Baixe The Ostrich (Avestruz) - ch7 f e outras Notas de estudo em PDF para zootecnia, somente na Docsity! Factors Affecting the Success of Commercial Incubation D.C. Deeming1 and A. Ar2 1Hatchery Consulting & Research, 17 Rowland Close, Wallingford, Oxfordshire OX10 8LA, UK; 2Department of Zoology, Tel Aviv University, Ramat Aviv 69978, Israel A review of scientific reports of fertility and hatchability of ostrich eggs incubated in different countries (Table 7.1) shows that results are highly variable. Fertility ranges from very poor (<50%) through to good (>85%), but there are no reports of fertility at the higher levels commonly experienced in the poultry industry (90–95%). As a consequence, hatchability of eggs incubated is poor with maxi- mal values at around 60%. Hatchability of fertile eggs is higher, but again at best only around 70% of fertile eggs are producing live chicks. There can be consid- erable variation in hatchability between farms (Deeming, 1995a) and between laying seasons (Philbey et al., 1991). Whilst it is certain that individual farming operations will achieve better results than shown in Table 7.1, these data do illus- trate two general problems in ostrich production, i.e. low fertility and reduced hatchability, which appear to be prevalent in farming operations worldwide. The aim of this chapter is to review those factors affecting successful artificial incuba- tion of ostrich eggs. Whilst the emphasis of this chapter is on problems with commercial, artifi- cial incubation, these can be better appreciated given an understanding of the natural nesting environment of the ostrich. Therefore, the chapter starts with a brief description of the environment in which nests are built by wild ostriches and some of the environmental parameters measured under incubating ostriches. The bulk of the chapter is based around the factors which affect commercial egg pro- duction and incubation and how these impact on hatchability. Where possible, suggestions are made about how existing problems could be investigated and pos- sibly solved. 7 © CAB International 1999. The Ostrich: Biology, Production and Health (ed. D.C Deeming) 159 Ch 6-7 23/6/99 9:53 am Page 159 160 D.C. Deeming and A. Ar Table 7.1. Examples of hatchability of ostrich eggs incubated in commercial operations around the world. Percentage Percentage Country hatchability hatchability of incubation Percentage of eggs of fertile (country of origin) fertility incubated eggs Reference South Africa – 50 – Smith et al. (1995) South Africa 72.9 61.8 – van Schalkwyk et al. (1996) South Africa 81.9 46.2 56.4 Cloete et al. (1998) South Africa 70 70–80 – Verwoerd et al. (1998) Great Britain 74.8 24.1 31.9 Deeming (1996a) Great Britain 42.6 27.9 48.5 Deeming (1996b) Great Britain 86.7 60.0 69.2 Deeming et al. (Namibia) (1993) Great Britain 67.9 39.0 58.2 Deeming et al. (Namibia) (1993) Great Britain 69.2 49.2 71.1 Deeming and (Namibia) Ayres (1994) Great Britain 77.8 37.2 51.5 Deeming (Zimbabwe) (1995a) Great Britain 82.4 47.6 57.7 Deeming (Bophuthatswana) (1996b) Great Britain 84.2 34.9 41.5 Deeming (the Netherlands) (1996b) Australia 51.3 58.4 – More (1997) Australia 67.9 45.5 67.0 More (1996b) Zimbabwe 30.0 3.3 11.1 Foggin and Honywill (1992) USA 63 – 66 Wilson et al. (1997) Israel 55 43 77 Ar and Gefen (1998) Israel 73 58 80 Anon. (1999) – not reported. Ch 6-7 23/6/99 9:53 am Page 160 Factors Affecting Commercial Incubation 163 increase in temperature and approach the temperature of the brood patch (Fig. 7.1; Swart and Rahn, 1988). Since the young embryo tends to float to the top of the yolk to a position just under the shell, its incubation temperature is almost certainly closer to that of the brood patch than to the mean egg temperature. The relative humidity measured in an ostrich nest averaged 41% (SD 5%, range 32–52%) with fluctuations which matched that of the relative humidity of the ambient air (Bertram and Burger, 1981b). The moisture content within the nest was higher than the ambient air (2.1 versus 1.6 kPa, respectively). Similar results were described by Swart et al. (1987), although the water content of the air in the nest during female, diurnal incubation, and the ambient air were lower (1.3 versus 0.99 kPa) than male, nocturnal incubation (1.57 versus 1.12 kPa for nest and air humidity, respectively). Swart et al. (1987) showed that although nest humidity was higher, about 60% of the variation in nest humidity could be explained by variations in ambient humidity. On average, the nest humidity throughout incubation recorded by Swart and Rahn (1988) was 1.76 kPa, com- pared with the ambient air of 0.63 kPa. These values are at the lower end of the range of nest humidities so far recorded for a variety of birds from a variety of nesting habitats (Rahn, 1991). Changes in the gaseous environment have not been recorded in an ostrich nest, but can be inferred from humidity changes between the nest and ambient, egg water loss and embryonic oxygen consumption. This calculation reveals a maximal decrease in nest oxygen of up to 0.1%. It can be assumed that any stale air that develops during the sitting behaviour of the adults is released once the adult stands to turn the eggs or to swap incubation duties with its partner. Egg Fig. 7.1. The relationship between egg temperature and day of incubation in infer- tile (❍) and fertile (●) ostrich eggs incubated by adult