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The performance of farmed ostrich eggs in eastern Australia
PREVENTIVE VETERINARY
MEDICINE
ELSEVIER
Preventive
Veterinary Medicine 29 (1996) 121-134
The performance of farmed ostrich eggs in eastern Australia Simon J. More
’
Farm Animal Medicine and Production, The Unioersity of Queensland, PO Box 125, Kenmore, Qld. 4069. Australia
Accepted 18 April 1996
Abstract A prospective observational epidemiological study was undertaken in the south-eastern region of Queensland in eastern Australia to collect accurate information on the performance of farmed ostriches, and to identify the most important constraints facing on-farm production. This paper (the second in a series of three) focuses upon the performance of 910 ostrich eggs laid on 12 farms in this region between 1 July 1993 and 30 June 1994. Each egg was observed from lay until it hatched, ‘was permanently removed from the incubator unhatched, or reached the 46th day of incubation without hatching (whichever occurred first). Eggs weighed on average 1301.9 g at lay, were stored for a mean of 3.7 days prior to the start of incubation, and lost an average of 15.5% of the initial set weight during the period of incubation. Overall fertility and hatchability percentages of 68.1% and 67.0%, respectively, were achieved. Laboratory examination was performed on some eggs that were infertile or failed to hatch. Although bacteria were isolated from some of these eggs, bacterial infection may not have been an important cause of incubation failure. Egg-level factors were examined for association with egg fertility and with egg hatchability using random-effects logistic regression modelling. There was no unconditional association between egg fertility and either egg weight at the start of incubation, the season of lay or the duration of egg storage prior to incubation. There was evidence, however, indicating a relationship between egg fertility and nonexamined pair and farm-level factors. Egg hatchability was conditionally associated with egg weight at the start of incubation, the percentage egg weight loss during incubation and the season of lay, and random pair-level extra-binomial variation was also demonstrated. The relationship between hatchability and weight loss was curvilinear, fertile eggs were most likely to hatch with weight loss during incubation of between 12 and 15% of the egg weight at the beginning of incubation. Keywords: Ostriches;
Australia; Eggs; Health and productivity profile; Productivity
’ Tel.: -- 61 7 3365 5767; fax: + 61 7 3365 5699; e-mail: [email protected]. 0167-5877/96/$15.00 Copyright PfI SO16’7-5877(96)01064-l
0 1996 Elsevier Science B.V. All rights reserved.
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1. Introduction A database of information is steadily developing on aspects related to ostrich eggs and egg incubation. For example, published work is available describing the microclimate of ostrich nests @wart et al., 1987; Swart and Rahn, 19881, key factors considered important in artificial incubation of ostrich eggs (Foggin and Honywill, 1992; Jensen et al., 1992, pp. 14, 39-42; Deeming, 19931, and the recent performance of ostrich eggs imported into the United Kingdom (Deeming et al., 1993). Nonetheless, there remains an important gap in our understanding of the current performance of ostrich eggs on commercial farms in Australia and elsewhere, and of factors that influence ostrich egg fertility and hatchability. The main objectives of this study were to collect accurate information on the performance of eggs laid by farmed ostriches in south-eastern Queensland, and to identify egg-level factors associated with the fertility and hatchability of these eggs.
2. Materials
and methods
2.1. Study design, data collection and management This work formed part of a larger study assessing the performance of ostriches in south-eastern Queensland. Information about the overall study design and general methods used for data collection, management and analysis are described elsewhere (More, 1996). Briefly, a prospective longitudinal observational study was undertaken for 18 months from 1 July 1993 on a convenience sample of 12 study farms located within 100 km of Brisbane, Queensland. The focus of the overall research effort was upon defined cohorts of hens, eggs and chicks, and it is from these individuals that measurements were collected relating to productivity. This paper describes those aspects of this work relevant to the performance of ostrich eggs. On each study farm, hens were enrolled into the ‘hen cohort’ if, at any time during the period 1 July 1993 to 30 June 1994, they were penned with at least one male bird for the purpose of producing fertile eggs. Enrolled birds were removed from the cohort if for any reason these conditions ceased to be met. The ‘egg cohort’ comprised all eggs laid by enrolled members of the hen cohort. These eggs remained under observation until each hatched, was permanently removed from the incubator unhatched, or reached the 46th day of incubation without hatching (whichever occurred first). The information that was collected for each egg included: egg identification and parentage, date and weight at lay, date and weight at ‘set’ (terminology used in the industry to refer to the start of incubation) (if set), weights throughout incubation, whether the egg was fertile, whether the egg hatched (defined as whether a live chick was transferred out of the hatcher or not) and date, weight and identification of the chick at hatching. The results of laboratory examination and culture were also recorded for those eggs that were not fertile or did not hatch, and which had been submitted for further laboratory examination, as was information about putative egg-level risk factors believed to be related to the fertility and hatchability of each egg. Producer-recorded information and opinion
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was used for each of the above items except the stage of embryonic death for those eggs that were fertile but failed-to-hatch, in which case a laboratory-based diagnosis was used. Incubated eggs were considered fertile or otherwise based upon the findings by producers, during candling. 2.2. Laboratory analyses Producers were encouraged to submit cohort eggs that were considered infertile or fertile but failed to hatch for further examination. Producers were reimbursed for courier costs associated with the submission of eggs, and laboratory examinations were provided at no cost. All eggs were examined initially by Andrew Badley (Farm Animal Medicine and Production, The University of Queensland, St Lucia) with subsequent bacteriological examination being undertaken in university (Veterinary Pathology, The University of Queensland, St Lucia) and government (Yeerongpilly Veterinary Laboratory, Yeerongpilly and Toowoomba Veterinary Laboratory, Toowoomba) veterinary laboratories. Eggs were opened using a high-speed cutting disc and the contents examined after emptying onto a flat tray. Eggs were considered infertile if there was no evidence of embryonic development. Embryonated eggs were classified as having suffered an early, mid or late death if the overall chick length was less than 6 cm, if the overall chick length was greater than 6 cm and the crown-rump length was less than 16 cm, or if the crown-rump length was greater than 16 cm, respectively. Microbial examination was only performed on those eggs with a detectable odour suggestive of bacterial involvement. A sterile swab was used to collect material from the inner surface of the shell membrane of infertile eggs and eggs with small embryos (less than 10 cm crown-rump length), or from the proventricular lumen of eggs containing larger embryos. The material was plated onto sheep blood agar and MacConkey’s agar (Oxoid Australia Pty Ltd, Heid.elberg West, Vie., Australia) and incubated at 37°C in an aerobic environment containing 5% CO, for 40-48 h. Bacterial isolates were identified using routine staining and biochemical techniques. 2.3. Data analysis Data analyses were performed using SAS release 6.04 (Statistical Analysis Systems Institute Inc., Cary, NC) and EGRET version 0.25.6 (Statistics and Epidemiology Research Corporation, Seattle, WA). The data were first examined using standard methods of descriptive statistics. Variables were described and subsequently handled appropriate to the result of a test for normality (using the Shapiro-Wilk statistic that tests the null hypothesis that the data was a random sample from a normal distribution). Standardisation of egg weights was undertaken with linear regression models after obtaining measurements of three standard weights using a Sartorius electronic scale (Sartorius, GmbH, Gbttingen, Germany) and the egg-weighing scales on each of the 12 farms. The relationship between the weight of eggs at lay and the start of incubation was examined using a multiple regression model. The model was evaluated by plotting the residuals against the predicted egg weight at set, and by examining the distribution of the residuals. Using data from all eggs weighed at lay or set and then again on days 7,
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14, 21, 28 and 35 of incubation, a repeated-measures analysis of variance was done to examine the differences in the weights of incubating eggs between hens and over time, and a post hoc test for linear trend was used to test the hypothesis of a linear loss of egg weight during incubation. Key production parameters were calculated to describe the performance of the egg cohorts on the 12 study farms. Fertility was calculated as the percentage of incubated eggs that were fertile, and hatchability as the percentage of fertile eggs that hatched. The intracluster correlation coefficient ( 3) was calculated as described by Donner (1993) and Donald and Donner (1988). The unconditional association between a number of independent egg-level variables and the dichotomous dependent variables describing fertility (coded fertile or not) and hatchability (hatch or not) was assessed using the x2 test for independence for categorical variables and the Wilcoxon rank-sum test for continuous variables (all continuous variables were non-normally distributed). The predictor variables unconditionally associated with each outcome variable at P < 0.30 were selected for inclusion into the relevant multivariable model. Separate multivariable models for fertility and hatchability were developed. In terms of these measures of egg productivity, clustering was suspected (the data had been collected from a number of different farms and from different pairs within each farm), and for this reason logistic binomial regression was used (McDermott et al., 1994). As suggested by Curtis et al. (1993), an ordinary logistic regression (OLR) model was developed initially, using methods described by Hosmer and Lemeshow (1989, pp. 82-134). Dummy variables were created for the categorical variables with more than two categories. Selected variables were introduced to each model in a forward stepwise manner, and were subsequently retained if the likelihood ratio statistic indicated a significantly improved (that is, P < 0.05) model fit (Curtis et al., 1993). Interaction terms and appropriate quadratic expressions of the selected variables were tested for significance. The variables selected using the ordinary logistic regression model were then introduced into a logistic binomial regression (LBR) model, and an additional non-negative parameter, u, was then introduced to model random pair effects. Random pair rather than farm effects were modelled, since this approach accounts for extra-binomial variation both between pairs and between farms, with farm being an unmeasured or unmodelled covariate nested within the random pair effects component of each model (Anonymous, 1993, Manual Addendum, pp. 100-101). The final P values and confidence intervals for the estimated coefficients are based on Wald’s test. Evidence for extra-binomial variation was determined using the likelihood-ratio test.
