Predictors of death and production performance of layer chickens in opened and sealed pens in a tropical savannah environment
© Shittu et al.; licensee BioMed Central Ltd. 2014
Received: 1 March 2014
Accepted: 3 September 2014
Published: 12 September 2014
Layer chickens are exposed to high risks of production losses and mortality with impact on farm profitability. The harsh tropical climate and severe disease outbreaks, poor biosecurity, sub-minimal vaccination and treatment protocols, poor management practices, poor chick quality, feed-associated causes, and unintended accidents oftentimes aggravate mortality and negatively affect egg production. The objectives of this study were to estimate the probability of survival and evaluate risk factors for death under different intensive housing conditions in a tropical climate, and to assess the production performance in the housing systems.
Daily mean mortality percentages and egg production figures were significantly lower and higher in the sealed pens and open houses (P < 0. 001) respectively. The total mean feed consumption/bird/day was similar for the open sided and sealed pens but the mean feed quantity per egg produce was significantly lower in the sealed pens ((P < 0.005). Seasons differently impacted on mortality with the hot-dry season producing significantly higher risk of mortality (61 times) and reduced egg production. Other parameters also differed except the egg production during the cold-dry season. Layers in sealed pens appear to have higher probability of survival and the Kaplan-Meir survival curves differed for each pen; ≥78 weeks old layer have higher probability of survival compared with the younger chickens and the 19-38 weeks age category are at highest risk of death (P < 0.001). The hazard-ratio for mortality of layers raised in sealed pens was 0.568 (56.8%).
Reasons for spiked mortality in layer chickens may not always be associated with disease. Hot-dry climatic environment is associated with heat stress, waning immunity and inefficient feed usage and increase probability of death with reduced egg production; usage of environmentally controlled building in conditions where environmental temperature may rise significantly above 25°C will reduce this impact. Since younger birds (19-38 weeks) are at higher risk of death due to stress of coming into production, management changes and diseases, critical implementation of protocols that will reduce death at this precarious period becomes mandatory. Whether older chickens' better protection from death is associated with many prophylactic and metaphylactic regimen of medications/vaccination will need further investigation.
Commercial egg-type poultry production is relatively low in northern Nigeria compared with the southern zones in view of a number of factors including but not limited to the climatic conditions which aggravate mortality and negatively affect egg production percentages. Other factors including severe disease outbreaks, poor biosecurity, sub-minimal vaccination and treatment protocols, poor management, practices, poor chick quality, feed-associated causes, and unintended errors and accidents similarly influence production . Laying hens are female chickens which are raised primarily for the purpose of commercial egg production. These birds and the breeder flocks are particularly at a higher risk of production losses, stress and pecking, higher disease incidents, inclement weather conditions and death due to long term exposures (≥72 weeks) to these factors on farms compared to meat type chicken .
A negative association has been established between mortalities and net profits associated with lower egg productions ,, and a mortality of up to 4% during weeks 1-8, 15% during rearing (9-20 weeks) and 12% during the laying period (21- ≥72 weeks) has been established as standards for the industry in a tropical climate . Ghodasara and colleagues  have broadly classified mortality in layer hens into three viz those associated with brooding stage (26.23%), growing stage (24.56%), and laying period (49.21%).
Diseases that have negatively impacted on production and increase mortality in layer hens include amongst others the following: Infectious bronchitis (up to 67% mortality and production losses) ,; Newcastle disease (between 51.5 and 60% mortality and 15% production losses) -; Coccidiosis (between 35.26 and 51.38% mortality and production losses respectively) ,; Infectious Bursal Disease (up to 40.4% mortality and production losses) ,.
Others include Fowl typhoid, fowl cholera, fowl pox, infectious laryngo-tracheitis, Marek's disease, Mycoplasma infections, infectious coryza, egg prolapse, aflatoxicoses, necrotic enteritis and E. coli infections and these can contribute between 1 - 20% mortality and egg production losses of up to 50% based on severity ,-.
Meanwhile, certain other highly fatal diseases with transboundary and trade limitation potentials like the highly pathogenic avian influenza particularly the H5 and H7 subtypes will cause between 80-100% mortality, with egg production down to near zero and a follow-up policy of depopulation.
