S1081: Nutritional Systems for Swine to Increase Reproductive Efficiency

(Multistate Research Project)

Status: Active

S1081: Nutritional Systems for Swine to Increase Reproductive Efficiency

Duration: 10/01/2023 to 09/30/2028

Administrative Advisor(s):

NIFA Reps:

Non-Technical Summary

Statement of Issues and Justification

Introduction:  Swine production is globally distributed. The U.S. is the world's third-largest swine producing country behind China and the E.U. The U.S. has been the world’s largest or 2nd largest exporter of pork and pork products, with exports being projected to be 23.5% of domestic commercial pork production (USDA, 2022a). Currently, U.S. pork production operations are heavily concentrated in the Midwest and eastern North Carolina. In 2021, an estimated 180 million hogs were slaughtered in the U.S. for an estimated gross on-farm value of $18.0 billion (USDA, 2022b). The average inventory as of September 1, 2022 was 73.8 million swine, of which 6.15 million were sows (USDA, 2022c). Swine production is driven by the fact that pork continues to be one of the major high-quality sources of protein in human diets and, because of its flavor, pork is the meat of choice worldwide with a U.S. average annual per capita consumption of 23.4 kg (USDA, 2022a).


Need as indicated by the stakeholders: Swine enterprises constitute a major source of on-farm income in the Southern Region of the U.S., and pork production in the Southern Region represents a significant portion of the U.S. pork production, especially sow production. The most rapidly growing component of swine production in the Southern Region has been in sow farms producing feeder pigs that are shipped to the Midwest for feeding to market weight. This trend is attributed to favorable environmental conditions, the availability of labor, and interest in contract swine production in the Southern Region, and to an availability of feedstuffs in the Midwest.


A primary factor affecting the profitability of swine production is sow productivity. Optimum nutrition of the sow is essential to maximize sow productivity and longevity. An ideal nutrition program should provide adequate nutrients to maximize sow productivity and health while minimizing the environmental footprint and feed costs. The continuing trend to increased litter size and intensive production schedules places biological demands on the sow that make high performance difficult to obtain and maintain without compromising sow health and wellbeing. An increase in the number of pigs marketed/sow/year through improved sow nutrition would result in increased profitability by allocating fixed sow costs over more pigs. However, increased productivity can decrease longevity in the herd without proper nutrition. Studies indicate that a sow must complete at least 3 parities before reaching a positive net present value (the point when she has covered purchase price and feed costs; Stalder et al., 2000, 2003).

The research committee of the National Pork Board (NPB) has identified improvements in sow nutrition as an area needing further research. The current S-1081 committee meets yearly with members of the American Feed Industry Association, the NPB, and representatives from large feed companies to survey their assessment of research priorities. The specific research objectives that we have chosen result directly from those meetings. All segments of the industry recognize sow productivity and nutrition as extremely important factors affecting profitability in swine production systems. In fact, sow longevity is one of the key research areas identified by the NPB for the use of the commodity checkoff monies that are appropriated for research. Although progress has been made in sow nutrition in the last 30 years, there is still a dearth of information relative to specific nutrient requirements of the very prolific and high milk-producing sows used today. Further research is greatly needed to accurately define the levels of various nutrients necessary for optimizing reproduction and lactation while minimizing nutrient excretion.

Importance of the work: It is extremely important to conduct research that provides solutions to potential sow nutrition and production problems and the impact that concentrated production systems have on the environment. Societal perspectives and governmental regulations place extreme pressures on pork production systems. Solutions to these issues must be provided so that swine production, a critically important component of agricultural productivity, will remain and continue to be an economically viable opportunity for our work force.