3. Results 3.1. General comments about ostrich egg management On the 12 study farms, the breeder pens were each inspected at least twice each day. Eggs were collected as soon as practical following discovery and moved to an egg handling facility. Prior to incubation, most eggs are stored for some time, and many
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were also fumigated and weighed. Ten farms had an artificial incubator on-site. During incubation, eggs were usually candled (and sometimes also weighed and/or fumigated) at least weekly. Eggs considered infertile or otherwise defective were normally quickly removed from the incubator. Producers moved eggs to a separate hatcher at approximately day 39 days of incubation. 3.2. Egg performance A total of 910 eggs were enrolled into the egg cohort. A further 53 eggs were laid during the study period by juvenile birds on two of the study farms. Five hundred and twenty four cohort eggs were weighed at lay (mean f SD 1301.9 f 113.2 g) and 294 of these eggs were also weighed at set. The linear relationship (F(2.z9,)= 31761, P < 0.001, adjusted r2 = 0.995) between the laid and set weight of the 294 eggs was Set weight = - 23.30 + 1.02( laid weight) - 1.67( days in storage prior to incubation) Set weights were recorded for 632 eggs (mean 1307.1 g, median 1331 g, range 983-1565 g) and were estimated for a further 201 incubated eggs (for which weights at lay but not at set were recorded), using the above formula. Seven hundred and three eggs (81.2% of those incubated) were fumigated at some time prior to and/or during incubation (using formaldehyde gas (471 eggs), Chickgard (Antec International, Sudbury, UK; 216 eggs) or Tek-Trol (Bio-Tek Industries, Atlanta, GA; 16 eggs). Egg weights at lay or set and at 7-day intervals during incubation until day 35 were recorded for 107 eggs laid by 8 different hens from 6 different study farms. These data were not available for any other eggs within the cohort. Results of a repeated-measures analysis of variance indicated that there was a significant difference in the weight of eggs laid by different hens (Fc7,99,= 18.15, P < O.OOl>, that egg weights differed significantly with time during incubation (Fc5,95)= 124.84, P < O.OOl), and that the effect of time on egg weight was significantly different for eggs laid by different hens = 2.54, P < 0.001). Furthermore, the change in egg weight over time followed %.402) a linear trend (F( ,.99j = 579.96, P < 0.001). The total egg weight loss during incubation
Eggs not
that were incubated (44)
Chicks point
dying
icks/embryos egg
at the
of hatch
during
(30)
dying incubation
(164)
Fig. 1. Owcome
of 910 eggs laid by ostrich hens on 12 farms in south-eastern
Queensland
during 1993/1994.
44 866 278 588 194 26 19 40 30 79 394
Not incubated Artificially incubated b Infertile Fertile Failed to hatch Early embryonic loss Mid embryonic loss Late embryonic loss Died at point of hatch Reason not investigated Hatched
26 (7) 19 (6) 40 (7) 9 (4) 0 (0)
25 (6)
0 (0)
No. confirmed (cultured) a 4.8 95.2 30.5 64.6 21.3 2.9 2.1 4.4 3.3 8.7 43.3
Percentage all eggs
Queensland of
during
32.1 68.1
Percentage of all incubated eggs
1993/1994
a Number of eggs submitted for laboratory examination (and from which bacterial isolation was attempted). b Includes nine eggs that were incubated naturally for the first 3- 19 days following lay (assuming incubation
n
Outcome
Table I Outcome of 910 eggs laid on 12 ostrich farms in south-eastern
commenced
67.0
33.0 13.4 9.8 20.6 15.5 40.7
Percentage of all fertile, failed-to-hatch eggs
on the day of lay).
Percentage of all fertile eggs
XJ. Mare/Preventive
Table 2 Outcome of ostrich eggs enrolled into the egg cohort on 12 farms in south-eastern according to farm of origin No. of eggs
All members of the egg cohort All eggs laid on farm A B C D E F G H
K L
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Queensland
in 1993/1994,
Fertility
Hatchability
Laid
Incubated
Fertile
Hatched
(%I a
(%o) b
910
866 c
588
394
68.1
67.0
61 36 68 64 156 15 85 31 185 125 46 38
49 34 65 62 147
33 31 38 17 131 0 76 20 117 78 20 27
22 21 29 8 113 49 13 68 63 3 5
67.3 91.2 58.5 27.4 89.1 0.0 90.5 64.5 63.2 63.4 48.8 71.1
66.7 67.7 76.3 47.1 86.3 64.5 65.0 58. I 80.8 15.0 18.5
84 31 185’ 123 41 38
a The percentage of incubated eggs that were fertile. b The percentage of fertile eggs that hatched. ’ Includes nine eggs that were incubated naturally for the first 3- 19 days following lay (assuming incubation commenced on the day of lay); six of these eggs were considered fertile, and two subsequently hatched
(expressed as the percentage weight loss between set and day 39 of incubation) was calculated for the 532 eggs weighed at either lay or set, and again at least once 21 or more days after set. The percentage egg weight loss between set and day 39 of incubation averaged 15.5% (median 14.9%, range 7.2-48.3%). The outcome of the cohort eggs is shown in Fig. 1. For a number of reasons (including egg imperfections (2 eggs), damage (12), soiling (5) or sale (1); perceived hen or cock immaturity (12); and recent poor breeding pair performance (12)), 44 eggs were not incubated. Eight hundred and fifty seven eggs were incubated artificially (mean length of storage between time of lay and the start of incubation was 3.7 (median 3, range O-14) days), and a further nine eggs were naturally incubated by one hen for 3-19 days after laying before being transferred to an artificial incubator. Five hundred and eighty eight of the incubated eggs were fertile, resulting in an overall fertility of 68.1% (Table 1). On each farm, the fertility of incubated eggs ranged from 0 to 91.2% (Table 2). Twenty five (9%) of the infertile eggs were submitted for laboratory examination, and bacterial isolation was attempted from six of these eggs. Three of the eggs were each infected with one bacterium (Enterobacter sakuzakii, Proteus stuartii or Pseudomonas maltophila); no bacteria were cultured from the other eggs. One hundred and ninety four of the fertile eggs failed to hatch. Ninety four (48%) were submitted for laboratory examination, and bacterial isolation was attempted from 24 eggs or full-term chicks. The following bacteria were isolated: Acinetobacter culcoaceticus (on three separate occasions from eggs suffering mid-embryonic mortality,
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late-embryonic mortality and death at point-of-hatch), Aeromonas hydrophilu (once; early-embryonic mortality), Citrobacter freundii (once; early-embryonic mortality), Klebsiella pneumoniae (once; death at point-of-hatch), Pseudomonas jluorescens (once; early-embryonic mortality), Pseudomonas pseudomalfei (once; early-embryonic mortality) and a dual infection of Staphylococcus spp. and Acinetobacter lwoffii (once; death at point-of-hatch). No bacteria were isolated from 15 submissions. Three hundred and ninety four eggs hatched on 11 farms, resulting in an overall hatchability of 67.0%. On each farm, the hatchability of fertile eggs ranged from 15.0 to 86.3% (Table 2). The mean length of incubation (the period between the start of incubation and hatch) for 392 of these eggs was 42.0 days (median 42 days, range 38-45 days). A further two eggs were partially incubated naturally, and the commencement date of incubation was not known. 3.3. Factors afSecting egg fertility and hatchability The intrafarm and intrapair within-farm correlation coefficients for fertility were both 0.13. The same coefficients for hatchability of fertile eggs were 0.10 and 0.08, respectively. Three egg-level variables were considered plausibly related to egg fertility: season of lay, duration of storage prior to incubation, and egg weight at the start of incubation.
