A review of the calculation of energy, protein and amino acid requirements for maintenance and egg production of breeding ostriches

SUMMARY

Few studies have modelled the nutrient requirements of breeding ostriches. Results obtained from historical studies were used here to determine the energy, protein and amino acid requirements of breeding ostriches for maintenance and egg production. Two energy scales were used to determine the energy requirement for egg production by ostrich breeders. The efficiency of metabolisable energy (ME) utilisation for energy deposition in the egg (k_o) was calculated as 0.8, and the ME required for egg production (ME_e) was 12.2 MJ/egg. The effective energy (EE) requirement for egg production (EE_e) and maintenance (EE_m) was calculated as 7.8 and 13.8 MJ/bird d, respectively. Average total daily balanced protein requirement (TP_t) was calculated as 160 g/bird d. Amino acid requirements for maintenance and egg production are given together with recommendations for the daily supply of each amino acid.

KEYWORDS:

• Ostrich
• energy
• protein
• nutrition
• egg production
• nutritional requirements
• amino acids

Introduction

Despite the fact that South Africa has had a thriving ostrich industry for over a century, little is known about the nutritional needs of these birds in comparison to other domestic animal species (Z. Brand et al. 2003). Currently, breeding ostriches are fed diets high in both energy and protein in an attempt to increase productivity (Z. Brand et al. 2003) but this practice may not maximise the profitability of the enterprise (Z. Brand et al. 2003). The ultimate objective when feeding any domestic species is to provide each animal with an economic optimum level of essential nutrients, thereby maximising the profitability of the enterprise (Azevedo et al. 2021). To achieve this objective in breeding ostriches, the potential laying performance of the strain must be determined, from which the required information regarding their nutrient requirements may be calculated (T. S. Brand et al. 2010). Modelling is an appropriate tool to determine both the nutrient requirements of animals and the optimum economic level of these nutrients to be included in the feed, but has rarely been used in the case of breeding ostriches (T. S. Brand and Gous 2006; Du Preez 1991), although studies conducted by Brand et al. (2014) have focussed on how to optimise feed nutrient contents by decreasing costs while maintaining bird performance.

A limited number of trials have been conducted to determine the requirements of ostriches for maintenance (T. S. Brand and Olivier 2011; Du Preez 1991) and egg production (T. S. Brand and Olivier 2011; Du Preez 1991). By describing the process of egg production similar to that used by Johnston and Gous (2003), energy requirements may be calculated using the approach of Gous and Nonis (2010), who calculated nutrient requirements for egg production of broiler breeders based on their internal cycle length and on the weights of yolk and albumen, which, in the case of the laying hen and broiler breeder, change as the hen ages.

Another issue to be addressed is the evaluation of the energy content of feed ingredients used by ostriches which have historically been derived from poultry assays and which have been shown to underestimate energy values for ostriches. Evidence is provided by the classical work of Cilliers (1994) who demonstrated that the ostrich exhibits higher TME_n values than domestic poultry for most of the common ingredients, and by Brand et al. (2000) who reported that ostriches exhibit higher ME values for the same diets when compared to energy values derived using pigs (25%) and poultry (17%). Also, true and apparent amino acid digestibilities of a high protein diet were shown to be higher for ostriches than for poultry (Cilliers et al. 1997). These underestimates may have led to obesity in ostrich hens and, in turn, to a depression in egg production during the laying period (Z. Brand et al. 2003). Badley (1997) has suggested that excess fat deposits can prevent eggs from being deposited in the oviduct. More response studies need to be performed in order for nutritionists to base their decisions on meaningful information rather than making use of unsuitable data (T. S. Brand et al. 2015).

In contrast to the above observations on the effect of excess energy intake on reproduction, a study conducted by Olivier et al. (2009) found that dietary energy intake above 25.5 MJ ME per bird day had no influence on egg production of breeding ostriches. Brand and Gous (2006) and Brand and Olivier (2011) suggested that the optimal intake of energy for breeding ostriches is 22 MJ ME per bird day. The daily allocation of feed needs to be restricted, however, as breeding ostriches tend to overconsume energy when fed ab libitum (T. S. Brand et al. 2010).

