Effects of dietary lysine levels on production performance, milk composition and plasma metabolites of the high-producing lactating sows

sow daily PUN and Regression analyses were performed to evaluate the linear and quadratic effects of dietary treatment and reproductive cycle. Multiple comparisons were made when the ANOVA indicated signicant differences. Tukey’s test was used in multiple comparisons of means to adjust the P-values when using a mixed model procedure for data analysis. Duncan’s test was used for One-way ANOVA. Repeated measures analysis of variance using the MIXED procedure of SAS was used to examine the responses of piglet performance, sow body weight, backfat, lactation feed intake, data of milk and plasma Data are presented as means and SEM. Statistical signicance was declared at indicated as and were declared at


Introduction
The modern sow has been selected for large litter size and milk production. In addition, balanced nutrition, more standardized management and effective disease control were widely used in sow production [1,2]. E ciency of reproduction signi cantly increased in the breeding herd [3]. However, sows need more nutrient levels to meet their lactation needs with a high litter size.
Compared with multiparous sow, primiparous sow not only meet the needs of lactation, but also meet the needs of their own growth and development [4]. Kim et al. (1999) reported that the weight of nursed individual mammary glands increases 50% during the 5-21 d lactation of primiparous sow [5]. Sow lactation feed intake often is not enough during lactation to meet the sow's energy and nutrient needs for maintenance and milk production, especially for parity 1 and 2 sows [6]. Research has shown that if these rst and second parity sows mobilize more than 15% of their protein mass during lactation, thus reducing subsequent reproductive e ciency and litter weaning weight [7]. Therefore, lactation nutrition level is more important for primiparous sow. This not only affected the lactation performance and the growth performance of piglets, but also affected the growth and development of sows, and further affected the litter number and life span of sow. Therefore, ensuring nutrient intake for rst-time sows will increase the productivity of the entire breeding population.
Sows can achieve and maintain high levels of milk production throughout her productive life if given adequate levels of energy and nutrients. The most critical nutrients for maintaining optimum lifetime milk productivity are energy and amino acids [8]. lysine has been considered the rst-limiting amino acid in corn-soybean meal diets for sows in lactation [9]. An adequate supply of lysine during lactation allowed sows to maximize milk production and subsequent reproductive performance [10]. Extensive studies have estimated total lysine requirements of sows during lactation between 37 g/d and 58 g/d [11][12][13][14][15]. Recent research showed that total lysine intakes of 70 g/day or 62 grams of SID lysine/day to optimize reproductive and milk production performance of sows [8]. Moreover, gilts eat 10 to 15% less than sows the percent SID lysine in the lactation must increase compared to a mature sow herd [8].
Increased supply of lysine in lactation could allow gilts to consume adequate amino acids to maximize reproductive performance and ensure body development. Therefore, the objective of this experiment was to evaluate the effect of dietary lysine levels for lactating primiparous sows on multiple reproductive cycles including piglet's performance, milk composition and nitrogen emissions.

Animals, dietary treatments, and Housing
A total of 160 primiparous Yorkshire sows were randomly allocated into four groups (40 sows each group). On d 110 of pregnancy, they were moved to the farrowing accommodation where they were housed in single farrowing pens. Four experimental diets containing different SID lysine levels of 0.84%, 0.94%, 1.04%, and 1.14% were provided to the gilts during lactation (Table 1). Methionine, threonine, tryptophan and valine were used to maintain ratios to lysine that were equal to those of the 0.84% Lys diet. Litter size was adjusted to 12 to 14 pigs per litter by 2 d after farrowing. On the day of farrowing, the sows were fed 1.5 kg, which was then successively increased (3 kg/d on d 1 and 2 of lactation; 4.5 kg/d on d 3 and 4 of lactation; ad libitum consumption from d 5 of lactation to weaning). Sows were fed two times daily (at 0900 and 1600 h.) and allowed ad libitum access to water from parturition until weaning. sows were weighed and backfat were determined within 24 h after farrowing and at weaning. All sows were fed with the same diet from weaning until the second insemination (day 0 of gestation), and then fed with the same gestation diets (3,000 kcal of ME/kg, SID Lys 0.70%) in individual stalls until farrowing. During the second reproductive cycle, repeat the treatment of the rst reproductive cycle.