birds. Redrawn using data from Swart and Rahn (1988). Ch 6-7 23/6/99 9:53 am Page 163 turning appears to be relatively infrequent in the ostrich, with the position of the eggs being regularly changed only when the male and female swap roles on the nest, although the adult will swivel on the nest, and sometimes stand and roll the eggs with its beak (Sauer and Sauer, 1966; Siegfried and Frost, 1974). Hatching in the nest occurs after an average incubation duration of 42–43 days and over 4–5 days (Sauer and Sauer, 1966; Jarvis et al., 1985a; Bertram, 1992). Jarvis (1994) reports that adult birds aid chicks in externally pipped eggs to hatch by crushing the shell with their sternum, but this report requires clarifi- cation. The chicks call out during hatching (Sauer and Sauer, 1966) although it is unclear to what extent this helps to synchronize the hatch of the clutch. It should be noted that if we accept the fact that in nature, population sizes are more or less stable, then on average each adult ostrich is replaced during its life span by only one other, and all others must perish as eggs, chicks or young. Thus different rules would apply to incubation success in nature, compared to artificial incubation ‘success’. COMMERCIAL ASPECTS OF EGG PRODUCTION Chemical composition of eggs Although the chemical composition of ostrich eggs has been described in some detail (Osuga and Feeney, 1968; Feeney and Allison, 1969; Deeming, 1993; Angel, 1995; Sales et al., 1996), very little research has been involved in deter- mining whether nutritional components of ostrich eggs influence hatchability, although cases of club-down, normally associated with riboflavin deficiency in the egg, have been reported (Deeming, 1997). One interesting project compared the lipid composition of egg yolks from farmed ostriches with that of yolks from eggs laid by birds living wild on a game ranch and with access to natural vegetation (Noble et al., 1996). There were no differences in the total lipids and proportions of lipid fractions, but significant differences were observed in the fatty acid profiles of the yolks. In the yolks from wild birds, all of the lipid fractions exhibited substantial concentrations of 18-carbon polyunsaturated fatty acids. In eggs from farmed birds the amounts of linolenic acid were only 10% of those observed in wild eggs. Similar levels of linolenic acid were also recorded in the yolks of ostrich eggs laid in Germany (Reiner et al., 1995). It has not been possible to date to link poor hatchability of farmed eggs with these differences in yolk composition, but it is known in poultry that an imbalance in essential fatty acids influences hatchability (Noble et al., 1986). Further research on nutritional deficiencies in ostrich embryos is required. 164 D.C. Deeming and A. Ar Ch 6-7 23/6/99 9:53 am Page 164 Egg size The ostrich egg is unusual for its large size, averaging 1545 g with a range of mass of 1–2 kg (Deeming, 1993). One sample of almost 17,000 eggs from Israel had an average mass of 1461 g (SD 163 g; Ar et al., 1996). This makes the ostrich egg the largest laid by a living bird, yet it is also one of the smallest in proportion to the body mass of the female (Rahn et al., 1975; Bertram, 1992; Deeming, 1993). This large egg size is interesting in a general zoological context, but it also has an important impact on commercial artificial incubation. The range in mean egg mass for all other species of birds (from hummingbird to emu) is around 700 g with a range in incubation periods of around 40 days. Allometric relationships (Rahn and Ar, 1974) suggest, however, that the duration of incubation of a 1500 g ostrich egg should be 58.8 days when based on egg mass alone, or 50 days when water vapour conductance of the eggshell is included in the calculation. By contrast, the ostrich embryo has an incubation period of 42 days and typical variation around this mean is only 2–3 days, despite the consid- erable range in egg mass. Therefore, compared with other birds, the duration of incubation is considerably shorter than predicted from egg mass. Ar and Gefen (1998) have shown that the rate of organ differentiation dur- ing the first half of incubation is fast, relative to that described for the fowl (Table 7.2). Moreover, the relative rate of growth during this period is much slower in the ostrich embryo. At 65% of the incubation duration, the ostrich embryo has attained a dry mass of only 19% of the dry hatchling mass, compared with a value of 32% in the fowl embryo. By contrast, during the last 35% of the incubation duration, the rate of mass gain is much faster than for other species. Furthermore, the ratio of true hatchling to residual yolk is 44:56 in the ostrich hatchling com- pared with 54:46 in the fowl (Ar and Gefen, unpublished observations, 1998). The ostrich embryo appears to have been able to accelerate development and hatches with relatively more residual yolk than typical precocial species. The rea- sons behind this strategy are unclear, but may lie with reduction of predation pressure on adults during incubation. The differences in egg mass in the ostrich have interesting implications for the rates of embryonic growth in individual eggs. Hatchling mass (65.6% of ini- tial egg mass) from a 1200 g egg would be 787 g, compared with 1180 g hatchling from a 1800 g egg. The difference in incubation period is only 3 days (Deeming et al., 1993). For