Technical feasibility of the research: The original Southern Multi-State Research Group (S-145) and the current group (S-1081; previously S-288, S-1012, S-1044, and S-1061) have made significant contributions in obtaining new knowledge and creating a better understanding of the nutritional needs of sows to improve reproductive efficiency. This Technical Committee has used the approach of: 1) defining high priority research areas in direct collaboration with stakeholders, 2) developing common protocols that are followed by all participating stations [with certain aspects rigidly followed by all participants and other aspects having flexibility for individual stations], 3) pooling the data, 4) drawing conclusions, 5) publishing the pooled results as abstracts at professional meetings and in peer-reviewed scientific journals, and 6) dissemination of research results through extension programming, trade magazines, and direct producer contact. Since its inception through 2022, the Committee has published 27 refereed publications, 2 conference proceedings, 43 abstracts, and 1 extension publication. These publications are the direct result of the collaborative research efforts of the Southern Multi-State Research Group. Also, in collaboration with the NCCC-42 Committee, the Committee has published two books and four book chapters. The first book published was entitled "Swine Nutrition" (now in its second hardcopy edition) and the second book published was entitled “Sustainable Swine Nutrition” (now in its second hardcopy edition and also available as an e-book). Committee members have been asked to speak at a number of producer and industry conferences to discuss research results. The S-1012 Committee was nominated by the Southern Region Department Heads for the NASULGC award for regional research. Over the last 20 years, participants in the Committee have clearly demonstrated that they can successfully collaborate in multi-state research. In addition, we meet annually with the NCCC-42 Committee, which is an informational exchange group working on swine nutrition. We have opened our objectives to their participation as well as participation by other institutions.

Justification for a multi-state approach: Sow research is well suited to a regional approach for three major reasons. First, in reproductive studies, large numbers of animals are required to generate meaningful data; individual experiment stations often do not have sufficient sow numbers for sow research. Progress in sow nutrition and management research is hampered by the large variation among sows in economically important reproductive traits (Aaron and Hays, 1991). In a summary of 7,925 farrowings in five herds, the coefficients of variation were 26-33% for total and live pigs farrowed and 36% for pigs weaned (Aaron and Hays, 2004). In contrast, the coefficients of variation for growth rate and feed efficiency were 5-8 and 4-7%, respectively, for pens of growing and finishing pigs. The number of replications needed to detect a 10% difference in litter size at birth and at weaning, at an 80% success rate and a 5% probability level, is 99 and 193 sows per treatment, respectively. Thus, it is difficult for individual experiment stations to generate the number of observations needed to reach statistically significant conclusions. Second, pooled results from several experiments conducted with a common protocol but under different environments provide valuable information from which broad inferences can be drawn, and more robust recommendations can be made. A third advantage of a multi-state approach is that the combined experience and expertise of several swine nutritionists can be focused on the most high-priority objectives. Additionally, a planned annual meeting provides opportunities to discuss new and old research findings.

Goals and impacts of the current research: The primary goal of this proposed project is to improve the reproductive performance of sows while increasing their retention in the herd, enhancing offspring robustness, and minimizing sow nutrient excretion. This research will include studies to evaluate sow lactation histidine requirements, enhancing lactation feed intake, and biological indicators of sow nutritional status, to both improve the reproductive performance of sows and improve the economic return to swine producers. Histidine is an amino acid that may become limiting to milk production when aggressive amino acid supplementation is used to take advantage of increasingly affordable synthetic amino acids and to reduce nitrogen excretion (done to minimize environmental impact). Low sow feed intake is a primary factor involved in low weaning weights of piglets and excessive body weight loss of the sow during lactation, resulting in greater time in returning to estrus. Finally, if biomarkers of nutritional status can be identified that are associated with the range of sow performance and sow wellbeing, they could potentially be used as early intervention indicators to more closely meet changing nutrient demands, limit excess protein excretion, and reduce diet production costs; however, there is currently too little data on this to begin its use for optimizing sow production and wellbeing. As sow health and productivity is increased by success in any of these objectives, there is potential for improvement in enterprise profitability. The results will also demonstrate responsiveness to societally-important issues of waste management and the environment as well as concerns about animal wellbeing in animal production.

Related, Current and Previous Work

Literature searches were made in the CSREES index, the CAB Index, the Index on Current Research in Pigs, and the Index of the Journal of Animal Science to locate past and current research in the three project areas.