As with the studies on excess energy intake on reproduction by Olivier et al. (2009), similar studies with dietary protein by Brand et al. (2015) revealed that diets ranging from 75 to 140 g/kg crude protein (CP), containing 2.9 to 5.8 g lysine/kg had no influence on reproduction of breeding ostriches. However, they cautioned against feeding higher levels than this as the excess protein might be detrimental to breeding birds.

It is evident from the above that the nutrient requirements of breeding ostriches are not well defined. The purpose of this review is an attempt to improve the estimates of energy and protein (amino acid) requirements of breeding ostriches, making use of published methods that have been applied to poultry. The chemical composition of eggs from studies performed by Brand (2002) and Olivier (2010) may be used to determine the protein and amino acid requirements for egg production, as well as the effective energy (EE) needed for albumen and yolk deposition, the latter making use of the theory developed by Emmans (1994). Protein and amino acid requirements for maintenance may be calculated from a model developed by Emmans (1989). Values from the study of Brand et al. (2005) may be used to determine the effective energy (EE) requirement for maintenance based on a model developed by Emmans and Fisher (1986). The efficiency of ME utilisation for deposition in the egg (k_o) and the ME requirement for egg production (ME_e) may be calculated according to the factorial method of Sakomura et al. (2009).

Changes occurring in the weight and composition of ostrich eggs

Brand (2002) and Olivier (2010) measured the weights and composition of ostrich eggs during different months of the breeding season, and the mean values obtained are given in Table 1. Small but significant increases occurred in egg (40.9 ± 10.8 g/month), albumen (28.5 ± 7.8 g/month) and yolk (8.37 ± 3.2 g/month) weights over the breeding season, but shell weight remained constant throughout. Egg protein content decreased (−1.30 ± 0.41 g/kg month) but the lipid content remained constant over the breeding season. These values may be used in more accurately calculating the energy, protein and amino acid requirements of ostriches during the breeding season.

Table 1. Changes in the weight and composition of ostrich eggs during the breeding season (Z. Brand 2002; Olivier 2010).

Month	Egg weight (g)	Albumen weight (g)	Yolk weight (g)	Shell weight (g)	Protein (g/kg)	Lipid (g/kg)
August	1264	668	322	274	116	99.3
September	1347	751	314	281	119	94.0
October	1370	752	329	285	121	104
November	1340	717	343	277	117	106
December	1424	792	348	281	113	97.8
January	1511	850	358	300	112	94.2
Mean	1376	755	336	283	116	99.2
SEM	45.6	32.3	13.7	8.69	1.59	3.99

Energy requirement for maintenance and egg production

Two approaches may be used to calculate the energy requirement of an average female ostrich during the breeding season. The first makes use of the classical ME system whilst the second, arguably more accurate, uses the effective energy (EE) scale proposed by Emmans (1994). Some of the information required for calculating the energy requirements of a breeding ostrich for maintenance and egg production is provided in Table 2, including the energy content (MJ/kg) of ostrich eggs (RE_e), the efficiency with which energy is deposited in the egg (k_o), the daily energy required for egg production (ME_e) and the total energy requirement per day (ME_t) (Olivier 2010). The latter is calculated according to the method of Sakomura et al. (2009) as described in Table 2. The ME_e calculation does not include the amount of energy needed for shell formation, since its contribution to the overall expenditure for egg production is small, although Du Preez (1991) included this in his calculation, suggesting that the energy required to deposit an egg shell was 1.2 MJ/kg.

Table 2. Estimates of the energy retained (RE_e)/kg in ostrich eggs, the efficiency with which energy is deposited in the egg (k_o), the daily energy required for egg production (ME_e) and the total energy requirement per day (ME_t) of breeding female ostriches during each month of the breeding season (Olivier 2010).

Month	REe^c(MJ/kg)	ko^d	MEe^e MJ/egg	MEe^f MJ/d	MEt^g MJ/d
August	6.74	0.79	12.1	6.0	20.6
September	6.65	0.78	12.1	6.0	20.6
October	7.07	0.83	12.1	6.0	20.6
November	7.03	0.83	12.1	6.0	20.6
December	6.61	0.75	12.1	6.0	20.6
January	6.44	0.76	12.1	6.0	20.6
Mean	6.90	0.80	12.2	6.0	20.6
Standard Deviation	0.1	0.1

^a,bMeans within column with different superscripts differ significantly (P < 0.05)

*Assuming a 210-day laying cycle, laying an egg every 2nd day

^cREe = 24.3 *OP + 39.6 *OL + 17.2 *OC (Chwalibog 1992). REe = retained energy in 1 kg egg in MJ; OP = protein content in eggs; OL = lipid content in eggs; OC = carbohydrate content in eggs; unit for content is kg/kg egg.

