“In piglets weaned at 27 days of age, the dietary standardized ileal digestible (SID) methionine-to-lysine ratio has no statistically significant effect on average daily weight gain.”

The current study indicated that increasing the dietary Met:Lys ratio (0.37–0.57) in the lactation diet had no significant effect on the overall performance of sows or the colostrum and milk composition, but it increased piglet mean BW and piglet ADG during the first week of lactation. There were no differences in piglet numbers, number of piglets weaned or piglet weights at day 14 after birth. However, the piglets reared on sows in the 0.37- or 0.57-ratio groups grew faster than those reared on sows in the control group (0.27) during the 1st week of lactation (P < 0.01). Only the piglets in the 0.47-ratio group had a significantly higher weaning weight than those in the 0.27-ratio group.

Nursery pigs would require a greater dietary SID Met to SID Lys ratio (SID Met:Lys) compared with growing-finishing pigs due to their immature immune systems and greater susceptibility to immune challenges. However, the results of the meta-analyses in this review do not support the assumption, which indicate that weaning stress alone does not increase the dietary SID Met:Lys unless nursery pigs are directly immune challenged. In general, SID Met:Lys ratios in nursery pig diets are formulated around values that have been shown to support optimal growth in dose–response studies.

The meta-regression evaluated methionine among four essential amino acids: “The present study used a meta-regression analysis to assess the relative ratio to lysine to maximize the feed efficiency of four essential amino acids (tryptophan, valine, isoleucine, and methionine) in pig diets.” In the results, the authors report that for methionine, increasing the SID Met:Lys ratio improved feed efficiency up to an optimum, after which no further improvements were observed, indicating that methionine level relative to lysine does affect performance metrics such as ADG and G:F in pigs, including data sets that involve nursery/young pigs.

During the first study (48 to 107 lb), average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (F/G) improved with increasing dietary lysine. Although no differences occurred in growth performance with increasing methionine ratio, there was a numeric improvement in growth performance for those pigs receiving diets containing 28% methionine relative to lysine. In summary, because of high ADFI observed in these studies, the dietary methionine levels used closely met or exceeded the pig’s requirement on a grams/day basis. Therefore, these data suggest that increasing dietary methionine does not improve pig performance.

Average daily gain increased quadratically (P < 0.05) as the concentration of SID lysine in the diets increased. Using the asymptote of the line, the optimal SID lysine requirement for maximizing average daily gain was estimated to be 1.08%. A linear increase (P < 0.05) was observed in the gain:feed ratio as the concentration of lysine in the diets increased. Averaging the values obtained by plotting the data for average daily gain and gain:feed yielded an estimated SID lysine requirement of 1.09%.

In this study, 120 pigs weaned at 21 d of age were fed diets with graded levels of methionine plus cystine while lysine was kept constant. Increasing Met+Cys improved (P<0.05) average daily gain and gain:feed from d 0 to 28 post‑weaning, and pigs fed the lowest Met+Cys level had reduced growth and impaired immune responses. The results suggest that the level of methionine‑containing sulfur amino acids relative to lysine significantly affects ADG and health in early‑weaned pigs.

In the model, lysine requirement was related to average daily gain and the G:F ratio. However, for the SID lysine requirement this does not have a direct significant effect, as the absolute performance level is accounted for in the model structure. The model predicts SID lysine requirements across a range of weaning ages and growth rates but does not specifically address methionine-to-lysine ratios.

The supplementation of L-valine to a valine-deficient basal diet led to an increase in growth and feed intake of weaned piglets. Estimated dVal:dLys for body weight gain was found to be 0.68 (EA, 95% of maximum response) and 0.67 (CLP) in the starter phase and 0.65 (EA, 95% of maximum response and CLP) in the total trial period. It is concluded that the supplementation of a valine-deficient basal diet for weaned piglets with L-valine improves the piglet’s weight gain and feed intake and that a dVal:dLys of 0.68 is recommended to optimize body weight gain.

“Methionine:lysine ratios ranged from 21.5 to 33.5%… Increasing dietary methionine increased average daily gain (ADG) during each week of the trial, with the maximum observed at approximately 0.50 and 0.39% methionine in the diets containing 1.8 and 1.4% dietary lysine, respectively (27.5% of lysine)… In conclusion, with either dietary lysine level used, maintaining methionine at 27.5% of lysine was required to maximize growth from d 0 to 21 postweaning.”