Sample collection and analysis
Blood samples were randomly taken from 16 sows immediately at parturition day and 21 d of lactation. Blood samples of 10 mL were collected from sows through ear vein. Then blood samples were centrifuged at 3,000 g at 4 °C for 15 min (Eppendorf centrifuge 5810R, Hamburg, Germany) to separate plasma. Colostrum and milk of 30 mL were collected from anterior, middle, and posterior teats from one side of the sow using a 50-mL centrifuge tube at 24 h and 14 d postpartum. All samples were stored at − 20 °C until analysis.
The milk composition was determined by near-infrared re ectance spectroscopy with a Foss Milkoscan FT+ (CombiFT + 200, Denmark). Before analysis, 5 mL thawed fresh milk per sample was aliquoted into a 50-mL centrifuge tube (sterilized), and 20 mL distilled water was added to dilute the sample [16]. Plasma urea nitrogen (PUN) level was analyzed using a commercial kit (Nanjing Jiancheng Bioengineering Institute, Nanjing, China), according to the manufacturer s instructions. The amino acid contents in colostrum, milk, plasma and diets were determined by ion-exchange chromatography (Amino Acid Analyzer L-8900, Hitachi, Tokyo, Japan) with post-column derivatization with ninhydrin.

Calculation and statistical analyses
Calculation and statistical analyses were conducted using SAS software (SAS 9.4, Inst, Inc., Cary, NC) with the individual sow as the experimental unit. PROC MIXED procedure was used in the analyses. Dietary treatment, reproductive cycle, the interaction between dietary treatment and reproductive cycle were speci ed as xed effects. Farrowing room was as a random effect. In the mixed model, the response variables were related measurements, including litter size, litter weight of born alive and weaning, sow daily feed intake during lactation, WEI, colostrum and milk compositions and amino acids levels, PUN and plasma amino acids levels. Regression analyses were performed to evaluate the linear and quadratic effects of dietary treatment and reproductive cycle. Multiple comparisons were made when the ANOVA indicated signi cant differences. Tukey's test was used in multiple comparisons of means to adjust the Pvalues when using a mixed model procedure for data analysis. Duncan's test was used for One-way ANOVA. Repeated measures analysis of variance using the MIXED procedure of SAS was used to examine the responses of piglet performance, sow body weight, backfat, lactation feed intake, data of milk and plasma parameters. Data are presented as means and SEM. Statistical signi cance was declared at P < 0.05 were indicated as signi cant difference, and tendencies were declared at 0.05 < P < 0.10.

Sow performance
Body weight and backfat thickness of sows at farrowing and at weaning did not differ among dietary treatments (Table 2). Weight loss and backfat thickness loss during lactation also did not differ.
Moreover, the dietary lysine levels did not affect feed intake of sows during lactation. However, WEI (P = 0.07) tended to be a quadratic effect by increasing dietary lysine level. Compared with the primiparous, sows had greater body weight (BW) (P < 0.01) and backfat thickness (P < 0.01) of sows at farrowing and at weaning, and weight loss (P = 0.01) during the second lactation period. Feed intake of the second lactation period signi cantly increased (P < 0.01). There was no interaction between dietary treatment and reproduction cycle on performance of sow. The effects of dietary lysine levels in lactation diet on piglet growth performance during lactation are shown in Table 3. No signi cant differences were found in litter size as well as litter and piglet weight at birth and after cross-fostering. Linear increase on survival rate of piglets (P = 0.03),weight of piglet (P = 0.04) at 21 d of lactation, and weight gain of piglet (P = 0.05). Compared with the rst lactation period, sows had greater litter size (P < 0.01), litter (P < 0.01) and piglet weight (P < 0.01) at birth and at weaning, and weight gain of piglet (P < 0.01) in the second lactation period. The signi cant diet × cycle interactions for survival rate of piglets (P = 0.03) and weight of piglet (P = 0.04) at 21 d of lactation.          were higher (P < 0.05) than those in the 0.84% Lys group. There was no interaction between dietary treatment and reproduction cycle on colostrum and milk amino acid concentrations of sow.