Table 3 Association Queensland
of egg-level variables with the hatchability in 1993/1994
Variable description
of 588 fertile ostrich eggs on 1 I farms in south-eastern
Percentage
Duration of storage prior to incubation < 3 days 3-7 days > 7 days
Percentage
a
Median
Percentage
a
Median 0.15 b
Season of lay Early (June-Aug.) Mid (Sept.-Feb.) Late (Mar.-May)
Egg weight at the beginning
P
Fertile eggs that did not hatch
Fertile eggs that hatched
14.6 43.7 8.7
5.1 24.0 3.9
21.3 43.1 2.9
12.0 18.7 1.9
0.28 b
’
of incubation
(g) d
egg weight loss during incubation
’
130.5 14.9
1268 16.2
0.02 c 0.004 e
a The percentage of all eggs with available data. b P value of x2 statistic. ’ Data were missing for six eggs that were initially incubated naturally. d Data were missing for 21 eggs that were not weighed at either the time of lay or at the start of incubation. ’ P value of Wilcoxon rank-sum statistic. f Percentage egg weight loss between set and day 39 of incubation; data were missing for 150 eggs that did not have both a recorded weight at lay or set and a weight 21 or mom days after the start of incubation.
SJ. More /Preventive Table 4 Logistic binomial Explanator-f
regression
Veferinary Medicine 29 (1996) 121-134
analysis of risk factors associated
variable
Intercept Egg weight at the beginning of incubation Percentage egg weight loss during incubation (Percentage egg weight loss during incubationI Season of lay b Early c Mid Late Extra-binomial variation
Regression - 9.463 0.054 0.497 -0.164 0 - 0.409 0.670
1.258
129
with egg batchability coefficient(b)
a
SE(b)
P
3.19 0.022 0.201 0.058
0.003 0.013 0.013 0.004
0.320 0.782 0.382
0.202 0.371 - d
a It is not valid to calculate odds ratios from regression coefficients when extra-binomial variation is demonstrated (Curtis et al., 1993). b Season of lay: early (June-August), mid (September-February). late (March-May). ’ The comparison group. d Not applicable. Ordinary logistic regression model: Deviance (G) = 464.8 (432.1 d.f.). Logistic binomial regression model: G = 441 .l (4321 d.f.). Each of the following interaction and quadratic terms was analysed using the ordinary logistic regression model and found to be non-significant: egg weightxegg weight loss, egg weightX season of lay, egg weight loss X season of lay, (egg weightJ2.
Within this egg cohort, however, none of these variables was unconditionally associated with fertility at P < 0.30, and a multivariable analysis was therefore not developed. Four egg-level variables were considered plausibly related to egg hatchability: season of lay, duration of storage prior to incubation, egg weight at the start of incubation, and percentage egg weight loss during incubation. Each of these variables was unconditionally associated with hatchability at P < 0.30 (Table 3). The relationship between hatchability and weight loss was not linear: fertile eggs were most likely to hatch with weight loss during incubation of between 12 and 15% of the egg weight at the beginning of incubation. The season of lay and length of storage were recorded for all 588 fertile eggs; however, information about the weight at lay or the start of incubation and weight loss during incubation was not available for 21 (4%) and 150 (26%) eggs, respectively. Data from 438 fertile eggs laid by 23 different breeding groups on ten farms were used in subsequent multivariable analyses. Using, an OLR model, three variables were conditionally associated with hatchability at P < 0.05: season of lay, egg weight at the start of incubation, and percentage egg weight loss during incubation. No interactions between these terms was found. A curvilinear relationship between hatchability and the percentage egg weight loss during incubation remained. A LBR model was created using these fixed variables and a random pair-effects parameter, and there was strong evidence of extra-binomial variation (P < 0.001; Table 4). In this latter model, the P values for each of the dummy variables describing the season of lay exceeded 0.20. Within this cohort, fertile eggs that were heavier at the start of incubation were more likely to hatch than lighter eggs. The likelihood of a fertile egg hatching was least for eggs that lost either excessively large or
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small amounts of weight during incubation. Fertile eggs were most likely to hatch if laid between March and May, were less likely if laid between June and August, and least likely if laid between September and February.
4. Discussion Throughout this study, heavy reliance was placed upon producers’ records and opinion for information about each cohort egg. This approach could not be avoided as a consequence of the observational nature and focus of this work; however, caution should be exercised when extrapolating aspects of this work to the reference population. In particular, results relevant to the repeated-measures analysis, the estimation of fertility and hatchability, and the laboratory examination of submitted eggs should each be viewed with care. Selection bias may limit the usefulness of the analyses that investigated weights of ostrich eggs during artificial incubation. As a result of the requirements of repeated-measures analyses, these investigations were restricted to data from 107 eggs from six farms. On the other farms in the study, eggs were weighed less frequently during incubation, if at all. It is conceivable that the producers on the 6 farms may also be more diligent than the other producers with farm practices other than the weighing of eggs. As a consequence, the eggs in this subset may not be representative of other eggs within the cohort. The accuracy of the measures of fertility and hatchability depends upon the candling skills of each of the participating producers. Throughout the commercial poultry industries, candling remains the accepted and most practical method to detect infertility in unhatched eggs; nonetheless, this procedure is a subjective diagnostic technique which is likely to be least accurate in the hands of the inexperienced. It is probable that some eggs suffering very early embronic mortality were mistaken by producers for infertile eggs. Therefore, the calculated fertility of 68.1% may under-represent, and hatchability of 67.0% over-represent the true level of these values of these farms. Although producers were asked to submit failed eggs to a participating laboratory, few eligible (9% of the infertile and 48% of the fail-to-hatch) eggs were submitted for examination during the study. All producers were equally encouraged to submit eggs for examination, and steps were taken to provide this service at no cost to each producer. However, because the courier services in different regions within the study area were of varying standards, this process was more convenient for some producers than others. Therefore, these submitted eggs represent a non-random sample of all infertile and fail-to-hatch eggs within the egg cohort, and care must be taken in generalising the laboratory results to the full cohort. The 910 eggs laid during the study collectively achieved fertility and hatchability percentages of 68.1 and 67.0, respectively. Although this phase generally proved less problematic than other phases of on-farm production, these figures are nonetheless disappointing in comparison with the results achieved by other commercial poultry industries (Tullett, 1990; Hodgetts, 1991). It is difficult to determine whether bacterial infection was an important cause of
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incubation failure on these farms because bacterial isolation was attempted on a biased subset of eggs. The subset is unrepresentative of all eggs that were infertile or failed-to-hatch because a number of the participating producers preferentially submitted eggs for examination that they believed may have been infected. It is also unrepresentative of the eggs submitted by producers for laboratory examination because microbial examination was only performed on eggs with a detectable odour suggestive of bacterial involvement. Nonetheless, because microbial examinations were conducted on the eggs that were most likely to be infected, the level of bacterial isolation (50% of the infertile eggs and 38% of the fail-to-hatch eggs and full-term chicks) is probably not excessive. For this reason, bacterial infection may not have been a common cause of incubation failure during the study period. On most of the study farms, the egg incubation facility was considered a high-security area with restricted access. Bacterial infection is considered an unlikely cause of incubation failure within the commercial poultry industry (Bruce and Drysdale, 1991). There has been a prolonged and intensive research effort to identify factors associated with the fertility and hatchability of eggs of commercial poultry species such as chickens and turkeys. In these species, determinants of egg fertility include climatic factors (including temperature, the length and intensity of the photoperiod, and altitude (Gangwar, 1983)), the fertility and mating behaviour of male birds (De Reviers and Williams, 1984; Fasenko et al., 19921, breeder age (Fasenko et al., 1992) and genetics (Gowe et al., 1993#), general management factors (Sainsbury, 1992, pp. 109-l 141, and egg-level variables (including weight and sequence position (Fasenko et al., 1992; Lemer et al., 1993)). Factors known to influence the