decrease in egg production and embryo viability in birds (EPA, 2006). Sauter and Steelo (1972) have suggested pesticide residues as a major cause of declining population of several wild species of birds. Poultry may be exposed to pesticides either by ingestion of contaminated feed or through use of pesticides in poultry house (Foster, 1974). This study evaluates the effect of dichorvos on the reproductive performance of egg laying hens.

Forty (40) day old black pullets were obtained from Zartec Farms, Ibadan, Nigeria. The birds were brooded under appropriate conditions until they were seven weeks old. The seven weeks old pullets (with an average weight of 557.5 ± 9.5 g) were then divided into four different groups containing ten birds each and housed in poultry pens at the livestock unit of the Department of Animal Science Technology, Federal University of Technology, Owerri. The four groups were fed ad libitum with commercial poultry feeds contaminated with 0.00, 0.01, 0.02 and 0.04% dichlorvos (w/v). The contamination was done by measuring out graded volumes of the pesticide and mixing it with the feed. Three different types of feeds were given to the birds during the period of the experiment. Starter feeds were given from day 1 to the 8^th week; growers mash was given from the 8^th week to first egg drop after which layers mash was given till the end of the experiment. The birds were maintained in a clean environment that ensures 12 h of light and 12 h of darkness. The control group had no pesticide added to their feed. The pesticide exposure was continued until nine weeks after the first lay of eggs. The birds were weighed before and at the end of the exposure while the feed intake was also recorded. Eggs were collected daily and weighed. Total proteins of the eggs were estimated by the method of Lowry et al., (1951). Egg yolk cholesterol was determined as described by Shen et al., (1982). The results were statistically analyzed using a one way analysis of variance (ANOVA).

It was observed that exposure to dichlorvos affected the weight of the birds. The body weight of the control birds were significantly higher (p<0.05) than those exposed to the pesticide. The percentage gain in weight is shown in Table 1. For the control, the percentage gain in body weight was 126.5% as against 68.75, 65.10 and 28.10% gain in weight recorded for 0.01, 0.02 and 0.04% contaminated diets, respectively.

There was a significant reduction (p<0.05) in feed intake in the birds fed on pesticide contaminated diet. The control group recorded an average feed intake of 924.59 ± 11.1g while those fed on 0.04% DDVP recorded an average feed intake of 600.32 ± 8.99 g throughout the period of the experiment (Table 1).

Exposure to dichlorvos affected the layer performance of the birds. The birds fed on normal diet (control group) laid their first egg when they were 18 weeks old (126 days), while those fed on 0.01 and 0.02% contaminated diets laid their first egg when they were 23 weeks old (161 days). Egg laying was further delayed in the birds exposed to 0.04% dichlorvos laying their first egg in the 36^th week. There was also a reduction in egg production in the birds fed on pesticide contaminated diet. A total of 544 eggs were laid by the birds over a period of nine weeks. Out of these, 52.75% were laid by the birds fed on normal diet (control) while the remaining 47.25% were laid by those fed on pesticide contaminated diet.

Analysis of the eggs from the birds exposed to different concentrations of dichlorvos showed that there was no significant effect on the weight of the eggs. There were also no significant differences (p > 0.05) in the weight of the egg shells between the control and those exposed to pesticide (Table 2). However, the highest egg shell weight (5.13 ± 0.36 g) was recorded in the group exposed to 0.04% pesticide. There was a slight reduction in the weight of egg white from 31.42 ± 0.45 g in the control to 29.35 ± 0.73 g in the birds exposed to pesticide (Table 2). This difference was also not statistically significant (p > 0.05). The differences in egg yolk weight was also not significant (p > 0.05), although the highest weight (14.59 ± 1.15 g) was recorded in the birds fed on 0.02% pesticide contaminated diet.

The protein content of the egg yolk was significantly (p<0.05) reduced from 161.54 ± 5.71 mg protein/gyolk in the control to 139.73 ± 3.42 mg protein/g yolk in the birds exposed to 0.04% pesticide (Table 3). There was also a significant (p<0.05) reduction in the protein content of egg white with increase in percentage pesticide contamination. A similar trend was also observed in the cholesterol content of the egg yolk (Table 3). The differences in cholesterol content of the egg yolk from the different groups were not significant (p > 0.05).

It has been reported that environmental contamination by agricultural chemicals (including pesticides) have adverse effects on the reproduction of exposed birds (Fry, 1995). There was a reduction in feed intake in birds exposed to dichlorvos. This is in line with the work of Pym et al., (1984). Our results showed that dichlorvos have negative effects on the reproductive performance of poultry birds. Sexual maturity was delayed in the birds fed on pesticide contaminated diet. This is reflected in the delay in egg laying time by as much as 18 weeks in the hens fed on 0.04% pesticide contaminated diet when compared with the control. The birds exposed to 0.01 and 0.02% pesticide, respectively, laid their first eggs on the same day (23^rd week) while those fed on 0.04% contaminated diet laid their first egg on the 36th week. Under normal circumstances, commercial egg layers will lay their first egg between 15-19 weeks.

Egg production was also affected by exposure to dichlorvos. The percentage egg production was significantly lowered (p<0.05) in the hens fed on pesticide contaminated diet when compared with the control. A number of factors affect the reproductive capacity of domestic fowl. These include quantity and quality of feeds, availability of clean water, proper lighting and cleanliness of the pens, among others. Exposure to environmental pollutants have been reported to have some adverse effects on fertility and reproductive capacity of laying hens. Summer et al., (1996) reported significant decreases in egg production, weight and fertility of adult hens exposed to halogenated hydrocarbons. Pribilincová et al. (1996) also reported a negative effect on the reproductive performance of laying hens exposed to phenyl mercury.

Results of this study suggest that exposure of poultry birds to dichlorvos could affect their reproductive success as well as the quality of eggs laid.

The authors are grateful to the Head of the Department of Animal Science Technology, Federal University of Technology, Owerri, Nigeria, for making their poultry pens available for this research.

The authors have not declared any conflict of interests.