the fowl egg, mass has no influence on embryonic mass at 12 days (Burton and Tullett, 1985) and so at 21 days of development the ostrich embryos in the two egg sizes will have a mass of around 19 g (Ar and Gefen, 1998). Therefore, the average rate of growth of these two embryos during the second half of development would be 37 g day–1 for the 1200 g egg, and 55 g day–1 for the 1800 g egg. This variation in embryonic growth rates poses the questions: how is the rate of embryonic growth controlled?; and how does the embryo ‘know’ how large its egg is (and hence its final body mass)? Such questions certainly apply in other species but the effects are exaggerated in the ostrich, making it ideal for research in this field. Factors Affecting Commercial Incubation 165 Ch 6-7 23/6/99 9:53 am Page 165 unable to find such a structure. The pores are complex, branching from a single opening on the inner side to a few openings on the outer side, usually within a shallow groove (Tyler and Simkiss, 1959; Tullett, 1978). Some authors (Sauer et al., 1975; Christensen et al., 1996; Richards and Richards, 1998b) report that a cuticle is present, but Sparks and Deeming (1996) suggest that it is absent and the outermost structure in the eggshell is the surface crystal layer, measuring approximately 4 µm thick. Careful examination of pub- lished scanning electron micrographs (Sauer et al., 1975; Christensen et al., 1996) would support this idea but there is scope for a detailed study to clarify this dis- agreement. Abnormal shells described by Sauer et al. (1975) were all commonly associ- ated with failure of normal embryonic development, although some shell defects were less serious. Eggs with shells exhibiting linear pore grooves did allow normal development. Failure in the processes of shell formation, producing either exces- sively thick or thin shells, compromises the ability of the shell to act as an appro- priate barrier between the embryo and the incubation environment. In particular, it affects shell gas and water vapour conductance and the efficiency of the shell to act as a physical barrier to microbial contamination (Deeming, 1995a, 1996a). Button et al. (1994) classified the quality of the shells of eggs which failed to hatch in a sample of 408 eggs. Around 20% of the eggs had rough shells, 16% had matt or chalky shells, 22% had thin shells (at the equator <1.47 mm) and 10% had high pore counts at the equator of the egg (>27.6 pores cm–2). Satteneri and Satterlee (1994) also found that high and low pore numbers in ostrich eggshells were associated with reduced hatchability. Unfortunately, it is often the case that almost all ostrich eggs collected are set to incubate irrespective of their quality. Deeming (1997) advocates that selection and management of hatching eggs is very important in determining the overall success of a commercial operation. Selection against and removal of cracked or deformed eggs, together with those having poor quality shells, i.e. chalky, wrinkled, or excessively dimpled, will in the long run increase the success rate of those eggs which are incubated. Egg production The success of ostrich farming depends largely on the production of fertile eggs. Nevertheless, there have been few reports of either rates of egg production or fer- tility of ostriches maintained on farms. Those reports which do describe these aspects of incubation (Table 7.1) show that, compared with poultry (Hodgetts, 1991), ostrich breeding has a long way to go to improve performance. Egg production per hen in Oudtshoorn, South Africa is only 50620 eggs during a June–February season of 120 laying days (Smith et al., 1995). The breed- ing performance of pairs of ostriches was recorded by van Schalkwyk et al. (1996) over five breeding seasons (1990–94) in South Africa. The average yearly egg production per hen was 55.5 eggs (SD 26.2) although the length of the breeding season differed from year to year. Egg production performance (EPP), i.e. the 168 D.C. Deeming and A. Ar Ch 6-7 23/6/99 9:53 am Page 168 number of eggs laid expressed as a percentage of the number of days on which eggs could be laid, was used to normalize results. The average EPP was 46.1% (SD 20.8%) with a range of 0–93.2% for individual pairs and shows a strong depend- ence effect. The EPP of individual pairings during the first breeding season could be used to predict the EPP in subsequent years (average EPP in subsequent years = 7.8160.763 EPP in first breeding season); EPP was negatively correlated with infertility and positively correlated with hatchability, and had a repeatability coef- ficient of 0.47. An EPP of 43.8% (SD 21.9%) was found for the same South African birds recorded from 1990–1996 (Cloete et al., 1998). The EPP of the breeding birds increased from an initial value of 30% at 2 years of age to a peak of 60% at 9 years, but showed a slow decline thereafter to around 45–50% at 17 years of age. EPP had a repeatability coefficient of 0.42. The bulk of egg production in Israel is between mid-February and September (Ar, 1996). Degen et al. (1994) reported that in a small experimental flock in southern Israel, over a 7-month laying season EPP was only 29.2%. Other stud- ies of productivity in commercial flocks in Israel record EPP of mature birds to be 54% (Anon., 1999). In Queensland, Australia, the bulk of the breeding season was between July and March (More, 1996a). The