hatchability of fertile eggs from commercial poultry Include aspects of the pre-incubation period (including the length of storage (Mayes and Takeballi, 1984; Y oo and Wientjes, 1991) and the type and method of fumigati’on (Whistler and Sheldon, 1989)), the physical characteristics of artificial incubation (incubator temperature, humidity, ventilation and hygiene, and egg orientation and turning (Bauer et al., 1990; Tullett, 1990; Wilson, 1991a)), breeder genetics (Gowe et al., 19931, nutrition (Whitehead, 1991) and age (Fasenko et al., 19921, and egg-level factors (including weight, weight loss during incubation and shell quality (Wilson, 1991b; Lemer et al., 1993)). As yet, there has been very little research about factors affecting the fertility and hatchability of farmed ostrich eggs; however, it is reasonahlle to expect many similarities with other commercial poultry species. Advice to ostrich producers is often given based upon our prior understanding of these commercial poultry species (see, for example, Doneley (1994) and Black (1995)). In this study, multivariable analyses were restricted to egg-level variables and their association with fertility and hatchability. It would benefit producers if farm and pair-level factors could also be examined because these are usually more readily amenable to manipulation by producers. However, because of the limited numbers of farms and productive breeding groups in this study, the statistical power of any such analyses in this work would have been very low. This would have greatly reduced the possibility of identifying determinants for fertility and hatchability, even if they were present. Clustering effects have been demonstrated in many recent explanatory epidemiological studies of animal populations (McDermott and Schukken, 1994). In this study I
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considered (due to genetics, management, nutrition, disease and environment) that eggs laid by one pair on a farm may possibly have been more alike in terms of outcomes such as fertility and hatchability than eggs laid by other pairs on the same farm, and by pairs on other farms within the study. For similar reasons, farm clustering effects may also have been present. The estimated intracluster correlation coefficients for both fertility and hatchability supported this view. None of the egg-level factors examined in this study were important in predicting the subsequent fertility of ostrich eggs. This was unexpected, given the relationship between fertility and a number of egg-level variables (including egg weight (Fasenko et al., 1992) and the time of lay within the egg laying cycle (Lemer et al., 1993)) in chickens and turkeys. On these 11 ostrich farms, hatchability was linked to three egg-level variables: the weight of eggs at the start of incubation, the amount of weight loss of eggs during incubation, and the season in which eggs were laid. Note, however, that the season of lay did not significantly affect egg hatchability after account was taken of random pair effects. Egg-weight loss during incubation has been considered an important factor in ostrich egg hatchability for some time, with too much or too little weight loss resulting in reduced hatchability. Indeed, Deeming et al. (1993) amongst others have suggested that a loss of approximately 15% of the initial weight to pipping (on about day 39) is physiologically optimal, and similar recommendations are made for other commercial poultry species (Lemer et al., 1993). The weight loss of eggs during incubation is affected by the porosity of the eggshell and the humidity of the air around the eggs (Deeming, 1993). The first of these factors may be caused by a range of problems, including hen immaturity, time of season and factors relating to nutrition, disease and genetics (Black, 1995), some of which may be difficult to influence. In contrast, the second factor can be readily manipulated by producers in modem artificial incubators, emphasising the need for producers to routinely monitor incubator humidity, both directly and by regularly weighing eggs during incubation. The current work also suggests a linear relationship between egg weight at the start of incubation and subsequent hatchability. This contradicts the findings from commercial chicken production, where it is recognised that intermediate sized eggs have the best hatchability, with small and large eggs hatching less well (Wilson, 1991b). Length of egg storage has previously been considered an important predictor of hatchability for ostriches (Jensen et al., 1992, p. 41) and other poultry species (Yoo and Wientjes, 1991); however, the current work could not duplicate this finding. Not surprisingly, there was clear evidence in this study of both farm and pair-withinfarm clustering with respect to both the fertility and hatchability of incubated eggs. Further epidemiological work on the role of specific pair and farm-level variables upon the fertility and hatchability of farmed ostrich eggs is clearly warranted.
Acknowledgements I gratefully acknowledge the cooperation and perseverance of the 12 ostrich producers. Andrew Badley and Joan Hendrikz are thanked for undertaking the egg laboratory
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examinations and providing statistical advice, respectively. The study received financial support from Triple D Ostrich Farms Pty Ltd, The University of Queensland and the Rural Industries Research and Development Corporation.
References Anonymous, 1993. EGRET Reference Manual first draft revision 4, EGRET Manual Addendum revision 3. Statistics and Epidemiology Research Corporation and Cytel Software Corporation, Seattle, WA. Bauer, F., Tullett, S.G. and Wilson, H.R., 1990. Effects of setting eggs small end up on hatchability and postha’rching performance of broilers. Br. Poult. Sci.. 31: 715-724. Black, D., 1995. Trouble-shooting reproductive diseases. In: Ostrich Odyssey 95, Proc. of the 5th Australian Ostrich Association Conference, Gold Coast, Australia, pp. 99- 104. Bruce, J. and Drysdale, E.M., 1991. Egg hygiene: routes of infection. In: S.G. Tullett (Editor), Avian Incubation, Poultry Science Symposium Series No. 22. Butterworth-Heinema, London, pp. 257-267. Curtis, C.R., Mauritsen, R.H., Kass, P.H., Salman, M.D. and Erb, H.N., 1993. Ordinary versus random-effects logistic regression for analysing herd-level calf morbidity and mortality data. Prev. Vet. Med., 16: 207-222. Deeming, D.C., 1993. The incubation requirements of ostrich (Struthio cam&s) eggs and embryos. In: Ostrich Odyssey, University of Sydney Post-graduate Committee in Veterinary Science, Proc. No. 217, pp. l-66. Deeming, DC., Ayres, L. and Ayres, F.J., 1993. Observations on the commercial production of ostrich (Struthio camelus) in the United Kingdom: incubation. Vet. Rec., 132: 602-607. De Reviers, M. and Williams, J.B., 1984. Testis development and production of spermatozoa in the cockerel (Galltis domesticus). In: F.J. Cunningham, P.E. Lake and D. Hewitt (Editors), Reproductive Biology of Poultry. Poultry Science Symposium Series No. 17. British Poultry Science Ltd. Longman Group, Harlow, UK, pa. 183-202. Donald, A. and Donner, A., 1988. The analysis of variance adjustment to chi-square tests when there is litter or herd correlation in surveys or field trials. Acta Vet. Stand. Suppl., 84: 490-492. Doneley, R.J.T., 1994. Investigation into infertility problems in the ostrich. In: Ostrich Odyssey 94, Veterinary Seminar Proc., Canberra, Australia, pp. 34-38. Domrer, A., 1993. The comparison of proportions in the presence of litter effects. Prev. Vet. Med., 18: 17-26. Fasenko, G.M., Hardin, R.T., Robinson, F.E. and Wilson, J.L., 1992. Relationship of hen age and egg sequerlce position with fertility, hatchability, viability, and preincubation embryonic development in broiler breeders. Poult. Sci., 71: 1374-1383. Foggin, CM. and Honywill, J., 1992. Observations on the artificial incubation of ostrich (Struthio camelus var. domesticus) eggs with special reference to water loss. Zimbabwe Vet. J., 23: 81-89. Gangwar, PC., 1983. Effect of climate on fertility and hatchability in poultry. Indian J. Poult. Sci., 18: 74-80. Gowe, R.S., Fait-full, R.W., McMillan, I. and Schmidt, G.S., 1993. A strategy for maintaining high fertility and hatchability in a multiple-trait egg stock selection program. Poult. Sci., 72: 1433- 1448. Hodgetts, B., 1991. Current hatchabilities in species of domestic importance and the scope for improvement. In: S.G. Tullett (Editor), Avian Incubation, Poultry Science Symposium Series No. 22. Butterwortl-Heinemama, London, pp. 139-144. Hosmer, D.W. and Lemeshow, S., 1989. Applied Logistic Regression. John Wiley, New York. Jensen, J.M., Johnson, J.H. and Weiner, S.T., 1992. Husbandry and Medical Management of Ostriches, Emus and Rheas. Wildlife and Exotic Animal TeleConsultants, College Station, TX. Lemer, S.P., French, N., McIntyre, D. and Baxter-Jones, C., 1993. Age-related changes in egg production, fertility, embryonic mortality, and hatchability in commercial turkey flocks. Poult. Sci., 72: 1025-1039. Mayes, F.J. and Takeballi, M.A., 1984. Storage of the eggs of the fowl (GuNus domesticus) before incubation: a review. World’s Poult. Sci. J., 40: l31- 140. McDermott, J.J. and Schukken, Y.H., 1994. A review of methods used to adjust for cluster effects in explanatory epidemiological studies of animal populations. Prev. Vet. Med., 18: 155- 173.