The housing system has been confirmed to have major impact on mortality and production based on previous studies ,. In Sweden, Fossum and colleagues  had confirmed that with a change from the conventional battery-cages to litter-based system, the submission of mortality from layer chicken farms increased, and Gerzilov et al.,  in their works in Bulgaria found out that mortality also spiked in young flocks on the litter compared to other housing systems and tend to normalize as the birds grow older. In the current study, we evaluated the effect of open-sided and sealed intensive housing systems, ages of birds in lay and seasons on production performance and risk of death in layer chickens under a tropical environment.
Descriptive statistics of layer chickens' mortality, egg production and feed consumptions
Chicken population per house (number)
Mortality in percentages
Egg production percentages
Total feed/layer/day (g)
Total feed/egg laid (g)
Mean ± SD
Mean ± SD
Mean ± SD
Mean ± SD
Mean ± SD
8414 ± 992
0.17 ± 0.25
60 ± 11
116 ± 3
202 ± 50
7874 ± 650
0.20 ± 0.27
59 ± 12
117 ± 5.7
210 ± 63
7452 ± 1713
0.16 ± 0.23
63 ± 10
117 ± 3.5
195 ± 44
7561 ± 2013
0.16 ± 0.19
67 ± 11
116 ± 3.2
180 ± 39
6447 ± 1834
0.18 ± 0.28
57 ± 10
116 ± 4.1
215 ± 51
12107 ± 602
0.04 ± 0.04
66 ± 10
119 ± 14
183 ± 29
14149 ± 625
0.06 ± 0.05
68 ± 17
116 ± 8.8
211 ± 194
11072 ± 1198
0.12 ± 0.13
74 ± 7.0
116 ± 3.1
160 ± 18
7573 ± 1661
0.18 ± 0.24
61 ± 12
117 ± 4.0
200 ± 51
12614 ± 1367
0.07 ± 0.08
69 ± 13
117 ± 10
187 ± 123
Although, the mean mortality percentages were lower that the standards for the industry (12% from 20 to 72 weeks), the values obtained here exclude those birds removed due to ill health, poor performance and aggressive behaviour all of which are removed by culling. The mean mortality percentages were significantly lower in the sealed pens (0.07%) compared with the open houses (0.18%) (P < 0. 001). Contrastingly, the egg production figures were significantly higher in the sealed pens compared to the open-sided ones (P < 0. 001, Table 1). The total mean feed consumption per bird per day was similar for the open sided and sealed pens but the mean feed quantity used to produce an egg was significantly lower in the sealed pens ((P < 0.005, Table 1).
Comparison of the effect of season on mortality, egg production and feed consumption
Mortality in percentages ± SD
Egg production percentages ± SD
Total feed/layer/day ± SD (g)
Total feed/egg laid ± SD (g)
Cold dry season
0.14 ± 0.25
0.09 ± 0.11
65.27 ± 11.27
65.48 ± 18.96
114.57 ± 2.86
117.10 ± 17.12
182.32 ± 41.32
226.0 ± 208.96
Hot dry season
0.35 ± 0.31
0.08 ± 0.11
50.16 ± 8.78
71.49 ± 7.01
120.14 ± 5.21
117.18 ± 3.52
248.84 ± 57.71
165.47 ± 16.71
Warm wet season
0.09 ± 0.08
0.06 ± 0.06
65.08 ± 8.54
70.30 ± 9.02
115.91 ± 1.76
116.39 ± 2.24
181.73 ± 28.37
168.63 ± 24.65
Cold dry season (O + S)
0.12 ± 0.21
65.35 ± 16.63
115.52 ± 10.79
198.80 ± 133.71
Hot dry season (O + S)
0.25 ± 0.29
58.16 ± 13.17
119.03 ± 4.86
217.57 ± 61.77
Warm wet season (O + S)
0.08 ± 0.07
67.04 ± 9.08
116.09 ± 1.97
176.82 ± 27.76
Survival and risk factors associated with layer poultry mortality
Multivariable Cox-proportional hazard model, with pen as a shared frailty, to determine covariates of time to poultry layer mortality in the Savannah region of Nigeria
Hazard ratio (95% CI)
0.568 (0.554 - 0.582)
61.499 (58.836 - 64.283)
12.013 (11.566 - 12.478)
19 - 38
39 - 58
0.329 (0.316 - 0.341)
59 - 78
0.032 (0.031 - 0.034)
0.011 (0.011 - 0.012)
0.911 (0.911 - 0.911)
Stocking density (linear)
1.053 (1.043 - 1.064)
Feed consumption (linear)
1.012 (1.011 - 1.013)
The stocking density varied widely in the analysis based on the observation of same house types but different flock populations resident in the buildings. There was an underutilization of full capacity of the different houses on the farm. Though we cannot immediately assess whether the stocking density directly impact on the production efficiency of the chickens and total egg laid but we confirmed that it did not influence the risk of mortality. Guo et al.  had confirmed that whilewhile stocking density does not affect egg production percentages and weight, it will cause increased feed consumption and lead to poorer feed efficiency. Other workers have suggested that strain of birds and stocking density impact on egg production, egg weight, egg output and mortality, and recommended a standard of 733 cm2 per hen under the tropical environment . From our observation, the stocking densities obtained for the different houses appeared low compared with the standards (450 cm2 or 69.8 inch2) recommended for the ISA Brown breed. The sealed house with environmental controlled buildings had significant higher production figures with lesser mortalities due to the fact that the chickens were kept in more clement conditions since weather conditions were automatically managed.