productivity of farmed ostrich hens in Australia was very low, with over 50% of hens maintained in pairs not laying any eggs dur- ing the season studied (July 1993 to June 1994). Only 910 eggs were laid in a year by 61 hens, and 88.8% were laid by birds maintained in pairs. Productivity of the 33 hens never maintained in a trio or a larger group was very low, and 18 of these birds did not lay during the study period. Productivity was higher for older hens. More (1997) recorded egg production of 38 hens and found that the average was only 2.4 eggs laid per hen per month (EPP=16%). In Britain, hen productivity was reported for the 1995 laying season on one farm with a total of 43 laying birds (Deeming, 1996a). EPP of the flock over the entire season was only 25.2%. The period of highest egg production was between mid-April and mid-September. Mean EPP values for individual breeding enclo- sures varied from 25.7 to 73.0%, with larger breeding groups producing fewer eggs per hen per week. EPP of individual birds generally ranged from 25.7 to 69.4% and only one hen (in a trio) was laying at close to her potential (EPP was 91.2%). Within trios, the two hens did not usually contribute equal numbers of eggs. Furthermore, increases in productivity were observed for individual birds from the 1994 to the 1995 season, and for individuals after one female was removed from an established trio of birds. Environmental stresses such as sudden rain or sudden heat spells can reduce egg laying temporarily (Anon., 1999). Such poor results for egg productivity will need to be addressed if ostrich farming is to become more profitable. Factors determining egg production have not been investigated, although the courtship behaviour of the ostriches appears to be important (Deeming and Bubier, Chapter 4), and this is certainly an area for further investigation. A few reports suggest that the pattern of egg laying may influence hatcha- Factors Affecting Commercial Incubation 169 Ch 6-7 23/6/99 9:53 am Page 169 bility. Wilson et al. (1997) report that, in the USA, hatchability of ostrich eggs dropped as the breeding season progressed. Deeming (1996a) showed that when egg numbers were low, i.e. at the start and end of the season, then hatchability was very low, although this may reflect low rates of fertility or unfavourable con- ditions in partially empty incubators. Fertility Reports from around the world (Table 7.1) suggest that the fertility of ostrich eggs is highly variable, with average values being low relative to other poultry species. Very few studies have examined the fertility of commercial ostriches and this cer- tainly is affecting development of the ostrich industry. Determination of true fer- tility is important, because candling at 14 days will show which eggs are not developing but does not indicate which eggs contain young dead embryos. A pilot study in Israel shows that some of the eggs considered infertile are actually fertile eggs in which the embryo died at a very young age (Ar, unpublished results, 1998). There is a need to open eggshells to confirm the absence of any embryonic development. Fertility of ostriches at the Oudtshoorn Experimental Station, South Africa, averaged 82.9%, but there was a range of 0–100% in those studied by van Schalkwyk et al. (1996). Cloete et al. (1998) reported that fertility was 81.9% in the same ostriches, recorded over a longer time period. In Australia, fertility for ostriches on 38 farms was an average of 51.3% (More, 1997). A smaller selection of 12 farms had slightly better results (68.1%), but fertility varied considerably between farms (27.4–91.2%; More, 1996b, 1997). On a British farm, weekly fertility rates during the 1995 season averaged 74.8% (SD=15.3%) for ostriches maintained in a variety of breeding situations (Deeming, 1996a). It started from a low of 50% at the beginning of the season, and rose to a plateau of over 80% by week 10 of laying, which was maintained until around week 26 of lay. Thereafter, fertility was variable, although high fer- tility was usually associated with high rates of lay. Over the season there was no significant correlation between the number of females in a breeding group and the fertility of the eggs produced in that group, although on average pairs and trios did produce more eggs per hen than birds in larger groups. The average weekly fertility of individual hens ranged from 32–100%. Factors determining the fertility of these ostriches have yet to be fully inves- tigated but will certainly include behavioural considerations. Deeming (1996a) reports that when in each of two enclosures one of the resident hens in the trios was killed, fertility of the eggs from the enclosure significantly increased. Mate compatibility and freedom from interference in courtship will be important fac- tors in determining whether mating takes place (Deeming, 1997; Deeming and Bubier, Chapter 4). Furthermore, Bubier et al. (1998) found that high rates of courtship behaviour of male ostriches towards humans were negatively correlated with fertility of eggs. Despite these studies, the reasons why fertility is generally 170 D.C. Deeming and A. Ar Ch 6-7 23/6/99 9:53 am Page 170 A common problem with ostrich incubation is the practice by many farmers of assisting chicks to externally pip and hence to hatch (Burger and Bertram, 1981; Jarvis, 1994; Deeming, 1997). Often these chicks are associated with