^dko = REe/[MEi-(MEm + MEg)] (Sakomura et al. 2009). ko = efficiency of ME utilization for deposition in the egg; MEi = ME intake/kg feed (MJ); MEm = ME requirement for maintenance per kg body weight (MJ); MEg = ME requirement for growth per kg body weight (MJ)

^eTMEe = ME needed for the formation of one egg (MJ) = REe/ko (Sakomura et al. 2009)

^fMEe = daily ME needed for egg production (MJ) = TMEe/2 d

^gMEt = total ME needed, MJ = MEe + MEm

**MEm (ME needed for maintenance) = 14.6 MJ/day (Cilliers 1994)

The average k_o for ostriches of 0.80, reported in Table 2, falls within the range reported for poultry, of between 0.60 and 0.85 (Chwalibog 1985; Luiting 1990; Sakomura et al. 2009), this never having previously been calculated for breeding ostriches. Differences in efficiency have been reported to be related to the rate of egg production, being lower when the rate of lay declines (Fisher et al. 1973; Wethli and Morris 1978). However, in a study by Olivier (2010), neither the age of the breeding birds nor the month of the breeding season influenced RE_e or k_o (p > 0.05) (Table 2). It thus appears that the energy required by ostriches to produce a kg of egg remains the same throughout the season.

The pattern of energy deposition in eggs (OE) was also investigated by Olivier (2010). In the period between August and December, there was no difference (p > 0.05) in the ratio of energy deposited in the egg as fat (OFE) and as protein (OPE). The age of the birds also had no influence on this ratio. This suggests that the ratio of yolk to albumen in the ostrich egg over the breeding season remains relatively constant.

From the above it appears that the efficiency of utilisation of an amino acid or of energy remains relatively the same amongst ostrich breeders irrespective of laying performance resulting in an ME requirement for egg production (ME_e) of 6.0 MJ/d (Table 2). Differences amongst individual females in their daily requirement for energy would therefore be in the amount used for maintenance. Based on a bird weighing 100 kg, the ME_m would be 14.6 MJ/day (Cilliers 1994). The calculated ME_t value of 20.6 MJ ME/d (Table 2) is similar to the value of 22 MJ ME/d proposed previously by Brand et al. (2003), and should be considered as the minimum daily energy requirement.

A more accurate estimate of the energy required by breeding ostriches may be obtained with the use of the effective energy (EE) scale (Emmans 1994). Using appropriate equations described in Table 3, the EE for maintenance (EE_m) was calculated as 13.8 MJ/bird d for a bird with a mature protein weight of 18.6 kg (Du Preez 1991). In this case, the maintenance requirement is based on body protein weight, which is more accurate than an estimate based on body weight, which includes body lipid. The EE_e requirement for the different months, with a mean of 7.8 MJ EE/bird/d, is presented in Table 3. These values apply to breeding female ostriches at all ages since the proportion of albumen and yolk in the egg, and hence egg composition, remains relatively unchanged at different ages (Olivier et al. 2009). This is different from broiler breeders, where the composition of eggs changes systematically with age (Johnston and Gous 2007).

Table 3. Mean effective energy (EE) requirement for the production of an egg (TEE_e), daily requirement for egg production (EE_e), and total EE requirement/d (EE_t) for each month of the breeding season (Olivier 2010).

Month	TEEe (MJ/egg)^a	EEe (MJ/d)^b	EEt (MJ/d)^c
August	14.3	7.2	21.0
September	15.3	7.6	21.4
October	15.5	7.8	21.6
November	15.2	7.6	21.4
December	16.2	8.1	21.9
January	17.1	8.6	22.4
Mean	15.6	7.8	21.6
SEM	1.0	0.5	0.5

^aTEE_e = total EE requirement for the formation of one egg (MJ) = EE_a + EE_y; EE_a = EE requirement for albumen formation (MJ) = 50 kJ/g protein; EE_y = EE requirement for yolk formation (MJ) = 50 kJ EE/g protein +56 kJ EE/g lipid (Emmans 1994).

