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Determining nutritional requirements by sex has been a research topic in numerous scientific works, both in the case of castrated males and entire males with the objective of more precisely meeting the requirements. In this article, the difference in requirements between entire males and females is quantified by analyzing the differential response of the standardized ileal digestible lysine to net energy ratio (SID Lys:NE) between both sexes. Our own experimental trials and a meta-analysis are used.
Empirical estimation of requirements
In a recent study (Aymerich et al., 2020) we compared the SID Lys requirements between entire males and females in the period from 70 to 105 kg LW. The results confirmed a differential linear response to increased SID Lys:NE between both sexes. Males showed a greater increase in growth and a greater reduction in feed conversion ratio (FCR) as the lysine-energy ratio increased. Likewise, modeling of requirements by sex showed that males required 17% or more SID Lys than females. While females required a SID Lys:NE ratio of 3.10-3.13 g/Mcal, males required 3.63-4.05 g/Mcal depending on the period (Figure 1).
Figure 1. Modeling the effect of lysine to energy ratio and average daily consumption in entire males and females between 70 and 105 kg LW (Aymerich et al., 2020).
Estimation of requirements by meta-analysis
The data from the above trials were combined with 13 other studies to analyze the effect of increasing SID Lys in entire males and females in the finishing period using a meta-analysis process (Aymerich et al., 2021). Figure 2 shows the results of the response in feed efficiency to the SID Lys:NE ratio and daily intake of SID Lys for each sex. The models show how performance differences are dependent on dietary amino acid levels, demonstrating that intact males require diets more concentrated in AA to reach their performance potential.
Figure 2. Modeling the effect of lysine to energy ratio and daily lysine intake in entire males (red) and females (blue) in finishing on the efficiency of feed conversion. (Aymerich et al., 2021).
Table 1 shows the optimal numerical results of the above models to achieve the desired performance level with each sex. It is interesting to note that for practically all relative performance levels males require between 0.14-0.16 % points more SID Lys in the feed, which represents an increase in requirements of 15-20%.
Table 1. Requirements in % SID Lys in a 2500 kcal NE/kg diet to achieve different target performance levels (Aymerich et al., 2021).
| Item | % of maximum performance | |||||
|---|---|---|---|---|---|---|
| 95 | 96 | 97 | 98 | 99 | 100 | |
| ADG | ||||||
| Entire males | 0.79 | 0.82 | 0.85 | 0.89 | 0.94 | 1.06 |
| Females | 0.65 | 0.68 | 0.71 | 0.75 | 0.80 | 0.92 |
| Feed efficiency | ||||||
| Entire males | 0.81 | 0.84 | 0.88 | 0.92 | 0.98 | 1.11 |
| Females | 0.67 | 0.70 | 0.73 | 0.77 | 0.82 | 0.95 |
Economic optimization
In addition to a different response in performance, the models obtained in the meta-analysis were used to determine the economic optimums for entire males and females according to the price context of synthetic AA and soybean meal between 70 and 100 kg LW. The results in the following table (Table 2) show that, depending on the price context and the production model, it will be more or less advantageous to differentiate the feeding of males and females. It is especially interesting that, in the context of low prices for soybean meal and AA, split-sex feeding is beneficial for males, since at higher SID Lys, they gain 1 kg more weight in the same period of time. However, in the context of high prices, the benefit comes especially from the reduction of the SID Lys in females, increasing the days in finishing, but with a more than € 1 higher margin per female.
Table 2. Economic modeling of SID Lys requirements by sex to obtain the highest margin per pig with fixed final weight or fixed time models.
| Fixed weight2 | Fixed time3 | |||
|---|---|---|---|---|
| Male | Female | Male | Female | |
| Context of low AA prices | ||||
| SID Lys:NE, g/Mcal | 3.68 | 3.09 | 3.96 | 3.38 |
| Var. days in finishing | -0.7 | 0.4 | - | - |
| Var. LW kg/pig | - | - | +1.0 | 0.0 |
| Var. IOFFC4, €/pig | +0.1 | +0.1 | +0.7 | 0.0 |
| Context of high AA prices | ||||
| SID Lys:NE, g/Mcal | 2.86 | 2.23 | 3.50 | 2.83 |
| Var. days in finishing | +1.7 | +3.3 | - | - |
| Var. LW kg/pig | - | - | +0.3 | -0.7 |
| Var. IOFFC4, €/pig | +0.23 | +1.29 | +0.04 | +0.42 |
1 Assuming maximum ADG to be 0.820 kg for females and 0.944 kg for males, both with an ADFI of 2.0 kg. Reference diet with 2540 kcal NE/kg and 0.85% of SID Lys. Initial weight of 70 kg in the simulations.
2 End weight of 100 kg.
3 Simulation considering 35 days in finishing
4 IOFFC: Income over feed and facility cost
In conclusion, feeding entire males and females differentially is an opportunity to minimize costs and maximize profits in finishing pigs (>70 kg BW). Moreover, dietary SID Lys concentration is positively related to crude protein (CP) content. Therefore, split-sex feeding is also an alternative for reducing nitrogen excretion by using diets with lower CP content, especially in females.