The hot-dry season presented with extremely higher mortality figures compared to the other months and the risk of death during this period was also very high. This season might impact negatively on production performance due to associated heat stress, waning immunity and feed qualities and quantities consumed during this period. Although the feed consumptions during this same period was slightly higher than for the other periods, the manager did indicated that more feed tends to be wasted in these months and more water were taken that in the colder and warmer months. The FAO has demonstrated that at a temperature above 28°C, egg production will significantly wane both in quantity and quality . During the hot dry period in North-west Nigeria, environmental temperature may rise up to 47°C. Anjum has similarly confirmed the effect of high environmental temperature on egg production in Pakistan .
It should be noted that an environmental temperature between 25-40°C will cause the bird to pant and may lead to heat stroke and eventual death . The climatic data for Gusau, Zamfara State was consistent with the above assertion and the findings in the study. A spike in mortality was associated with the hot-dry months of February to May, the months when average minimum/maximum temperatures and relative humidity values were as follows: (Jan: Temp = 7-40°C, Rel. humidity = 19%; Feb: Temp = 10-41°C, Rel. humidity = 16%; March: Temp = 12-43°C, Rel. humidity = 14%; April: Temp = 15-44°C, Rel. humidity = 21%; May: Temp = 20-47°C, Rel. humidity = 37%; June: Temp = 18-46°C, Rel. humidity = 50%) . The effect of the above observation is an increasing production cost per egg produced and reduced profitability. It will thus be necessary to mitigate the excessive effect of heat stress on these birds during these months.
Since the sealed pen type with environmental controls appear to reduce the risk of death in layer birds by about half, it will become important to promote this housing type in sub-Saharan Africa where climatic conditions get extreme at certain period of the year.
It is noteworthy to state that younger birds (19-38 weeks) are at higher risk of death compared with older chickens (Table 3). They are at least three times more likely to die compared with those in age 39-58 weeks or 91 times more compared with those in age category ≥78 weeks. Previous workers have suggested that between 52 and≤ 100% of the lead causes of death (collibacillosis, flock cloacal cannibalism, coccidiosis and lymphoid leucosis) of layer chicken in Sweden is associated with chickens at young age of between 20 and 39 weeks . While we did not evaluate the causes of death, those diseases and conditions mentioned above and egg prolapse, chronic respiratory disease, infectious coryza, toxicities have been determined as the lead cause of death elsewhere ,. It remains to be determined if the same lead causes of death are responsible for the mortalities seen in the flock in this study. However, it will be critical to implement management protocols that will reduce death through other means at this precarious period of the bird life when they are exposed to stress of coming into production, changed environment, changed feed, and other forms of stressful conditions. While it is clear that the older birds were better protected from death, it is probable that the many prophylactic and metaphylactic regimen of medications/vaccination they have undergone within the last months were responsible for this observation.
Environmental controlled sealed buildings positively influenced egg production percentages and reduced mortality in a hot humid tropical climate. The chickens eat less quantities, waste more feeds and has reduced production efficiency during the hot dry environmental temperatures. Since younger chickens (19-38 weeks) are at higher risk of death compared with older chickens, due to stress of coming into production, management changes and diseases, critical implementation of management protocols that will reduce death through other means at this precarious period becomes mandatory. Effort should be intensified to distinctly identify disease-associated and production/management-associated deaths in layer chickens as reasons for spiked mortality in layer chickens may not always be associated with disease. This is important in order to reduce the burden of drug administration in production animals.