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McDermott, J.J., Schukken, Y.H. and Shoukri, M.M., 1994. Study design and analytic methods for data collected from clusters of animals. Prev. Vet. Med., 18: 175-191. More, S.J., 19%. The performance of farmed ostrich hens in eastern Australia. Prev. Vet. Med., 29: 107-120. Sainsbury, D., 1992. Poultry Health and Management. Chickens, Turkeys, Ducks, Geese and Quail, 3rd edn. Blackwell Scientific Publications, London. Swart, D. and Rahn, H., 1988. Microclimate of ostrich nests: measurements of egg temperature and nest humidity using egg hygrometers. J. Comp. Physiol. B, 157: 845-853. Swart, D., Rahn, H. and de Koch, J., 1987. Nest microclimate and incubation water loss of eggs of the African ostrich (Srruthio camelus var. domesticus). J. Exp. Zool., SuppI., 1: 239-246. Tullett, SC., 1990. Science and the art of incubation. Poult. Sci., 69: I- 15. Whistler, P.E. and Sheldon, B.W., 1989. Bactericidal activity, eggshell conductance, and hatchability effects of ozone versus formaldehyde disinfection. Poult. Sci., 68: 1074- 1077. Whitehead, C.C., 1991. Nutrition of the breeding bird and developing embryo. In: S.G. Tullett (Editor), Avian Incubation, Poultry Science Symposium Series No. 22. ButterworthHeinema, London, pp. 227-238. Wilson, H.R., 1991a. Physiological requirements of the developing embryo: temperature and turning. In: SC. Tullett (Editor), Avian Incubation, Poultry Science Symposium Series No. 22. Butterworth-Heinema, London, pp. 145-156. Wilson, H.R., 1991b. Interrelationships of egg size, chick size, posthatching growth and hatchability. World’s Poult. Sci. J., 47: 5-20. Yoo, B.H. and Wientjes, E., 1991. Rate of decline of hatchability with preincubation storage of chicken eggs depends on genetic strain. Br. Poult. Sci., 32: 733-740.
MEDICINE
ELSEVIER
Preventive
Veterinary Medicine 29 (1996) 121-134
The performance of farmed ostrich eggs in eastern Australia Simon J. More
’
Farm Animal Medicine and Production, The Unioersity of Queensland, PO Box 125, Kenmore, Qld. 4069. Australia
Accepted 18 April 1996
Abstract A prospective observational epidemiological study was undertaken in the south-eastern region of Queensland in eastern Australia to collect accurate information on the performance of farmed ostriches, and to identify the most important constraints facing on-farm production. This paper (the second in a series of three) focuses upon the performance of 910 ostrich eggs laid on 12 farms in this region between 1 July 1993 and 30 June 1994. Each egg was observed from lay until it hatched, ‘was permanently removed from the incubator unhatched, or reached the 46th day of incubation without hatching (whichever occurred first). Eggs weighed on average 1301.9 g at lay, were stored for a mean of 3.7 days prior to the start of incubation, and lost an average of 15.5% of the initial set weight during the period of incubation. Overall fertility and hatchability percentages of 68.1% and 67.0%, respectively, were achieved. Laboratory examination was performed on some eggs that were infertile or failed to hatch. Although bacteria were isolated from some of these eggs, bacterial infection may not have been an important cause of incubation failure. Egg-level factors were examined for association with egg fertility and with egg hatchability using random-effects logistic regression modelling. There was no unconditional association between egg fertility and either egg weight at the start of incubation, the season of lay or the duration of egg storage prior to incubation. There was evidence, however, indicating a relationship between egg fertility and nonexamined pair and farm-level factors. Egg hatchability was conditionally associated with egg weight at the start of incubation, the percentage egg weight loss during incubation and the season of lay, and random pair-level extra-binomial variation was also demonstrated. The relationship between hatchability and weight loss was curvilinear, fertile eggs were most likely to hatch with weight loss during incubation of between 12 and 15% of the egg weight at the beginning of incubation. Keywords: Ostriches;
Australia; Eggs; Health and productivity profile; Productivity
’ Tel.: -- 61 7 3365 5767; fax: + 61 7 3365 5699; e-mail: [email protected]. 0167-5877/96/$15.00 Copyright PfI SO16’7-5877(96)01064-l
0 1996 Elsevier Science B.V. All rights reserved.
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1. Introduction A database of information is steadily developing on aspects related to ostrich eggs and egg incubation. For example, published work is available describing the microclimate of ostrich nests @wart et al., 1987; Swart and Rahn, 19881, key factors considered important in artificial incubation of ostrich eggs (Foggin and Honywill, 1992; Jensen et al., 1992, pp. 14, 39-42; Deeming, 19931, and the recent performance of ostrich eggs imported into the United Kingdom (Deeming et al., 1993). Nonetheless, there remains an important gap in our understanding of the current performance of ostrich eggs on commercial farms in Australia and elsewhere, and of factors that influence ostrich egg fertility and hatchability. The main objectives of this study were to collect accurate information on the performance of eggs laid by farmed ostriches in south-eastern Queensland, and to identify egg-level factors associated with the fertility and hatchability of these eggs.