eggs which have lost insufficient or too much moisture during incubation (Ar et al., 1996). Furthermore, Deeming and Ayres (1994) report high mortality of assisted chicks with poor growth rates in the survivors. Although adult ostriches were reported to help crack their eggs after external pipping (Jarvis, 1994), this prac- tice should be discouraged in breeding programmes because there is a danger that weak genetic lines, which are unable to hatch unassisted, are perpetuated in the breeding stock instead of being identified and culled. FACTORS AFFECTING COMMERCIAL HATCHABILITY Microbial contamination In a study of near-term embryonic mortality of ostrich eggs in South Africa, bac- terial contamination was observed in 13.4% of the eggs examined and the organ- isms isolated were typical of soil and faecal environments (Brown et al., 1996). In Australia, 26.5% of dead-in-shell eggs had microbial contamination, most of which were of a faecal origin (Button et al., 1994). Eggs incubated in Zimbabwe showed a high incidence of microbial contamination, with 45% of infertile and 67% of dead-in-shell eggs being infected with a variety of soil and faecal bacte- ria and fungi (Foggin and Honywill, 1992). Eggs imported into Britain from Zimbabwe also showed a high rate of microbial contamination (Deeming, 1995a); 22.8% of all eggs and 36.3% of fertile eggs were infected with bacteria and/or fungi, although the contamination rate of eggs varied according to which farm supplied the eggs. The organisms isolated were all of soil and faecal origin. Deeming (1996b) found that eggs from a variety of sources (Bophuthatswana (South Africa), the Netherlands and Britain) had rates of microbial contamina- tion of 18–20%. The incidence of fungal contamination was higher in eggs in which the embryo had survived to the end of development. In Britain the con- tamination rates of eggs laid during the 1995 season were very high, with a third of all eggs incubated being contaminated (Deeming, 1996a). The proportion of eggs contaminated varied between breeding enclosures (2.8–60.7%) and between individual hens (7.0–57.7%). On a weekly setting basis of fertile eggs there was a significant negative correlation (r=–0.873, P<0.001, DF=23) between percentage hatchability and percentage contamination (Deeming, 1996a). Shell properties are very important in determining the risk of microbial con- tamination. Shells with high conductance values have a higher risk of contamina- tion. Deeming (1996b) found that in two of three samples, eggs with microbial contamination had average percentage mass loss values which were significantly higher than those observed for uncontaminated eggs. This relationship is confirmed Factors Affecting Commercial Incubation 173 Ch 6-7 23/6/99 9:53 am Page 173 by a significant correlation between the percentage of eggs containing microbial growth in different water loss categories (Deeming, unpublished data, 1998). Sanitation programmes for ostrich eggs are invariably based on procedures employed for poultry eggs, and vary from egg washing through to simply brushing off soil (Deeming, 1997). van Schalkwyk et al. (1998) compared hatchability of eggs disinfected using exposure to UV light, or washed with either a peroxide- or quaternary ammonium-based product. The UV light maximized hatchability, but it was not different to that obtained from untreated controls. The washing disin- fection procedures appeared to depress hatchability by 6–10% due to increased early mortality (van Schalkwyk et al., 1998). Given their shell characteristics, there is a need for additional research to develop an appropriate sanitation pro- gramme for ostrich eggs. Pre-incubation storage The importance of storage of ostrich eggs to the ostrich industry cannot be under- estimated. The seasonal nature of laying in ostriches requires that eggs be stored in the beginning and at the peak of the laying season to overcome incubator capacity problems (Ar, 1996). In the wild, a clutch of ostrich eggs often numbers between 10 and 12 eggs (Bertram, 1992). This means that the first egg laid in the clutch has spent 18–22 days exposed to the elements before incubation is started. In a farming environ- ment, regular removal of eggs means that the laying season extends over several months. This is characterized by low egg production by the flock at the beginning and end of the season, with a peak in production during the middle of the season (Ar, 1996; Deeming, 1996a). It is often the case that eggs are stored for up to a week before being artificially incubated. At the start and end of the season, stor- age may be even longer in order to ensure that sufficient eggs are set. Research into the effects of pre-incubation storage is limited and most com- mercial practice in relation to storage is derived from the poultry industry. Although the difference only approached significance, ostrich eggs collected in the evening had lower mortality than eggs collected during the following morn- ing (van Schalkwyk, 1998), confirming the suggestion of Deeming (1995a, 1996a, b, 1997) that ostrich eggs should be collected soon after laying so as to minimize the incidence of microbial contamination and other factors which reduce egg quality. Egg position (i.e. with the air space at the top or bottom, or with eggs held with long axis horizontal) did not affect embryonic mortality of ostrich eggs stored for up