Description of studied farm
The farm is located in Bakura (12°09′N, 5°54′E), a low poultry density area of the Savannah region of northwest Nigeria, away from human residential areas. The distance of the farm from the nearest highway is 5 km and approximately 13 km away from the neighbouring town. Each pen contained adequate number of cages to accommodate the population of chickens per house. The farm has 12 pens, 6 of which are open-sided pens and the remaining 6 are sealed pens. These pens are in three rows: The first row has 4 open pens, the second row has 5 with 2 open pens and 3 sealed and the last row has 3 sealed pens. The temperature and relative humidity control in the open pens are influenced directly by the supervening climatic conditions while the sealed pens are environmentally controlled automatically by thermostat-controlled monitors. All the pens were equipped with automatic 3-tiers battery cages with facilities including feeders, nipple drinkers, litter conveyor belts beneath each tier and a feed silo. The birds (ISA Brown) were placed in the house before the onset of lay at 13-14 weeks and remained therein till end of lay (90 weeks) in open pens and (100 weeks) in sealed pens. This variation in end of lay age is taken as the farm's standard. The chickens were dewormed at week 8 and 14 and were vaccinated against the following pathogens: Mareks disease, Newcastle disease, Infectious bursal disease, Fowl pox, Fowl typhoid and Egg drop syndrome. The lighting program, the farm strictly adheres to the ISA Brown management protocols. The birds had 16 hours (maximum) of photoperiod as at the time the egg production reached 50% till the end of lay. Body weight, day and night length were considered in the determination of photoperiod length. In 2010, a prospective cohort study was conducted between 1st January and 31st December using longitudinal data from apparently healthy layer chickens from this regional commercial poultry farm. This farm largely supplies eggs and culled layers in northwest Nigeria. As at the time of data collection, only 5 open and 3 sealed pens were fully functional, giving a total of 8 pens used for the study. All records of daily, weekly and monthly activities were obtained through a data retrieval system on each chicken in the pens. Follow-ups documents and pathology reports/mortality records on each bird were obtained where necessary to exclude ambiguous data and iatrogenic causes of death within the period of study (365 days). All records of daily mortalities were also evaluated by either of the 2 resident poultry veterinarians to confirm their authenticities.
Summary of variables used in statistical analyses for identifying covariates for layer poultry performance in opened and sealed housing systems in the Savannah region of Nigeria
The month during which total daily egg production was recorded (January - December)
The season during which total daily egg production was recorded (cold-dry, hot-dry or warm-wet). This variable was generated from the Month variable.
Unique identifier for pen (1 - 8)
Unique identifier for the type of pen (opened or sealed)
Age of layers during which total daily egg production was recorded (19 - 101 weeks)
Total number of layers or stocking density on daily record
Total number of dead layers on daily record
Percentage dead of layers on daily record
Quantity of feed (gram) required per layer per day
Quantity of feed (gram) required per egg production per day
Total feed required (kilogram) per layer per day
The actual feed (kilogram) fed to layers per day
Total number of eggs produced per day
Total number of crates of eggs produced per day
Percentage daily egg production
Data analysis and model building
In the study, the unit of study is the pen and mortality rate is based on the number of dead chicken per house per day divided by the number of birds housed per pen. All statistical analyses were performed using Stata® v. 10 (StataCorp, College Station, Texas 77845 USA). Continuous variables were analysed using descriptive statistics: minimum, maximum, mean and median values. Daily mortality counts were modeled using survival analysis. The Kaplan-Meier estimator was used to estimate the probability of survival and median survival time and the log-rank test was used to compare survival curves.
While the random effects was in Z i ⋅ ω.
It was expected that observations with equal value of a variable in the data that identifies the group are assumed to have shared frailty. Across groups, the frailties are assumed to be gamma-distributed latent random effects that affect the hazard multiplicatively, or, equivalently, the logarithm of the frailty enters the linear predictor as a random offset. The results for the Cox proportional hazard model were reported as HR with its corresponding P-values and 95% CI. For each covariate, HR relative to the chosen reference category was calculated in order to show their relationships.
Although the observational data does not involve the handling and manipulation of animals, we ensured that all the “Five Freedom of Webster” (Freedom from hunger or thirst, Freedom from discomfort, Freedom from pain, injury or disease, Freedom to express (most) normal behaviour, Freedom from fear and distress) were taken care of in the farm. Similarly, the project was approved by the Research Committee of the Faculty of Veterinary Medicine, Usmanu Danfodiyo University, Sokoto. We also obtained and got clearance from the management of the farm to proceed and publish the outcome of the research.