Objective 1: Examination of supplemental histidine on the lactation performance of sows: There are no published reports on the impact of additional histidine in sow lactation diets on the performance of sows or their piglets. There are some anecdotal reports among industry personnel of improved weaning weights associated with increased levels of histidine in the lactation diet.  However, in the dairy literature there are numerous reports showing improved lactation performance with the supplementation of histidine to dairy cows fed low metabolizable protein diets (Giallongo et al., 2015, 2016; Doelman et al., 2008). Dairy cows fed diets that were deficient in histidine were found to have decreased dry matter intake, milk protein, milk yield and blood hemoglobin (Giallongo et al., 2017). 

Swine lactation diets are normally formulated on a standardized ileal digestible (SID) lysine basis with other amino acids being set in specific ratios to lysine.  This would be similar to the metabolizable protein concept in dairy cows. The 2012 NRC for swine lists the ratio for histidine in sow lactation diets to be 0.38 of the lysine content. Analysis of sows’ milk indicates that the histidine content ranges from 0.41-0.47 of the lysine content from day 5-26 of lactation (Daza et al., 2004). Sow lactation diets formulated using the ideal protein concept make use of added synthetic amino acids to replace soybean meal and to help reduce nitrogen excretion by the animal.  In most of these formulations, histidine is the 3rd or 4th limiting amino acid and often is the first amino acid that is not supplemented in the diet using synthetic amino acids. With the continuous improvement in the productivity of sows and the increased milk demand that results from larger litters, additional research is needed to understand the amino acid needs for milk production. With the histidine:lysine ratio in milk being higher than the current NRC dietary recommendations, it is possible that the dietary histidine concentration is limiting milk production.  It is interesting to note that cows fed higher levels of histidine also showed higher dry matter intake (Giallongo et al., 2017). A secondary benefit to increasing histidine in the lactation diet of sows may be increased feed intake that could result in increased overall milk production and increased growth of the piglets.

Objective 2: Biomarkers related to sow metabolic status, piglet vigor, and reproductive potential: There is a concerning trend for increased sow mortality and reduced preweaning piglet survivability. Based on PigChamp data, average sow mortality is currently at 14%, pre-wean mortality at 15%, and post-wean mortality at 10% (Schull, National ASAS Meeting, 2020). This mortality rate appears independent of sow herd size according to a recent review of Metafarms Sow Farm data suggesting this is a multi-factorial issue not specific to large farms where limited on-farm labor potentially contributes to herd health concerns (Eckberg, 2022). The specific causes of sow mortality are highly varied and include bacterial and viral disease as well as general sow health issues related to gastrointestinal health, heart and locomotive issues, kidney and urinary tract inflammation, reproductive failure and prolapses (Supakorn et al., 2019). In a review of sow mortality in a 5,200-sow herd based on postmortem analysis, sow death in gestation and lactation made up 80% of all on-farm deaths (Sanz et al., 2007). Locomotor disturbances, gastric ulcers, and urinary disturbances represented close to 60% of reasons for death or euthanasia. However, most data on sow mortality are based on educated ‘guess’ by on-farm staff. In this case, Metafarms data reports that 52% of sow death reasons are listed as ‘unknown’ and 50% of sows euthanized are due to structural issues. This suggests that general sow health is declining in commercial sow herds; however, clear benchmarks to define sow health are not currently available. These benchmarks are needed to establish improved sow herd management and feeding protocols to address sow mortality and enhance overall sow herd health and productivity. Rectal and vaginal prolapses have been a focus with respect to sow mortality, instigating an extensive survey of over 100 sow farms representing 400,000 sows which determined that 21% of sow mortality was attributed to pelvic organ prolapse (Ross, 2019). This work also noted 39% of sow deaths were categorized as ‘unknown/other’. Perineal score was identified as a high-risk factor contributing to prolapse rate. In further investigation, serum markers of inflammation were greater for sows identified as having a high perineal score (i.e. high risk of prolapse) compared to sows with a low perineal score in late gestation (Kiefer et al., 2021). This work supports the hypothesis that poor (or lower) sow metabolic status contributes to sow mortality and emphasizes the need to identify other biological markers that can be used to establish new sow management and feeding protocols to enhance sow health and lower sow mortality rate.