^bEE_e = daily EE requirement for egg production (MJ) = TEE_e/2 d.

^cEE_t = total EE requirement (MJ) = EE_e + EE_m, where EE_m = daily EE required for maintenance (MJ) = M_E ∙ BP_m^0.73 = 13.8 MJ/d (Emmans and Fisher 1986); M_E = 1.63 MJ/unit day; BP_m = mature body protein weight excluding feathers, kg.

In calculating the energy required for egg production using the ME system, Chwalibog (1992) and Sakomura et al. (2009) used the energy contained in yolk, albumen and carbohydrate (24.3, 39.6 and 17.2 MJ/kg, respectively, Table 2) whereas Emmans (1994) and Gous and Nonis (2010), when applying the EE system, used values of 50 and 56 MJ/kg for the amount of energy required for the deposition of protein and lipid, respectively, and ignored the carbohydrate fraction, which is negligible. These latter amounts take account of the amount of energy deposited plus the work done in the process of depositing these components. Taking account of a higher energy requirement for maintenance when using the EE approach of Emmans (1994) and of the work done in depositing the yolk and albumen in the egg results in a higher estimate of the requirement for EE than for ME (20.6 MJ ME vs. 21.6 MJ EE). This reflects a difference in approach in calculating the requirement and not a difference in the amount of energy required.

Protein and amino acid requirements for maintenance and egg production

The total daily amount of protein required by a breeding ostrich (TP_t) includes that for maintenance (TP_m) and for egg production (TP_e). TP_m was calculated as 70 g/d using the equation of Emmans (1994) and a value of 18.6 kg for BP_m (Table 4). The TP_t, presented for each month of the breeding season in Table 4, averaged 160 g protein/d, remaining fairly constant throughout the season. The mean value for the protein content of an egg (Table 4) is lower than that calculated by Du Preez (1991) (127 vs. 138 g/egg) whereas the values for TP_m are virtually the same (70 vs. 72 g/d, respectively).

Table 4. The crude protein (CP) content of an egg, the total amount of protein required for the production of an egg (TPE_e), the daily protein requirement for egg production (TP_e), and that required daily for maintenance (TP_m) plus egg production (TP_t) during each month of the breeding season (g) (Olivier 2010).

Month	CP content of egg (g)^a	TPEe (g)^b	TPe (g/d)^c	TPt (g/d)^d
August	114	163	81.4	151
September	127	182	90.9	161
October	132	188	94.1	164
November	125	179	89.4	159
December	129	184	91.9	162
January	135	193	96.4	166
Mean	127	181	90.7	160
Standard deviation	7.3	10.4	5.2	5.2

^aMean Crude Protein (CP) content of egg on an as is basis.

^bTPEe = total protein requirement for the formation of one egg (g) = CP/0.7 (0.7 being the assumed efficiency of utilisation of dietary protein for egg production).

^cTPe = daily protein requirement for egg production (g) = TPEe/2 d.

^dTPt = total protein requirement, g = TPm + TPe where TPm = total protein requirement for maintenance = 0.008. BPm0.73 = 70 g/d.

The protein requirement reported above would apply to a well-balanced mixture of essential and non-essential amino acids. The required balance may be estimated by accounting for the amino acid contents of the yolk and albumen (Gous and Nonis 2010, Table 5), the relative proportions of yolk and albumen in ostrich eggs (Z. Brand 2002; Olivier 2010, Table 2), an efficiency factor (Emmans and Fisher 1986 suggested a value of 0.85 for amino acids), and the amount of each amino acid required for maintenance, using the equation of Emmans and Fisher (1986), Table 5. A breeding ostrich hen was assumed to contain 18.6 kg body protein at maturity (Du Preez 1991) when calculating maintenance requirements, and to be producing an egg as described in Table 1. Using appropriate values from Lunven et al. (1973), from Emmans and Fisher (1986) and from Fisher and Gous (2009), the amino acid contents of yolk and albumen (mg/g) were estimated and are given in Table 5. Assuming an efficiency of utilisation of amino acids of 0.85 (Emmans and Fisher 1986), the intake of each amino acid required to meet these requirements was then calculated. As an egg is laid, on average, every second day, the requirement was halved, and together with the requirement for maintenance (AA_m), the daily requirement for each amino acid (AA_t) was calculated and is given in Table 5. These values are in good agreement with those of du Preez (1991), while the AA_m values are marginally lower than those reported by du Preez (1991).

