Given that the appropriate SID Lys requirement plays an important role in reducing feed costs with maintaining animal performance, this study was conducted to determine the optimal SID Lys requirements of growing-finishing pigs under commercial conditions. To avoid the possible compensatory growth effect [16, 17], the pigs used in the three experiments demonstrated herein were obtained from three different batches.
In Exp. 1 and 2, growth performance was markedly affected by the SID Lys supply. For 30 to 60 kg growing pigs in Exp. 1, there was significant difference in ADFI among groups. The impact of different SID Lys levels on feed intake of ad libitum fed pigs was not consistent in the literature. Several studies revealed that Lys supply did not affect the feed intake [18], while other reports showed that Lys restriction resulted in reduced [19] or elevated feed intake [20]. This difference appears to be partly due to the different method used to formulate Lys-restricted feeds, by varying proportions of feed ingredients or using synthetic AA to create restriction [1]. Moreover, dietary fiber (wheat bran) or soy oil used in the experimental diets may also affect feed intake. According to Yin et al. [20], gut microbiome may also contribute to the potential mechanism of lysine restriction-mediated feeding behavior.
In Exp.1, the lowest F:G was obtained by pigs fed the 1.03% SID Lys diet while the highest F:G was obtained at 0.75 and 1.13% SID Lys. Notably, F:G showed both linear and quadratic reduction with increasing SID Lys inclusion. Our results were in line with the results by [1], where the authors observed that, for 25 to 50 kg pigs, lower G:F ratio was observed in pigs fed the reduced SID Lys diet. However, in terms of ADG, these authors reported that ADG significantly increased as SID Lys increased while no difference in ADG for growing pigs was found in our study.
For early finishing pigs from 60 to 90 kg in Exp. 2, maximum ADG and ADFI were achieved by pigs consuming the 0.87% SID Lys diet. In addition, ADG increased linearly and quadratically as SID Lys increased, while ADFI increased in a quadratic manner and a tendency to increase with increasing SID Lys in the diet. There was a linear and quadratic improvement in F:G as SID Lys increased, and the lowest F:G was obtained by pigs fed the 0.79 and 0.87% Lys diet. These results were in accordance with previous studies [10, 18], who showed that Lys restriction had significantly influence on growth performance traits such as ADG, ADFI and G:F ratio. Moreover, data compiled by Cloutier et al. [1] also revealed that ADG for finishing pigs (70 to 100 kg) significantly increased with increasing Lys levels while no difference was observed with respect to ADFI and G:F ratios.
For 90 to 120 kg late finishing pigs in Exp. 3, there was no evidence for difference in terms of ADG, ADFI, and F:G, suggesting that the performance of pigs from 90 to 120 kg was less affected by SID Lys restriction and the lowest SID Lys (0.53%) seemed to be sufficient for this group of pigs. A previous study carried out by Ma et al., [21] showed that increasing SID Lys could improve ADG and FCR (feed conversion ratio) both in linear and quadratic manner for late finishing gilts fed low crude protein. Another research also found that increasing SID Lys increased ADG and ADFI quadratically in finishing pigs weighing greater than 100 kg, while marginal improvements in F:G were observed with increasing SID Lys [12]. The inconsistency from the present study could be due to differences in Lys and crude protein levels, animal gender, weight, as well as experimental conditions.
In the current study, the appropriate Lys requirement was estimated with a linear-break point model and a quadratic model using ADG and F:G as the response criteria. Noteworthy, we noticed that different statistical models could yield different requirement estimates, in accordance with previous studies [22,23,24]. Based on broken-line models, the dietary SID Lys levels for the minimum F:G was 0.88%, lower than the NRC [13] recommendations of 0.94% for 30 to 60 kg growing pigs. Breakpoint for ADG and F:G of 60 to 90 kg finishing pigs both occurred at 0.79% SID Lys, which was the same as the current NRC recommendations. In contrast, when using a quadratic model, the optimum SID Lys levels to minimize F:G of 30 to 60 kg pigs were 0.99%, slightly higher than the current NRC [13] recommendations. A previous study carried out by Ho et al. [25] showed that the optimal SID Lys requirement for 30-50 kg pigs was 1.10%, which was also higher than the NRC [13] recommendations. Consistently, for 60 to 90 kg pigs, estimation of the required Lys for ADG and F:G was 0.83 and 0.84% using quadratic regression, which was greater than the NRC recommendations.
Interestingly, we found that F:G resulted in higher optimum SID Lys requirement than ADG in this study. Similar observations where F:G gave higher estimates of Lys requirement compared to ADG have been reported previously in the literature [2, 26]. Hence, these results may indicate that the Lys requirement would differ depending on the response criteria. More nutrients would be partitioned towards maintenance requirement when bodyweight increase was more towards visceral organs [2]. Therefore, even as maximal growth was attained, there could still be a metabolic demand for Lys [27]. Furthermore, in the present study, we found that the estimated Lys requirements were notably lower using broken-line models than quadratic models. In accordance with this, previous work also observed that broken-line models always resulted in lower estimates compared with quadratic models [28, 29]. As mentioned in previous studies [21, 30], the Lys requirement was often underestimated using broken-line regression model, because the breakpoint was selected as the minimum nutrient requirement for the theoretical average pig. However, on the contrary, the quadratic model was used to estimate the Lys requirement to reach 100% of the maximum response, which usually appeared to overestimate the nutritional requirement for pigs [30, 31]. For quadratic model, the arbitrary selection of 90% or 95% of the maximum response was probably aimed at meeting the requirement of most of the animals in a population [32]. Therefore, taking the average of the two models would be more closer to the requirement of the pigs or economic optimum.
Taken together, the SID Lys requirement for pigs from 30 to 60 kg bodyweight was 0.94%, for pigs from 60 to 90 kg was 0.81 to 0.82%, and for pigs from 90 to 120 kg was 0.53% depending on the response criteria with performance maximized. Using the estimated growth performance equations provided may aid swine nutritionists to determine the most economical Lys levels in actual diet formulation for a given situation.