2. Materials
and methods
2.1. Study design, data collection and management This work formed part of a larger study assessing the performance of ostriches in south-eastern Queensland. Information about the overall study design and general methods used for data collection, management and analysis are described elsewhere (More, 1996). Briefly, a prospective longitudinal observational study was undertaken for 18 months from 1 July 1993 on a convenience sample of 12 study farms located within 100 km of Brisbane, Queensland. The focus of the overall research effort was upon defined cohorts of hens, eggs and chicks, and it is from these individuals that measurements were collected relating to productivity. This paper describes those aspects of this work relevant to the performance of ostrich eggs. On each study farm, hens were enrolled into the ‘hen cohort’ if, at any time during the period 1 July 1993 to 30 June 1994, they were penned with at least one male bird for the purpose of producing fertile eggs. Enrolled birds were removed from the cohort if for any reason these conditions ceased to be met. The ‘egg cohort’ comprised all eggs laid by enrolled members of the hen cohort. These eggs remained under observation until each hatched, was permanently removed from the incubator unhatched, or reached the 46th day of incubation without hatching (whichever occurred first). The information that was collected for each egg included: egg identification and parentage, date and weight at lay, date and weight at ‘set’ (terminology used in the industry to refer to the start of incubation) (if set), weights throughout incubation, whether the egg was fertile, whether the egg hatched (defined as whether a live chick was transferred out of the hatcher or not) and date, weight and identification of the chick at hatching. The results of laboratory examination and culture were also recorded for those eggs that were not fertile or did not hatch, and which had been submitted for further laboratory examination, as was information about putative egg-level risk factors believed to be related to the fertility and hatchability of each egg. Producer-recorded information and opinion
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was used for each of the above items except the stage of embryonic death for those eggs that were fertile but failed-to-hatch, in which case a laboratory-based diagnosis was used. Incubated eggs were considered fertile or otherwise based upon the findings by producers, during candling. 2.2. Laboratory analyses Producers were encouraged to submit cohort eggs that were considered infertile or fertile but failed to hatch for further examination. Producers were reimbursed for courier costs associated with the submission of eggs, and laboratory examinations were provided at no cost. All eggs were examined initially by Andrew Badley (Farm Animal Medicine and Production, The University of Queensland, St Lucia) with subsequent bacteriological examination being undertaken in university (Veterinary Pathology, The University of Queensland, St Lucia) and government (Yeerongpilly Veterinary Laboratory, Yeerongpilly and Toowoomba Veterinary Laboratory, Toowoomba) veterinary laboratories. Eggs were opened using a high-speed cutting disc and the contents examined after emptying onto a flat tray. Eggs were considered infertile if there was no evidence of embryonic development. Embryonated eggs were classified as having suffered an early, mid or late death if the overall chick length was less than 6 cm, if the overall chick length was greater than 6 cm and the crown-rump length was less than 16 cm, or if the crown-rump length was greater than 16 cm, respectively. Microbial examination was only performed on those eggs with a detectable odour suggestive of bacterial involvement. A sterile swab was used to collect material from the inner surface of the shell membrane of infertile eggs and eggs with small embryos (less than 10 cm crown-rump length), or from the proventricular lumen of eggs containing larger embryos. The material was plated onto sheep blood agar and MacConkey’s agar (Oxoid Australia Pty Ltd, Heid.elberg West, Vie., Australia) and incubated at 37°C in an aerobic environment containing 5% CO, for 40-48 h. Bacterial isolates were identified using routine staining and biochemical techniques. 2.3. Data analysis Data analyses were performed using SAS release 6.04 (Statistical Analysis Systems Institute Inc., Cary, NC) and EGRET version 0.25.6 (Statistics and Epidemiology Research Corporation, Seattle, WA). The data were first examined using standard methods of descriptive statistics. Variables were described and subsequently handled appropriate to the result of a test for normality (using the Shapiro-Wilk statistic that tests the null hypothesis that the data was a random sample from a normal distribution). Standardisation of egg weights was undertaken with linear regression models after obtaining measurements of three standard weights using a Sartorius electronic scale (Sartorius, GmbH, Gbttingen, Germany) and the egg-weighing scales on each of the 12 farms. The relationship between the weight of eggs at lay and the start of incubation was examined using a multiple regression model. The model was evaluated by plotting the residuals against the predicted egg weight at set, and by examining the distribution of the residuals. Using data from all eggs weighed at lay or set and then again on days 7,
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14, 21, 28 and 35 of incubation, a repeated-measures analysis of variance was done to examine the differences in the weights of incubating eggs between hens and over time, and a post hoc test for linear trend was used to test the hypothesis of a linear loss of egg weight during incubation. Key production parameters were calculated to describe the performance of the egg cohorts on the 12 study farms. Fertility was calculated as the percentage of incubated eggs that were fertile, and hatchability as the percentage of fertile eggs that hatched. The intracluster correlation coefficient ( 3) was calculated as described by Donner (1993) and Donald and Donner (1988). The unconditional association between a number of independent egg-level variables and the dichotomous dependent variables describing fertility (coded fertile or not) and hatchability (hatch or not) was assessed using the x2 test for independence for categorical variables and the Wilcoxon rank-sum test for continuous variables (all continuous variables were non-normally distributed). The predictor variables unconditionally associated with each outcome variable at P < 0.30 were selected for inclusion into the relevant multivariable model. Separate multivariable models for fertility and hatchability were developed. In terms of these measures of egg productivity, clustering was suspected (the data had been collected from a number of different farms and from different pairs within each farm), and for this reason logistic binomial regression was used (McDermott et al., 1994). As suggested by Curtis et al. (1993), an ordinary logistic regression (OLR) model was developed initially, using methods described by Hosmer and Lemeshow (1989, pp. 82-134). Dummy variables were created for the categorical variables with more than two categories. Selected variables were introduced to each model in a forward stepwise manner, and were subsequently retained if the likelihood ratio statistic indicated a significantly improved (that is, P < 0.05) model fit (Curtis et al., 1993). Interaction terms and appropriate quadratic expressions of the selected variables were tested for significance. The variables selected using the ordinary logistic regression model were then introduced into a logistic binomial regression (LBR) model, and an additional non-negative parameter, u, was then introduced to model random pair effects. Random pair rather than farm effects were modelled, since this approach accounts for extra-binomial variation both between pairs and between farms, with farm being an unmeasured or unmodelled covariate nested within the random pair effects component of each model (Anonymous, 1993, Manual Addendum, pp. 100-101). The final P values and confidence intervals for the estimated coefficients are based on Wald’s test. Evidence for extra-binomial variation was determined using the likelihood-ratio test.
3. Results 3.1. General comments about ostrich egg management On the 12 study farms, the breeder pens were each inspected at least twice each day. Eggs were collected as soon as practical following discovery and moved to an egg handling facility. Prior to incubation, most eggs are stored for some time, and many
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were also fumigated and weighed. Ten farms had an artificial incubator on-site. During incubation, eggs were usually candled (and sometimes also weighed and/or fumigated) at least weekly. Eggs considered infertile or otherwise defective were normally quickly removed from the incubator. Producers moved eggs to a separate hatcher at approximately day 39 days of incubation. 3.2. Egg performance A total of 910 eggs were enrolled into the egg cohort. A further 53 eggs were laid during the study period by juvenile birds on two of the study farms. Five hundred and twenty four cohort eggs were weighed at lay (mean f SD 1301.9 f 113.2 g) and 294 of these eggs were also weighed at set. The linear relationship (F(2.z9,)= 31761, P < 0.001, adjusted r2 = 0.995) between the laid and set weight of the 294 eggs was Set weight = - 23.30 + 1.02( laid weight) - 1.67( days in storage prior to incubation) Set weights were recorded for 632 eggs (mean 1307.1 g, median 1331 g, range 983-1565 g) and were estimated for a further 201 incubated eggs (for which weights at lay but not at set were recorded), using the above formula. Seven hundred and three eggs (81.2% of those incubated) were fumigated at some time prior to and/or during incubation (using formaldehyde gas (471 eggs), Chickgard (Antec International, Sudbury, UK; 216 eggs) or Tek-Trol (Bio-Tek Industries, Atlanta, GA; 16 eggs). Egg weights at lay or set and at 7-day intervals during incubation until day 35 were recorded for 107 eggs laid by 8 different hens from 6 different study farms. These data were not available for any other eggs within the cohort. Results of a repeated-measures analysis of variance indicated that there was a significant difference in the weight of eggs laid by different hens (Fc7,99,= 18.15, P < O.OOl>, that egg weights differed significantly with time during incubation (Fc5,95)= 124.84, P < O.OOl), and that the effect of time on egg weight was significantly different for eggs laid by different hens = 2.54, P < 0.001). Furthermore, the change in egg weight over time followed %.402) a linear trend (F( ,.99j = 579.96, P < 0.001). The total egg weight loss during incubation
Eggs not
that were incubated (44)
Chicks point
dying
icks/embryos egg
at the
of hatch
during
(30)
dying incubation
(164)
Fig. 1. Owcome
of 910 eggs laid by ostrich hens on 12 farms in south-eastern
Queensland
during 1993/1994.
44 866 278 588 194 26 19 40 30 79 394
Not incubated Artificially incubated b Infertile Fertile Failed to hatch Early embryonic loss Mid embryonic loss Late embryonic loss Died at point of hatch Reason not investigated Hatched
26 (7) 19 (6) 40 (7) 9 (4) 0 (0)
25 (6)
0 (0)
No. confirmed (cultured) a 4.8 95.2 30.5 64.6 21.3 2.9 2.1 4.4 3.3 8.7 43.3
Percentage all eggs
Queensland of
during
32.1 68.1
Percentage of all incubated eggs
1993/1994
a Number of eggs submitted for laboratory examination (and from which bacterial isolation was attempted). b Includes nine eggs that were incubated naturally for the first 3- 19 days following lay (assuming incubation
n
Outcome
Table I Outcome of 910 eggs laid on 12 ostrich farms in south-eastern
commenced
67.0
33.0 13.4 9.8 20.6 15.5 40.7
Percentage of all fertile, failed-to-hatch eggs
on the day of lay).
