Introduction

Optimizing nutrient composition in pig diets is essential for maximizing growth performance and ensuring high-quality meat production (Rodrigues et al., 2022). Among the essential amino acids, tryptophan and lysine are of particular importance due to their role in protein synthesis and the overall growth of pigs (Lin et al., 1988; Liao et al., 2015). However, the conventional approach of formulating diets with balanced levels of tryptophan and lysine may not always be the most costeffective or efficient strategy.

Recent research has explored the efficacy of tryptophan relative to lysine in finishing pig diets as a means to enhance performance and meat quality while minimizing feed costs (Ma et al., 2016). This strategy involves manipulating the amino acid ratio to better match the animal's specific nutrient requirements, thereby potentially improving nutrient utilization and growth efficiency.

The influence of tryptophan-to-lysine ratios on pig performance has gained considerable attention (Aftab, 2012). Tryptophan is a precursor for serotonin, a neurotransmitter that plays a crucial role in regulating feed intake and behavior (Zhang et al., 2007). By decreasing the level of tryptophan relative to lysine, it is hypothesized that feed intake can be modulated, potentially leading to improvements in growth performance. Additionally, tryptophan is known to interact with the immune system, and altering its ratio to lysine could have implications for immune function and overall health status of finishing pigs (Gao et al., 2018; Huang et al., 2021).

Meat quality is another important aspect that is influenced by the nutritional composition of pig diets (Ellis and McKeith, 1999). Amino acid ratios, including tryptophan and lysine, have been found to affect carcass traits, meat composition, and sensory attributes (Ma et al., 2020). Decreasing the relative level of tryptophan to lysine may impact the deposition of intramuscular fat, which is associated with marbling and tenderness (Tous et al., 2014). Understanding the effects of these dietary modifications on meat quality parameters is essential for producing pork products that meet consumer preferences and market demands.

Given the potential benefits and consequences associated with decreasing tryptophan levels relative to lysine, there is a need for a comprehensive evaluation of its efficacy on the performance and meat quality of finishing pigs. This research study aims to determine that the lysine and tryptophan (Lys : Trp) ratio would contribute to growth performance and meat quality in finishing pigs.

Material and Methods

The research protocol (DK-2-2104) for this work was approved by the Dankook University Animal Care and Use Committee in Cheonan, South Korea.

Animals and diets

A total of 144 crossbred finishing pigs (Duroc × [Yorkshire × Landrace], 17 weeks old) with an average body weight of 70.6 ± 3.9 kg were randomly distributed to four dietary treatment groups based on body weight (BW) and sex. Each treatment had nine replicate pens with four pigs (two barrows and two gilts) per pen. Different lysine : tryptophan ratios, such as 1 : 0.175, 1 : 0.160, 1 : 0.145, and 1 : 0.130, were fed to the pigs in the four treatments. The diets were formulated to meet or surpass the NRC (2012) nutrient requirements (Table 1). The temperature was thermostatically maintained at 25℃ during the experiment, while the humidity was controlled at 60%. All the pigs were kept in an environmentally maintained facility equipped with a barn with slatted plastic floor. To ensure that the animals had unlimited access to feed and water during the experiment, a one-sided feeder and nipple drinker were equipped.

Sampling and measurements

Growth performance

Each pig’s average daily gain (ADG) was assessed at the beginning, week 5, and overall to determine its ADG. The feed consumption and residuals were recorded to measure the average daily feed intake (ADFI) and feed conversion ratio (FCR).

Nutrient digestibility

At the end of the study, the apparent total tract digestibility (ATTD) of nutrients was determined. To determine the digestibility of the nutrients, chromium oxide (Cr₂O₃, 0.25%), an indigestible marker, was introduced to the diet seven days before fecal collection. Rectal massage was performed to collect feces from at least 2 pigs (1 gilt and 1 barrow) in each pen and specimens were combined by pen put on the ice box, transported to the laboratory, and stored at -20℃. Samples from the feces and feed were placed in an oven at 70℃ for 72 hours. Specimens were then crushed to pass through a 1-mm sieve and gathered. The method of the AOAC (2000) was used to analyze the dry matter (DM), nitrogen (N), and digestible energy (DE) of feed and feces samples. UV spectrophotometry (UV-1201, Shimadzu Corporation, Japan) was used to determine the presence of chromium in the specimens. Using a Parr 6100 bomb calorimeter (Parr Instrument Company, USA), the heat of combustion in the samples was measured to determine the amount of DE. The following formula was used to calculate the apparent total tract digestibility (ATTD).

ATTD of nutrients (%) = [1 − (Nf × Cd)/(Nd × Cf)] × 100, where Nf stands for nutrient concentration in excreta (% DM), Nd stands for nutrient concentration in diet (% DM), Cf stands for chromium concentration in excreta (% DM) and Cd stands for chromium concentration in diet (% DM).

Back fat thickness, lean meat percentage

The back fat thickness and lean percentage of all live pigs (n = 36 per treatment) were measured 5 cm from the right hand side of the midline from 3 different sites (shoulder, mid back, and loin at a position directly above the point of elbow, last rib, and last lumbar vertebra, respectively) before the commencement and at the end of week 4 and week 8 of the experiment using a real-time ultrasound instrument (Piglog 105, SFK Technology, Denmark). The mean value was taken and used for subsequent statistical analysis.