SA, RAA and MAS compiled the data used in this study. FOF and HAW advised on the study design. SA and FOF carried out all statistical modelling of the data. SA and FOF did the literature search for related published papers. All authors contributed to the writing, reading and approval of the manuscript for submission.
We thank the management and owner of Rufa’i Farm, Bakura, Zamfara State of Nigeria, for providing access to farm data and who was happy to know our research findings and recommendations. We are grateful to the University of Pretoria for providing the forum to publish the work in an open access journal.
- Farooq M, Mian MA, Durrani FR, Syed M: Prevalent diseases and mortality in egg type layers under subtropical environment.Livest Res Rural Dev 2002. Available at: ., [http://www.lrrd.org/lrrd14/4/faro144.htm]
- Sorensen P: Selection, Environment of Layer, and Response on Nesting Behavious. In Proceedings of the 19th World Poultry Congress, Volume 2. Amsterdam, the Netherlands: World’s Poultry Science Association, Netherlands Branch; 1992:409–412.Google Scholar
- North MO: Breeder Management. Commercial Chicken Production Manual. The Avi. Publishing Company. Inc, Westport, Connecticut; 1984:240-243. 298-321Google Scholar
- Farooq M, Mian MA, Faisal S, Durrani FR, Arshad M, Khursid A: Status of broiler breeders stocks in Abbotabad and Mansehra. Sarhad J Agr. 2001, 17 (4): 489-495.Google Scholar
- Food and Agriculture Organization of the United Nations (FAO): Egg Production. Ch. 1. In Egg Marketing: A Guide for the Production and Sale of Eggs, Food and Agriculture Organization Agricultural Services Bulletin No. 150, 2003. Rome: FAO. Available at: . Accessed on February 26, 2014., [http://www.fao.org/docrep/005/y4628e/y4628e03.htm]
- Ghodasara DJ, Joshi BP, Jani PB, Gangopadhyay RM, Prajapati KS: Pattern of mortality in chicken. Indian Vet J. 1992, 69 (10): 888-890.Google Scholar
- Butcher GD, Winterfield RW, Shapiro DP: An outbreak of nephropathogenic H13 Infectious bronchitis in virus. Avian Dis. 1990, 34 (4): 916-921. 10.2307/1591383.View ArticlePubMedGoogle Scholar
- Rikula UL, Sihvonen L, Rossow L: Infectious bursal disease in Finland. Suomen Elainlaakarilehti. 1993, 99 (7-8): 457-461.Google Scholar
- Lambert C, Kabar AC: Egg drop syndrome (EDS 76) in New Caledonia, first diagnosis and control.Revue d' Elevage-et-de-Medecine Veterinaire-de-Nouvelle Caledonie 1994, 14(7):12.Google Scholar
- Anjum AD, Hassan S, Arbi GS: Infectious bursal disease in chickens. Pakistan Vet J. 1993, 13 (2): 54-58.Google Scholar
- Savic V: Epidemiology of intensive poultry production in Croatia during the last 35 years. Stocarstvo. 1999, 53 (6): 449-459.Google Scholar
- Demir S: Coccidios in Bursa region of chicken factors and their propagation (Bursa bölgesi tavuklarında coccidiose etkenleri ve bunların yayılışı) (in Turkish). J Vet Facul Uludağ University (Uludağ Univ Vet Fak Derg). 1992, 2: 143-151.Google Scholar
- Singh KCP, Verma SK, Prasad CB: Occurrence of infectious bursal disease in Chickens, isolation and clinico-pathology. Indian J Virol. 1994, 10 (2): 83-89.Google Scholar
- Amin S, Shafique KA, Arshad M, Rahman SU: Epidemiology Studies on Infectious Bursal Disease in Poultry. Proceedings of the National Seminar on Epidemiology of Livestock and Poultry Diseases. College of Vet. Sci, Lahore, Pakistan:1995.Google Scholar
- Taylor RD, Jones GPD, Murison RD: Effect of Calcium Feeding on The Expression of Marek’s Disease. In Proceedings of 1999 Australian Poultry Science Symposium, Volume 11. Sydney, NSW: University of Sydney; 1999:128–131.Google Scholar
- Nicholls TJ: Marek's disease in sixty week-old laying chickens.Aust Vet J 1984, 61(7):243. doi:10.1111/j.1751-0813.1984.tb06005.x.View ArticlePubMedGoogle Scholar