Percentage of all fertile eggs
XJ. Mare/Preventive
Table 2 Outcome of ostrich eggs enrolled into the egg cohort on 12 farms in south-eastern according to farm of origin No. of eggs
All members of the egg cohort All eggs laid on farm A B C D E F G H
K L
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Queensland
in 1993/1994,
Fertility
Hatchability
Laid
Incubated
Fertile
Hatched
(%I a
(%o) b
910
866 c
588
394
68.1
67.0
61 36 68 64 156 15 85 31 185 125 46 38
49 34 65 62 147
33 31 38 17 131 0 76 20 117 78 20 27
22 21 29 8 113 49 13 68 63 3 5
67.3 91.2 58.5 27.4 89.1 0.0 90.5 64.5 63.2 63.4 48.8 71.1
66.7 67.7 76.3 47.1 86.3 64.5 65.0 58. I 80.8 15.0 18.5
84 31 185’ 123 41 38
a The percentage of incubated eggs that were fertile. b The percentage of fertile eggs that hatched. ’ Includes nine eggs that were incubated naturally for the first 3- 19 days following lay (assuming incubation commenced on the day of lay); six of these eggs were considered fertile, and two subsequently hatched
(expressed as the percentage weight loss between set and day 39 of incubation) was calculated for the 532 eggs weighed at either lay or set, and again at least once 21 or more days after set. The percentage egg weight loss between set and day 39 of incubation averaged 15.5% (median 14.9%, range 7.2-48.3%). The outcome of the cohort eggs is shown in Fig. 1. For a number of reasons (including egg imperfections (2 eggs), damage (12), soiling (5) or sale (1); perceived hen or cock immaturity (12); and recent poor breeding pair performance (12)), 44 eggs were not incubated. Eight hundred and fifty seven eggs were incubated artificially (mean length of storage between time of lay and the start of incubation was 3.7 (median 3, range O-14) days), and a further nine eggs were naturally incubated by one hen for 3-19 days after laying before being transferred to an artificial incubator. Five hundred and eighty eight of the incubated eggs were fertile, resulting in an overall fertility of 68.1% (Table 1). On each farm, the fertility of incubated eggs ranged from 0 to 91.2% (Table 2). Twenty five (9%) of the infertile eggs were submitted for laboratory examination, and bacterial isolation was attempted from six of these eggs. Three of the eggs were each infected with one bacterium (Enterobacter sakuzakii, Proteus stuartii or Pseudomonas maltophila); no bacteria were cultured from the other eggs. One hundred and ninety four of the fertile eggs failed to hatch. Ninety four (48%) were submitted for laboratory examination, and bacterial isolation was attempted from 24 eggs or full-term chicks. The following bacteria were isolated: Acinetobacter culcoaceticus (on three separate occasions from eggs suffering mid-embryonic mortality,
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late-embryonic mortality and death at point-of-hatch), Aeromonas hydrophilu (once; early-embryonic mortality), Citrobacter freundii (once; early-embryonic mortality), Klebsiella pneumoniae (once; death at point-of-hatch), Pseudomonas jluorescens (once; early-embryonic mortality), Pseudomonas pseudomalfei (once; early-embryonic mortality) and a dual infection of Staphylococcus spp. and Acinetobacter lwoffii (once; death at point-of-hatch). No bacteria were isolated from 15 submissions. Three hundred and ninety four eggs hatched on 11 farms, resulting in an overall hatchability of 67.0%. On each farm, the hatchability of fertile eggs ranged from 15.0 to 86.3% (Table 2). The mean length of incubation (the period between the start of incubation and hatch) for 392 of these eggs was 42.0 days (median 42 days, range 38-45 days). A further two eggs were partially incubated naturally, and the commencement date of incubation was not known. 3.3. Factors afSecting egg fertility and hatchability The intrafarm and intrapair within-farm correlation coefficients for fertility were both 0.13. The same coefficients for hatchability of fertile eggs were 0.10 and 0.08, respectively. Three egg-level variables were considered plausibly related to egg fertility: season of lay, duration of storage prior to incubation, and egg weight at the start of incubation.
Table 3 Association Queensland
of egg-level variables with the hatchability in 1993/1994
Variable description
of 588 fertile ostrich eggs on 1 I farms in south-eastern
Percentage
Duration of storage prior to incubation < 3 days 3-7 days > 7 days
Percentage
a
Median
Percentage
a
Median 0.15 b
Season of lay Early (June-Aug.) Mid (Sept.-Feb.) Late (Mar.-May)
Egg weight at the beginning
P
Fertile eggs that did not hatch
Fertile eggs that hatched
14.6 43.7 8.7
5.1 24.0 3.9
21.3 43.1 2.9
12.0 18.7 1.9
0.28 b
’
of incubation
(g) d
egg weight loss during incubation
’
130.5 14.9
1268 16.2
0.02 c 0.004 e
a The percentage of all eggs with available data. b P value of x2 statistic. ’ Data were missing for six eggs that were initially incubated naturally. d Data were missing for 21 eggs that were not weighed at either the time of lay or at the start of incubation. ’ P value of Wilcoxon rank-sum statistic. f Percentage egg weight loss between set and day 39 of incubation; data were missing for 150 eggs that did not have both a recorded weight at lay or set and a weight 21 or mom days after the start of incubation.
SJ. More /Preventive Table 4 Logistic binomial Explanator-f
regression
Veferinary Medicine 29 (1996) 121-134
analysis of risk factors associated
variable
Intercept Egg weight at the beginning of incubation Percentage egg weight loss during incubation (Percentage egg weight loss during incubationI Season of lay b Early c Mid Late Extra-binomial variation
Regression - 9.463 0.054 0.497 -0.164 0 - 0.409 0.670
1.258
129
with egg batchability coefficient(b)
a
SE(b)
P
3.19 0.022 0.201 0.058
0.003 0.013 0.013 0.004
0.320 0.782 0.382
0.202 0.371 - d
a It is not valid to calculate odds ratios from regression coefficients when extra-binomial variation is demonstrated (Curtis et al., 1993). b Season of lay: early (June-August), mid (September-February). late (March-May). ’ The comparison group. d Not applicable. Ordinary logistic regression model: Deviance (G) = 464.8 (432.1 d.f.). Logistic binomial regression model: G = 441 .l (4321 d.f.). Each of the following interaction and quadratic terms was analysed using the ordinary logistic regression model and found to be non-significant: egg weightxegg weight loss, egg weightX season of lay, egg weight loss X season of lay, (egg weightJ2.
Within this egg cohort, however, none of these variables was unconditionally associated with fertility at P < 0.30, and a multivariable analysis was therefore not developed. Four egg-level variables were considered plausibly related to egg hatchability: season of lay, duration of storage prior to incubation, egg weight at the start of incubation, and percentage egg weight loss during incubation. Each of these variables was unconditionally associated with hatchability at P < 0.30 (Table 3). The relationship between hatchability and weight loss was not linear: fertile eggs were most likely to hatch with weight loss during incubation of between 12 and 15% of the egg weight at the beginning of incubation. The season of lay and length of storage were recorded for all 588 fertile eggs; however, information about the weight at lay or the start of incubation and weight loss during incubation was not available for 21 (4%) and 150 (26%) eggs, respectively. Data from 438 fertile eggs laid by 23 different breeding groups on ten farms were used in subsequent multivariable analyses. Using, an OLR model, three variables were conditionally associated with hatchability at P < 0.05: season of lay, egg weight at the start of incubation, and percentage egg weight loss during incubation. No interactions between these terms was found. A curvilinear relationship between hatchability and the percentage egg weight loss during incubation remained. A LBR model was created using these fixed variables and a random pair-effects parameter, and there was strong evidence of extra-binomial variation (P < 0.001; Table 4). In this latter model, the P values for each of the dummy variables describing the season of lay exceeded 0.20. Within this cohort, fertile eggs that were heavier at the start of incubation were more likely to hatch than lighter eggs. The likelihood of a fertile egg hatching was least for eggs that lost either excessively large or
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small amounts of weight during incubation. Fertile eggs were most likely to hatch if laid between March and May, were less likely if laid between June and August, and least likely if laid between September and February.