Table 5. The amino acid content in the yolk and albumen of an ostrich egg (AAC_e), the total amount required per egg (AA_e), the daily amount required per egg (DAA_e), the requirement for maintenance (AA_m), and the total amino acid requirement/d (AA_t) (Olivier, 2010).

Amino acid	Content, mg/g, in
	Yolk^a	Albumen^b	AACe ^c (g)	AAe ^d (g)	DAAe ^e (g/d)	AAm^f (g/d)	AAt ^g (g/d)
Arg	13.8	6.6	9.62	11.3	5.66	4.85	10.5
Ile	11.1	6.6	8.73	10.3	5.13	2.85	7.98
Lys	15.2	7.6	10.8	12.7	6.35	5.35	11.7
Met	5.6	4.8	5.51	6.5	3.24	1.78	5.02
Thr	9.9	5.4	7.44	8.8	4.38	3.00	7.38
Trp	3.9	2.3	3.05	3.6	1.79	0.71	2.50

^aFrom Gous and Nonis (2010); mean yolk weight of 336 g (Table 2).

^bFrom Gous and Nonis (2010); mean albumen weight of 755 g (Table 2).

^cAAC_e = amino acid content of an egg on as is basis.

^dAA_e = amino acid requirement for formation of one egg (g) = AAC_e/0.85 (0.85 being assumed efficiency of utilisation of amino acids for egg production).

^eDAA_e = daily amino acid requirement for egg production (g) = AA_e/2 d.

^fAA_m = amino acid required for maintenance, g = 0.008 ∙ BP_m^−0.27 ∙ BP ∙ AAC (kg/d). 0.008 kg = protein required per maintenance unit (Emmans 1989); BP = body protein weight, kg excluding feathers; BP_m = mature protein weight, kg; AAC = amino acid content in BP excluding feathers, g/kg.

^gAA_t = total amino acid requirement (g) = AA_m + DAA_e.

Assuming that female ostrich breeders consume 2.5 kg dry matter/d and that they lay an egg every 2.6 days (Olivier 2010), the concentration of crude protein and of each amino acid in the feed may be calculated from the above requirements. For example, the CP content of the feed would need to be 160/2.5, or 64 g/kg and lysine, 11.7/2.5 = 4.68 g/kg.

Conclusion

Previous studies have revealed that a daily ration below 22 MJ ME per bird per day will result in lower egg production (Z. Brand et al. 2003), while the current calculations suggest that the minimal total energy requirement to produce an average size egg is 20.6 MJ ME/d or 21.6 MJ EE/d. Experiments by Brand et al. (2010) concluded that the energy required to produce an ostrich egg appears to be the main constraint to egg production during breeding, as different concentrations of dietary protein had no significant effect on laying performance. Average TP_t was calculated as 160 g/d, or 64 g/kg. Amino acid requirement for maintenance (AA_m) and egg production (AA_e) calculated here are comparable with values presented by Du Preez (1991).

Of the two energy scales used in this study to determine the energy requirement for egg production, the EE, or net energy, scale is superior to the ME scale as it provides a more accurate estimate of the energy available for production purposes (Emmans 1994; Gous and Nonis 2010). Mean values for EE_m and EE_e were calculated as 13.9 and 7.8 MJ/bird d, respectively. To make use of these values, the EE scale must be applied in place of the ME scale.

The current study provides more information than has previously been published on the energy and amino acid requirements of breeding ostriches. Changes in albumen and yolk weights during the breeding season enabled more accurate estimates of the amino acid and energy requirements to be made over time, although these changes were shown to be small. Because of the large daily intake of feed by breeding females, the energy and amino acid contents required in the feed are low, casting doubt on the economic advantage of making minor adjustments to the feed each month during the breeding season. Nevertheless, the data presented here will assist producers to feed their animals more closely to their actual requirements.

Disclosure statement

No potential conflict of interest was reported by the author(s).