Statistical Analysis

General linear model approaches were used to evaluate the data in a completely randomized-block design (SAS Institute Inc., USA). Pen was used as an experimental unit. Standard errors of the mean were used to describe variation in data. The effects of the various Lys : Trp ratios in the diet were examined using linear and quadratic polynomial contrasts, with p < 0.05 denoting significance and p < 0.10 showing trends.

Results

The effect of decreasing the level of tryptophan in growth performance of finishing pig diet is described in Table 2. Reducing the level of tryptophan did not show any significant effects (p > 0.05) in ADG, ADFI, and FCR during the whole experimental period. Similarly, no differences were observed in the nutrient digestibility of DM, N, and DE in pigs (Table 3).

As shown in Table 4, the decreasing level of tryptophan linearly decreased the back-fat thickness at overall period (p = 0.038) and reduced at week 5 (p = 0.007). Also, the lean meat percentage (LMP) showed a tendency to linearly increase (p = 0.097) at initial and significantly increased at the end of the experiment (p = 0.045).

Discussion

The objective of the present study was to determine the effects of decreasing levels of dietary apparent ileal digestibility (AID) Lys : Trp ratio in finishing pig diets on their performance and carcass quality. Several previous studies have reported varying optimal Lys : Trp ratios. For instance, a study by Liu et al. (2019) using a broken-line model showed that the optimum AID Lys : Trp ratios for maximal weight gain and minimal feed conversion were 0.171, 0.183, and 0.184, respectively in finishing pigs weighing 20 to 50, 50 to 80, and 80 to 110 kg at the start of the experiment when fed low crude protein diets ad libitum. In another study, Gonçalves et al. (2018) reported that the estimated AID Lys : Trp ratio for 30 to 125 kg gilts ranged from a minimum of 0.169 for maximum FCR to 0.235 for maximum ADG, using a linear-plateau model. A study by Zhang et al. (2012) conducted on growing pigs observed AID Lys : Trp ratio optimums ranging from 0.197 to 0.236, where the AID Lys : Trp ratio needed to maximize ADG was at least 0.220 for 0.250 to 50 kg pigs. Xie et al. (2014) observed that estimates of the optimum AID Trp to Lys ratios were 0.203, 0.197, and 0.214 for weight gain, and feed conversion ratio, respectively, using a broken-line model. However, results from the present study have shown that the ratio of lysine and tryptophan (1 : 0.175, 1 : 0.160, 1 : 0.145, and 1 : 0.130) did not show any significant effect on ADG, ADFI, and FCR in finishing pigs. The inconsistent result showed due to difference in Lys : Trp ratio, different body weights, age of the animal, and different breeds used in the study.

The tryptophan-lysine ratio is considered important for digestibility in finishing pig diets due to its role in supporting efficient growth, nutrient utilization, and overall health in pigs. Lys : Trp ratios are both essential amino acids required by pigs for protein synthesis (Lin et al., 1988). A balanced tryptophan-lysine ratio can positively influence gut health and digestion (Gao et al., 2020). Tryptophan is associated with the production of important gut hormones that regulate feed intake and gastrointestinal motility (Roager and Licht, 2018). This can impact the pig's ability to digest and absorb nutrients effectively. However, in the present study, decreasing the AID Lys : Trp ratio in the finishing diet did not show any significant effect on DM digestibility, N, and energy digestibility. The digestibility of nutrients is not solely dependent on the Trp to Lys ratio. Digestibility is influenced by a complex interplay of various nutrients. While the tryptophan-lysine ratio is important for specific physiological functions, other nutrients like fiber, fat, and various amino acids also play crucial roles in digestion (Xu et al., 2021; Lee et al., 2022). The absence of a negative effect on digestibility might indicate that other nutrients in the diet were already well-balanced, compensating for any potential influence of the tryptophan-lysine ratio.

Normally, a pig deposits proportionally fatter and less muscle as it approaches its mature body size and weight. For heavier pigs (130 kg), Smit et al. (2018) showed reduced lean yields (59 - 61%). However, the present study showed that the backfat thickness tended to decrease at week 5, and back-fat thickness decreased at the end of the experiment, whereas LMP linearly tended to increase initially and increased at the end of the experiment with decreasing trp-lys ratio in the finishing diet. Tryptophan, as a precursor of serotonin, influences feed intake and appetite regulation. A reduction in the Lys : Trp ratio may have contributed to a decrease in serotonin production, which, in turn, could have led to a decrease in feed intake and subsequently reduced fat deposition (Yabut et al., 2019). This aligns with studies suggesting that tryptophan depletion can modulate lipid metabolism and adipose tissue accumulation (Osawa et al., 2011). The current study showed that ADFI reduced numerically, for this reason, backfat thickness was reduced. Furthermore, lysine, as a key amino acid for protein synthesis and muscle development, can influence lean tissue accretion (Liao et al., 2015). A lower Lys : Trp ratio might have facilitated an increased availability of lysine relative to tryptophan, enhancing protein synthesis and lean muscle deposition. This could explain the observed increase in lean meat percentage.

Conclusion

Even though the Trp to Lys ratio did not influence ADG, FCR, or ADFI, it had a notable effect on back-fat thickness and LMP. Our findings indicate that a Trp : Lys ratio as low as 0.13 had a significant impact on these particular aspects.