to 6 days (van Schalkwyk, 1998). Swart (1978) showed that ostrich eggs stored at temperatures of 20–23°C for up to 14 days showed little decrease in viability, whereas longer periods of storage caused a significant depression in hatchability. In one batch of ostrich eggs from the Netherlands, Deeming (1996b) showed that as storage period (exact condi- tions not known because the eggs were supplied by a commercial source) increased, the hatchability of eggs set decreased from around 60% at 9–10 days’ 174 D.C. Deeming and A. Ar Ch 6-7 23/6/99 9:53 am Page 174 storage to zero at 17 days. Deeming (1996b) showed that for 12–14 days of stor- age there was only 50% hatch of fertile eggs, and this was associated with high early mortality (1–7 days) rather than late mortality. Wilson et al. (1997) showed a similar decline in hatchability in two sets of eggs stored for varying lengths of time, with increasing storage periods causing an increase in the age of dead embryos. In Israel, storage of eggs at 15–16°C for periods of 1–7 days showed that hatchability of fertile eggs was depressed by up to 4%, for eggs stored for either 1 day or for 6–7 days (Ar and Gefen, 1998). This depression in hatchability was associated with an increase in early mortality. Ar and Gefen (1998) suggest that ostrich eggs may benefit from a storage period of only 3–4 days. By contrast, the hatchability of eggs stored at room temperature (mean of 20°C) for up to 7 days was not significantly decreased by the length of storage (Deeming, personal obser- vations, 1998). van Schalkwyk (1998) showed that storage temperature was a key factor in determining embryonic viability. Embryonic mortality was lower at a storage temperature of 17°C than at 25°C, although at neither temperature was mortality affected by a 12 h period of incubation before storage. The temperature of storage also affected the size of the blastoderm after 7 days’ storage, with the diameter doubling with each degree increase in storage temperature from 25 to 27°C (van Schalkwyk, 1998). As has been reported by Meir and Ar (1998) for poultry eggs, there appear to be beneficial effects of pre-heating of ostrich eggs prior to storage (Brand et al., 1998). Heating to 36°C for 4 h prior to less than 6 days’ storage at 17°C signifi- cantly increased hatchability by around 8%, by significantly reducing the per- centage of late deaths. By contrast, pre-warming of stored eggs to 25°C for 16 h prior to incubation had no significant effect on hatchability. Temperature The incubator temperature adopted for artificial incubation of ostrich eggs has usually been determined in the past with little scientific investigation. Deeming (1993) found that most studies used an incubation temperature of between 36.0 and 36.5°C, although eggs have been successfully incubated at temperatures ranging from 35.0–37.0°C. Incubation of eggs at a variety of temperatures between 36.0 and 36.7°C has shown that incubation at 36.4°C allows 50% of all chicks to hatch at 42 days (Ar et al., 1996). The duration of incubation to 50% hatch (I, days) is primarily a function of incubator temperature (T, °C) in forced draught incubators (I=1875.80–98.06T11.31T2; Ar et al., 1996). Ostrich eggs incubated in a home-made room kept at 35°C had an incubation period longer by 2–3 days as compared to incubation at 36°C (Jarvis et al., 1985b). A higher starting temperature (37°C) during single-stage incubation reduced the average incubation period by almost 3 days over a constant temperature of 36.0°C (Deeming et al., 1993). In nature, the top of the ostrich egg adjacent to the brood patch averages Factors Affecting Commercial Incubation 175 Ch 6-7 23/6/99 9:53 am Page 175 by Ar et al. (1996) who found that the average conductance of almost 10,000 eggs was 1173.8 (SD=347.3) mgH2O day–1 kPa–1. Deeming (1995a) measured water vapour conductance of 250 eggs to be 945.8 (SD=232.5) mgH2O day–1 kPa–1. The water vapour conductance of a 1500 g ostrich egg is predicted to be 1100 mgH2O day–1 kPa–1 (Ar and Rahn, 1978). As was pointed out by Deeming (1995a), mass specific water vapour conductance (Gsp) should be calculated so as to take egg mass into account. Average values for Gsp are 664.7 (Deeming, 1995a) and 785.3 mgH2O day–1 kg–1 kPa–1 (Ar et al., 1996). Both low and high values for Gsp were associated with high embryonic mortality (Deeming, 1995a; Ar et al., 1996). The duration of incubation is also affected by Gsp, with high-conductance shells allowing faster development than average and low-conductance shells (Fig. 7.3). The loss of water vapour from an egg is dependent on both the water vapour conductance of the eggshell and the partial pressure difference between the inside and outside of the egg (Ar et al., 1974). In practical terms the shell con- ductance is relatively fixed, although a decrease of around 5% on weeks 3 and 4 of incubation, then rising to 11% above initial values at the end of incubation, was found only in fertile eggs (Ar et al., 1996). Since the water vapour pressure inside the egg is always saturated at egg temperature, the changes in calculated shell conductance (based on incubator temperature) may stem from changes in egg temperature as development proceeds. The loss of water vapour from an egg is controlled by adjusting the humidity of the air in the nest or incubator. In most incubators, there is a single humidity setting, matched to hopefully