- Dhillon AS, Roy P, Lauerman L, Schaberg D, Weber S, Bandli D, Wier F: High mortality in egg layers as a result of necrotic enteritis. Avian Dis. 2004, 48 (3): 675-680. 10.1637/7113.View ArticlePubMedGoogle Scholar
- Sandoval VE, Terzolo HR, Blackall PJ: Complicated infectious coryza outbreaks in Argentina. Adv Agr Sci. 1999, 6 (1): 65-77.Google Scholar
- Reece RL, Beddome VD, Barr DA: Diseases diagnosed in replacement layer and breeder chicken flocks in Victoria, Australia, 1977 to 1985. Vet Rec. 1986, 119 (19): 471-475. 10.1136/vr.119.19.471.View ArticlePubMedGoogle Scholar
- Bains BS: Yolk Sac Infection. A Manual of Poultry Diseases. F. Hoffmann-La Roche and Co. Limited Company, Basle, Switzerland;1979:85-105.Google Scholar
- Qu FQ, Yang SQ, Jiu B, Zhang DZ, Li HW, Liu S, Chen SY: Diagnosis and control of swollen head syndrome in layers.Chin J Vet Med 1997, 23(7):23.Google Scholar
- Zanella A, Alborali GL, Bardotti M, Candotti P, Guadagnini PF, Martino PA, Stonfer M: Severe Escherichia coli O111 septicemia and polyserositis in hens at the start of lay. Avian Pathol. 2000, 29 (4): 311-317. 10.1080/03079450050118430.View ArticlePubMedGoogle Scholar
- Mukhopadhyay HK, Paul WM, Dorairajan N, Chandran NDJ: Drop in egg production due to aflatoxin B1 contamination in feed. Int J Animal Sci. 2000, 15 (1): 123-126.Google Scholar
- Fossum O, Jansson DS, Etterlin PE, Vågsholm I: Causes of mortality in laying hens in different housing systems in 2001 to 2004.Acta Vet Scand 2009, 51:3. Available at: . doi:10.1186/1751-0147-51-3., [http://www.actavetscand.com/content/51/1/3]View ArticleGoogle Scholar
- Gerzilov V, Datkova V, Mihaylova S, Bozakova N: Effect of poultry housing systems on egg production. Bulgarian J Agr Sci. 2012, 18 (6): 953-957.Google Scholar
- Guo YY, Song ZG, Jiao HC, Song QQ, Lin H: The effect of group size and stocking density on the welfare and performance of hens housed in furnished cages during summer. Anim Welfare. 2012, 21: 41-49. 10.7120/096272812799129501.View ArticleGoogle Scholar
- Benyi K, Norris D, Tsatsinyane PM: Effects of stocking density and group size on the performance of white and brown Hyline layers in semi-arid conditions. Tropl Anim Health Prod. 2006, 38 (7-8): 619-624. 10.1007/s11250-006-4417-1.View ArticleGoogle Scholar
- Kekeocha CC: Pfizer Poultry Production Handbook. Macmillan Publishers Ltd, London: 1985.Google Scholar
- MyWeather2: Local Weather: Gusau Climate History (Historical Monthly Averages for January -June). In Available at: . Assessed on February 26, 2014., [http://www.myweather2.com/City-Town/Nigeria/Gusau/climate-profile.aspx?month=1]
- Anjum AD: Prevalence of the poultry disease in and around Faisalabad and their relation to weather. Pakistan Vet J. 1990, 10 (1): 42-48.Google Scholar
- Boado E, Laurent E, Herrera C, Quintero D, Canovas A: Prevalence of the major diseases in different categories of poultry during the different seasons of the year. Revista Cubana de Ciencia Avicola. 1991, 18 (3): 257-262.Google Scholar
- Abrahamsson P, Tauson R: Performance and egg quality of laying hens in an aviary system. J Appl Poultry Res. 1998, 7 (3): 225-232. 10.1093/japr/7.3.225.View ArticleGoogle Scholar
- Farooq M: Prevalent Diseases and Mortality in Egg Type Layers. In Available online at: . Accessed on February 24, 2014., [http://www.priory.com/vet/egg.htm]
- Hosmer DW, Lemeshow S: Logistic Regression for Matched Case-Control Studies. Applied Logistic Regression. Edited by: Hosmer DW, Lemeshow S. Wiley, New York; 2000:223-259. 10.1002/0471722146.View ArticleGoogle Scholar
- Therneau TM, Grambsch PM: Modeling Survival Data: Extending the Cox Model. Springer Science and Business Media, New York, USA: 2000.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.