4. Discussion Throughout this study, heavy reliance was placed upon producers’ records and opinion for information about each cohort egg. This approach could not be avoided as a consequence of the observational nature and focus of this work; however, caution should be exercised when extrapolating aspects of this work to the reference population. In particular, results relevant to the repeated-measures analysis, the estimation of fertility and hatchability, and the laboratory examination of submitted eggs should each be viewed with care. Selection bias may limit the usefulness of the analyses that investigated weights of ostrich eggs during artificial incubation. As a result of the requirements of repeated-measures analyses, these investigations were restricted to data from 107 eggs from six farms. On the other farms in the study, eggs were weighed less frequently during incubation, if at all. It is conceivable that the producers on the 6 farms may also be more diligent than the other producers with farm practices other than the weighing of eggs. As a consequence, the eggs in this subset may not be representative of other eggs within the cohort. The accuracy of the measures of fertility and hatchability depends upon the candling skills of each of the participating producers. Throughout the commercial poultry industries, candling remains the accepted and most practical method to detect infertility in unhatched eggs; nonetheless, this procedure is a subjective diagnostic technique which is likely to be least accurate in the hands of the inexperienced. It is probable that some eggs suffering very early embronic mortality were mistaken by producers for infertile eggs. Therefore, the calculated fertility of 68.1% may under-represent, and hatchability of 67.0% over-represent the true level of these values of these farms. Although producers were asked to submit failed eggs to a participating laboratory, few eligible (9% of the infertile and 48% of the fail-to-hatch) eggs were submitted for examination during the study. All producers were equally encouraged to submit eggs for examination, and steps were taken to provide this service at no cost to each producer. However, because the courier services in different regions within the study area were of varying standards, this process was more convenient for some producers than others. Therefore, these submitted eggs represent a non-random sample of all infertile and fail-to-hatch eggs within the egg cohort, and care must be taken in generalising the laboratory results to the full cohort. The 910 eggs laid during the study collectively achieved fertility and hatchability percentages of 68.1 and 67.0, respectively. Although this phase generally proved less problematic than other phases of on-farm production, these figures are nonetheless disappointing in comparison with the results achieved by other commercial poultry industries (Tullett, 1990; Hodgetts, 1991). It is difficult to determine whether bacterial infection was an important cause of
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incubation failure on these farms because bacterial isolation was attempted on a biased subset of eggs. The subset is unrepresentative of all eggs that were infertile or failed-to-hatch because a number of the participating producers preferentially submitted eggs for examination that they believed may have been infected. It is also unrepresentative of the eggs submitted by producers for laboratory examination because microbial examination was only performed on eggs with a detectable odour suggestive of bacterial involvement. Nonetheless, because microbial examinations were conducted on the eggs that were most likely to be infected, the level of bacterial isolation (50% of the infertile eggs and 38% of the fail-to-hatch eggs and full-term chicks) is probably not excessive. For this reason, bacterial infection may not have been a common cause of incubation failure during the study period. On most of the study farms, the egg incubation facility was considered a high-security area with restricted access. Bacterial infection is considered an unlikely cause of incubation failure within the commercial poultry industry (Bruce and Drysdale, 1991). There has been a prolonged and intensive research effort to identify factors associated with the fertility and hatchability of eggs of commercial poultry species such as chickens and turkeys. In these species, determinants of egg fertility include climatic factors (including temperature, the length and intensity of the photoperiod, and altitude (Gangwar, 1983)), the fertility and mating behaviour of male birds (De Reviers and Williams, 1984; Fasenko et al., 19921, breeder age (Fasenko et al., 1992) and genetics (Gowe et al., 1993#), general management factors (Sainsbury, 1992, pp. 109-l 141, and egg-level variables (including weight and sequence position (Fasenko et al., 1992; Lemer et al., 1993)). Factors known to influence the hatchability of fertile eggs from commercial poultry Include aspects of the pre-incubation period (including the length of storage (Mayes and Takeballi, 1984; Y oo and Wientjes, 1991) and the type and method of fumigati’on (Whistler and Sheldon, 1989)), the physical characteristics of artificial incubation (incubator temperature, humidity, ventilation and hygiene, and egg orientation and turning (Bauer et al., 1990; Tullett, 1990; Wilson, 1991a)), breeder genetics (Gowe et al., 19931, nutrition (Whitehead, 1991) and age (Fasenko et al., 19921, and egg-level factors (including weight, weight loss during incubation and shell quality (Wilson, 1991b; Lemer et al., 1993)). As yet, there has been very little research about factors affecting the fertility and hatchability of farmed ostrich eggs; however, it is reasonahlle to expect many similarities with other commercial poultry species. Advice to ostrich producers is often given based upon our prior understanding of these commercial poultry species (see, for example, Doneley (1994) and Black (1995)). In this study, multivariable analyses were restricted to egg-level variables and their association with fertility and hatchability. It would benefit producers if farm and pair-level factors could also be examined because these are usually more readily amenable to manipulation by producers. However, because of the limited numbers of farms and productive breeding groups in this study, the statistical power of any such analyses in this work would have been very low. This would have greatly reduced the possibility of identifying determinants for fertility and hatchability, even if they were present. Clustering effects have been demonstrated in many recent explanatory epidemiological studies of animal populations (McDermott and Schukken, 1994). In this study I
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considered (due to genetics, management, nutrition, disease and environment) that eggs laid by one pair on a farm may possibly have been more alike in terms of outcomes such as fertility and hatchability than eggs laid by other pairs on the same farm, and by pairs on other farms within the study. For similar reasons, farm clustering effects may also have been present. The estimated intracluster correlation coefficients for both fertility and hatchability supported this view. None of the egg-level factors examined in this study were important in predicting the subsequent fertility of ostrich eggs. This was unexpected, given the relationship between fertility and a number of egg-level variables (including egg weight (Fasenko et al., 1992) and the time of lay within the egg laying cycle (Lemer et al., 1993)) in chickens and turkeys. On these 11 ostrich farms, hatchability was linked to three egg-level variables: the weight of eggs at the start of incubation, the amount of weight loss of eggs during incubation, and the season in which eggs were laid. Note, however, that the season of lay did not significantly affect egg hatchability after account was taken of random pair effects. Egg-weight loss during incubation has been considered an important factor in ostrich egg hatchability for some time, with too much or too little weight loss resulting in reduced hatchability. Indeed, Deeming et al. (1993) amongst others have suggested that a loss of approximately 15% of the initial weight to pipping (on about day 39) is physiologically optimal, and similar recommendations are made for other commercial poultry species (Lemer et al., 1993). The weight loss of eggs during incubation is affected by the porosity of the eggshell and the humidity of the air around the eggs (Deeming, 1993). The first of these factors may be caused by a range of problems, including hen immaturity, time of season and factors relating to nutrition, disease and genetics (Black, 1995), some of which may be difficult to influence. In contrast, the second factor can be readily manipulated by producers in modem artificial incubators, emphasising the need for producers to routinely monitor incubator humidity, both directly and by regularly weighing eggs during incubation. The current work also suggests a linear relationship between egg weight at the start of incubation and subsequent hatchability. This contradicts the findings from commercial chicken production, where it is recognised that intermediate sized eggs have the best hatchability, with small and large eggs hatching less well (Wilson, 1991b). Length of egg storage has previously been considered an important predictor of hatchability for ostriches (Jensen et al., 1992, p. 41) and other poultry species (Yoo and Wientjes, 1991); however, the current work could not duplicate this finding. Not surprisingly, there was clear evidence in this study of both farm and pair-withinfarm clustering with respect to both the fertility and hatchability of incubated eggs. Further epidemiological work on the role of specific pair and farm-level variables upon the fertility and hatchability of farmed ostrich eggs is clearly warranted.
Acknowledgements I gratefully acknowledge the cooperation and perseverance of the 12 ostrich producers. Andrew Badley and Joan Hendrikz are thanked for undertaking the egg laboratory
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examinations and providing statistical advice, respectively. The study received financial support from Triple D Ostrich Farms Pty Ltd, The University of Queensland and the Rural Industries Research and Development Corporation.
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