achieve a 13% 178 D.C. Deeming and A. Ar Fig. 7.3. The effect of mass-specific water vapour conductance (Gsp) on the length of incubation. Mean Gsp(A)=100.265.90, N=1098; mean Gsp(C)=130.565.96, N=268. Ar and Gefen, unpublished results (1998). Ch 6-7 23/6/99 9:53 am Page 178 mass loss up to pipping for the average egg. Hence, eggs with low- and high-con- ductance eggshells will lose insufficient or excessive amounts of water, respec- tively. Matching the humidity of the incubator to the shell conductance is a way to optimize weight loss of individual eggs. Ar et al. (1996) showed that lowering humidity for low-conductance eggshells increased hatchability by around 5%, whereas around 9% more eggs with high-conductance shells hatched when humidity was increased. The problem with high-conductance eggshells lies in the excessive mass loss from the egg, leading to dehydration of the contents and mortality of the embryo. High-conductance eggshells are also more prone to microbial contamination (Fig. 7.4), which almost certainly lowers hatchability further. Often eggs with high mass loss have very poor quality eggshells and should not have been set in the first instance (Deeming, 1997). Such eggs can be saved in some cases by par- tially taping strips over the shell (Ar, unpublished observations, 1998). For nor- mal incubation humidity conditions, better egg selection to remove those eggs with poor quality shells will reduce the problem of egg dehydration. Low mass loss of ostrich eggs is considered a problem by many authors, because the relatively low humidity required to achieve the appropriate mass loss can be difficult to achieve in artificial incubation (Burger and Bertram, 1981; Ley Factors Affecting Commercial Incubation 179 Fig. 7.4. The distribution of mass specific water vapour conductance (Gsp) for all eggs and those with microbial contamination. Incubation temperature, 36.0°C; mean=42.8 days; SD=1.0, N=1018. Ar and Zemach Ostriches, unpublished observations (1998). Ch 6-7 23/6/99 9:53 am Page 179 et al., 1986; Deeming, 1993, 1997; Brown et al., 1996). The consequence of low mass loss is high mortality associated with heavy, oedemic embryos (Philbey et al., 1991; Deeming, 1995a, 1997; Brown et al., 1996). Malpositions, particularly where the embryo is incorrectly orientated towards the air space, are also com- mon in low-mass-loss eggs (Deeming, personal observations, 1994–1996). Oxygen and carbon dioxide exchange Insufficient water loss produces, among other things, a small air cell which may bring about difficulties in the hatching process. Such embryos have problems with internal pipping and the embryo makes an external pip hole closer to the air space pole of egg (Deeming, 1995b). Ar (1991) has suggested that a small air cell may prevent complete gas filling of the lungs and airsacs of the hatching embryo. Furthermore, Ley et al. (1986) found that most of the oedemic embryos examined had died before internal pipping, and considered suffocation to be the reason for death. Reiner and Dzapo (1995) showed that the rate of oxygen consumption (VO2) is associated with the vitality of the embryos in the few days before hatch- ing and could be used to predict whether individual embryos would hatch. Similarly, Tazawa et al. (1998) have shown that the heart rate of embryos likely to die is reduced. Low mass loss from eggs certainly causes problems due to the water retention in the embryo but, in addition, the low shell conductance restricts the oxygen uptake of the embryo and CO2 emission from it. The oxygen consumption rate of ostrich embryos follows a sigmoidal pattern with time, typical of precocial birds with a dip in VO2 a few days before internal pipping (Hoyt et al., 1978; Meir and Ar, 1990; Reiner and Dzapo, 1995; Smith et al., 1995; van Schalkwyk, 1998; Ar and Gefen, 1998). At its peak, VO2 averages around 100–120 ml kg–1 h–1 (Ar, 1996). Reiner and Dzapo (1995) found that embryos with high values for VO2 had shorter incubation periods. The diffusion of both oxygen and carbon dioxide is limited by the conduc- tance of the shell and the pressure difference of the gases between the inside and outside of the egg. Although not investigated in ostrich eggs as yet, it is known that below a certain eggshell conductance, fowl embryos become oxyconformers, i.e. their oxygen uptake is limited by the oxygen flux that can diffuse across the shell (Tullett and Deeming, 1982; Burton and Tullett, 1983; Ar et al., 1991). Higher-conductance shells do not restrict oxygen diffusion and so the embryos appear to regulate their rate of oxygen consumption. In this context it is inter- esting to note that humidity adjustment during incubation, which is capable of correcting the water problem, had less effect on embryonic mortality in the low- conductance ostrich eggs than in the high-conductance eggs (Ar et al., 1996). The respiratory quotient (RQ) of ostrich eggs is initially high (1.7 on day 8), probably due to the CO2 retained within the egg from its time in oviduct, but it gradually drops to 0.65 on days 22–28 and stabilizes at 0.68 during the last third of development (Fig. 7.5; Ar and Gefen, 1998). However, Meir and Ar (1990) reported an RQ in the last week of incubation of 0.79, and there is no plausible reason to explain the difference. 