The bulletin emphasizes that amino acid ratios relative to lysine are critical: "Other amino acids also must be maintained in the proper ratio to lysine to achieve optimal performance. For example, research at Kansas State University has shown that the correct methionine:lysine ratio is as important as the total lysine level in the diet." Table 3 lists "Minimum true ileal digestible (TID) amino acids relative to lysine" and gives methionine at 28% of TID lysine and Met & Cys at 58% of TID lysine, indicating that deviations from these ratios can limit performance in weaned pigs.

Formulation of starter diets for weaned piglets to lower-than-usual lysine decreases the requirement for expensive protein sources while having minimal effect on performance but it is crucial to pay attention to the levels of other amino acids relative to lysine. Suggested ratios relative to lysine are 58 per cent for methionine and cysteine, 62 per cent for threonine, 65 per cent for valine and 52 per cent for isoleucine (60 per cent if high levels of blood products are used). Although lysine requirements of nursery pigs have increased in recent years and vary with environmental conditions and genotype, when expressed relative to growth rate, empirical studies in recent years have consistently found the requirement to be 19 g per kg of gain.

Nursery pigs (initial BW 5.5 kg; approximately 19–21 days of age) were used to determine the SID sulfur amino acid to lysine ratio. Increasing the SID SAA:Lys ratio from 50 to 70% improved (linear, P<0.05) ADG and G:F, with the requirement for maximum ADG estimated near 60% SID SAA:Lys. The data demonstrate that the relative level of sulfur amino acids (including methionine) to lysine significantly affects growth rate in very light nursery pigs.

Discussing sulfur amino acids, the article states: "It appears that the TSAA:lysine ratio for growing pigs is between 55 and 60 per cent and may increase slightly as pigs reach market weight." It also notes that for nursery pigs, the minimum requirement for threonine relative to lysine is approximately 60 to 62% in the nursery stage and that ratios are used to meet optimal growth. Although this piece focuses mainly on growing and finishing pigs, the presentation of optimal total sulfur amino acid (which includes methionine) to lysine ratios implies that the balance of methionine (and sulfur amino acids) relative to lysine affects growth performance rather than being neutral.

Although this experiment focused on isoleucine, it provides context for amino acid:lysine ratios in segregated early-weaned pigs (SEW) of similar age to 27-d weaning: “These data suggest that the isoleucine requirement for the SEW pig is approximately 60% of lysine on an apparent digestible basis.” The study found that varying the Ile:Lys ratio significantly affected growth performance, demonstrating that, in SEW piglets, changing standardized ileal digestible essential amino acid:lysine ratios can alter average daily gain.

The abstract notes: “Two experiments evaluated the effects of increasing standardized ileal digestible (SID) Ile:Lys ratio on growth performance of nursery pigs.” The modeled responses showed that ADG and feed efficiency changed with increasing SID Ile:Lys, and the authors identified an optimal range. While the study is on isoleucine, it supports the broader concept that in nursery pigs, standardized ileal digestible essential amino acid:lysine ratios have statistically detectable effects on growth performance, including ADG.

This experiment evaluated true ileal digestible (TID) lysine and threonine concentrations for 80- to 130-lb and 170- to 230-lb pigs. The authors report effects of increasing TID lysine and threonine on ADG, ADFI, and F/G, and derive optimal threonine-to-lysine ratios (for example, "a TID threonine-to-lysine ratio of 67% seems optimal"). Methionine is not varied independently in this study; methionine and other amino acids were held at adequate ratios relative to lysine, so no effect of methionine-to-lysine ratio on ADG is reported in this weight class.

Pigs and feeders were weighed at the beginning and end of each phase to calculate average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency. Increasing SID Arg:Lys up to an optimal range improved ADG and feed intake, but further increases provided no additional benefits and in some cases slightly reduced performance. The bulletin concludes that there is an optimal SID Arg:Lys range for nursery pigs and that exceeding this range does not significantly improve growth performance.

Across commercial and academic formulations for nursery pigs weaned around 21–28 days of age, the standardized ileal digestible methionine to lysine (SID Met:Lys) ratio is typically set near 55–60% of lysine when methionine plus cysteine are considered together, and around 30–40% for methionine alone. Within this commonly recommended range, most dose–response trials report limited or no statistically significant improvements in average daily gain when methionine is increased above requirement, provided that lysine is not limiting.