Discussion
In the present study, lysine level in the lactation diet did not affect sow lactation body weight, backfat changes and feed intake. Some studies support most of these ndings. Sow average daily feed intake (ADFI), backfat changes, and litter size at weaning were not affected by increases in lysine from 0.80 to 1.06% [17]. Yang et al. (2009) reported that increasing in lysine from 1.02 to 1.34% did not affect body weight change, backfat change, feed intake and lysine intake during lactation of primiparous sow [4]. Touchette et al. (1998) reported that lysine intake during lactation decreased quadratically weight loss and loin eye area loss, while did not affect number of pigs weaned, litter growth rate, sow backfat loss, or WEI of primiparous sows [15]. Our study show that 0.94 and 1.04% lysine diets reduced WEI. Richert et al. (1996) reported that increasing lysine in lactation results in reduced BW loss during lactation [18]. Parity had a signi cant effect on sow body weight, backfat thickness and feed intake during lactation. It seemed that the requirements of the still growing primiparous sows were greater than those of the multiparous sows during this period.
Our study demonstrates that increasing dietary lysine increased survival rate of piglets, weight of piglet and weight gain of piglet at 21 d of lactation. These ndings were in agreement with the results reported by Chen (1978) who evaluated the levels of lysine during gestation [19]. An addition of more than this synthetic amino acid could increase preweaning mortality and decrease the number of piglets weaned [19]. Similar ndings on good litter performance nursed by sows fed higher lysine diets were reported [20,21,22]. The piglet weaning weight of the present study were similar to those observed by Santos et al.
(2006) who reported no improvement on litter performance with greater Lys intake [23]. However, previous studies reported that sows fed 0.67%, 0.86%, 1.06% or 1.25% of apparent digestible lysine diet during a 17-d lactation did not nd any difference on litter performance [15]. In the present study, the number of pigs born and weaned was similar for primiparous and multiparous sows, but greater litter weight at weaning and growth rate were observed in litters nursed by multiparous sows than primiparous sows. Yang et al. (2009) have also indicated that modern genotype multiparous sows have greater reproductive performance at born and at weaning than primiparous sows [4]. However, in our previous research, Lys levels (0.95% and 1.10%) over two consecutive lactations did not affect average weight at weaning, litter weight at weaning, little weight gain, and litter growth rate in multiparous sows [24]. This could be the probable reason for lysine intake levels meet lactation requirements in multiparous sows. Therefore, this indicates that modern genotype sows need more nutrients for lactation, especially less than 3 parities of sows.
Providing sows with su cient amounts of nutrients during lactation is crucial for optimal performances from the lactating sows and consequently the health and growth of the newborn piglets [25]. While insu cient nutrient intake caused body composition loss for maintain milk production [26]. Furthermore, excessive mobilization of body tissues results in low milk production, delayed return to estrus postweaning [27]. In the present study, the clear effect of dietary lysine levels in increasing milk contents of fat, protein, total solids and solid not fat agrees with the previous observations [4,19]. This could be a key reason for piglets have better lactation performance due to difference in the sow milk composition with lysine levels increasing during lactation. concentrations re ected protein (amino acids) mobilization in lactating sows [28]. As dietary concentration of the Lys increases, muscle tissue loss decreases, and PUN concentration is increased [20,29]. In our study, sows fed high-lysine diets had higher PUN concentration at d 21 of lactation. Similar ndings on PUN concentration by sows fed higher lysine diets [4,17]. Dietary concentration of the lysine increases, muscle tissue loss decreases and, PUN concentration is increase [4]. Furthermore, greater PUN may indicate less protein utilization, excess N, or more transamination or deamination [30]. Thus, PUN concentration may be increased if sows more protein or amino acids than the requirement.
The relationship between the amino acid adequacy of a diet and the plasma amino acid levels is well documented [18,31]. Further dietary additions of this amino acid result in rapid accumulation in blood plasma of animals [32]. In our study, evaluated the rst and second parity sows fed different lysine levels, keeping the ideal amino acids ratios constant by adding Met, Thr, Val, and Trp increased plasma amino acid levels by measured. In addition, the milk amino acid change was similar for plasma amino acid levels. This further suggests that good litter performance nursed and subsequent reproductive performance by sows fed higher amino acid diets over two consecutive lactations in primiparous sows.

Conclusion
Although increasing lysine (balance the other amino acids) levels for lactating sows was no difference in sow body conditions and performance of piglets at birth. However, increasing dietary lysine levels in lactation was bene cial for survival rate of piglets, litter weight, piglet weight and ADG at weaning. Moreover, milk compositions were changed, increasing amino acid concentration by dietary lysine levels which is an important reason for the performance improvement of piglets. Therefore, when implementing strategies for improving reproductive e ciency, higher dietary amino acids levels during lactation must be considered, especially for primiparous sows.