180 D.C. Deeming and A. Ar . . . . Ch 6-7 23/6/99 9:53 am Page 180 transfer into the hatcher. The similarities in the relative compositions of fowl and ostrich eggs (Carey et al., 1980; Deeming, 1993) suggest that the effects of not turning during incubation will be similar in both species. Most modern incubators have automatic mechanisms for turning eggs dur- ing incubation, although the angle and frequency of turn can vary considerably (Deeming, 1993). Turning through 630–45° at hourly intervals almost certainly satisfies normal development, but little research has been carried out on the importance of turning frequency and angle of turning in ostrich eggs. van Schalkwyk (1998) showed that as the angle of hourly rotation increased (from 60° to 90° via 10° increments), the level of both early and late embryonic mor- tality decreased. The orientation of the egg during incubation also appears to have an influence. Incubating eggs with their long axis horizontal for 2–3 weeks and then re-positioning the eggs with their long axis vertical for the rest of incu- bation, has beneficial effects on hatchability over positioning the eggs either ver- tically or horizontally throughout development (Smith et al., 1995; van Schalkwyk, 1998). This pattern of egg positioning during incubation has a signif- icant effect on hatchability if the angle of turn is relatively small (60°) but the advantage is lost when the angle of turn is 90° (van Schalkwyk, 1998). Ostrich eggs have low hatchability if not turned during incubation (van Schalkwyk, 1998) but do appear to be relatively tolerant of low turning fre- quency. Wilson and Eldred (1997) showed that hatchability of eggs turned eight times a day was not significantly different from that of eggs turned 24 times a day. Low-frequency turning is particularly effective if the turning angle is large. Hand turning of eggs through 180° only twice daily appears to be sufficient to produce reasonable hatchability, although this is lower than for eggs turned hourly (van Schalkwyk, 1998). The difference in hatchability appears to be related to higher early embryonic mortality in the hand turned eggs (van Schalkwyk, 1998). Hand turning requires that the egg is unrestrained in the incubator and can adopt a natural orientation. In this way, as water loss proceeds during incubation, the air space end of the egg becomes less dense and this end of the egg, which is lighter, tends to rise, and probably aids the embryo to orientate towards the air space (Deeming, 1997). A characteristic problem associated with lack of egg turning is low hatcha- bility and residual albumen in the fowl egg (Tullett and Deeming, 1987). Badley (1996) showed that the apparent size of the extra-embryonic membranes as assessed by candling at day 35 of incubation of ostrich eggs, correlated with both the amount of residual albumen in dead-in-shell eggs and with hatchability. If the light area at the ‘sharp pole’ of the egg (i.e. the opposite end from the air space) was 0–10% of the shell area, then hatchability was above 87% of the eggs set. If the area was 20%, then hatchability fell to 75% and none of the eggs with larger clear areas hatched. Although hatchability figures were lower, the same pattern was observed for eggs incubated in Britain (Fig. 7.6; Deeming, personal observa- tions, 1998). The increase in the light area corresponds to which stage of embry- onic development individual eggs have achieved; eggs with a light area of 20% or more almost certainly contain an embryo which has slowed in development or Factors Affecting Commercial Incubation 183 Ch 6-7 23/6/99 9:53 am Page 183 has already died. Whether the extra-embryonic membrane development failure is the result or the cause of slow development early in incubation is a question that is not yet answered. IN CONCLUSION Research into incubation problems in ostrich eggs is still in its infancy and there are many areas of interest which require considerable work. These include: nutri- tional factors and breeder bird management in order to maximize fertility and hatchability; appropriate egg sanitation and storage protocols; appropriate incu- bation conditions, particularly temperature and turning; and the relationship between egg mass and embryonic growth. Any research needs to take into account the large variability in mass, shell properties and fertility with ostrich eggs, because these can influence experimental designs and hence the signifi- cance of any results. Uniformity of egg quality will not only improve the experi- mental assessment of incubation problems, but will also reap improvements in hatchability in commercial operations. The different requirements of the embryo in terms of temperature, ventila- tion, turning, water loss rate and O2 and CO2 pressures during different phases of incubation, which are starting to emerge from present knowledge, may indicate 184 D.C. Deeming and A. Ar Fig. 7.6. The relationship between the percentage of the shell which is dark (indicating the presence of an embryo) when candled at 35 days and percentage hatchability of fertile eggs. Deeming, unpublished observations (1998). Ch 6-7 23/6/99 9:53 am Page 184 that in the future the method of single-stage incubators, where conditions of incubation change to fit embryonic demands at different stages of development, will prevail. ACKNOWLEDGEMENTS Thanks to Ms Ann Belinsky for her skilled help during the preparation of this chapter. REFERENCES Angel, C.R. 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