Piglet survivability is similarly a multi-factorial issue where piglet weight at birth and colostrum intake are key factors contributing to pig survival. In the classic work of Quiniou and colleagues based on over 900 litters and 12,000 piglets, piglet birth weight decreased as litter size increased with an average piglet birth weight of 1.26 kg for litters of ≥17 piglets; further, piglets with 1.20 to 1.40 kg birth weight had a 91% likelihood of survival to day 7 of age. According to PigChamp data, the average litter size of US sow farms is 15.6 piglets (Schull, National ASAS Meeting, 2020). While data specific to average piglet birth weight across US herds is not readily available, a review of Danish sow herds with litter sizes equal to or greater than US herds, the average piglet birth weight was 1.25 kg; however, preweaning mortality in US sow herds continues to rise suggesting that factors other than birth weight are contributing to a reduction in piglet vigor. Recent work has demonstrated that hemoglobin concentration status of the sow can be connected to stillbirth probability and that the stillbirth rates were negatively correlated with hemoglobin levels of the sow (Bhattari et al., 2018, 2019). This further supports the relationship between sow metabolic status, piglet vitality and reproductive performance (Hu et al., 2019). Therefore, it is necessary to characterize sow metabolic status across a range of parities and barn environments to better establish connections to piglet vigor and sow reproductive potential. This is needed to refine current sow management and feeding protocols to improve overall sow health and positively impact sow and piglet livability.

Objective 3: Effect of a sensory additive during lactation on sow feed intake and reproductive performance: Sow feed intake during lactation is crucial to the maintenance of sow body condition, milk production, and litter performance. Studies have shown that sows, especially primiparous sows, do not consume enough feed during lactation to meet their energy and amino acid requirements. Moreover, it has been extensively demonstrated that excessive weight loss during lactation compromises subsequent reproductive performance (Hughes, 1993; Tantasuparuk et al., 2001).  Clowes et al. (2003) reported that sows mobilizing more than 12% of their overall protein mass during lactation subsequently face reproductive difficulties and produce litters with reduced weights. Second parity sows tend to have lower farrowing rates or reduced litter size (Hoving et al., 2010; Saito et al., 2010) which can influence litter size in subsequent parities (Hoving et al., 2011). Thaker and Bilkei (2005) observed that weaning-to-service-intervals increased (P < 0.05) when lactation weight losses were above 5% for parity 1 sows. Lactation weight losses of 10% or greater also exerted a negative (P < 0.001) effect on subsequent total-born litter sizes in parity 1 versus parity 2-5 sows. In addition, Sasaki and Koketsu (2008) reported that in sows with a high lifetime efficiency and longevity, fewer sows had a decrease in litter size between 1st and 2nd parity compared with sows with a low lifetime efficiency and longevity. Adequate feed intake is therefore important to improve reproductive performance, particularly among sows during their first lactation, to improve lifetime breeding efficiency and the average parity of breeding herds as well as reduce the gilt replacement cost in commercial herds.  

Sensory additives (both olfactory and gustatory) have often been suggested as additives to improve feed intake and, potentially, nutrient digestibility and gut health. Antibacterial phytochemicals such as essential oils represent a concentrated form of phytogenic feed additives, containing mainly the active ingredients of plant secondary metabolites (Snow et al., 2016). There are more than one hundred kinds of essential oils (Wei and Shibamoto, 2007). Several studies have shown multiple properties of phytogenic feed additives including antibacterial, antiviral, antifungal, antioxidant, digestive stimulants, immunomodulators, hypolipidemic agents, and heat stress alleviators (Zhai et al., 2018). Li et al. (2012) found that feeding a mixture of two essential oil products, thymol and cinnamaldehyde, to young pigs enhanced growth performance, increased nutrient digestibility, promoted lymphocyte proliferation, and decreased E. coli in the cecum, colon, and rectum. The enhanced growth is consistent with previous observations showing that weight gain in young pigs was significantly increased by essential oils compared with unsupplemented diets (Cho et al., 2006). Lien et al. (2007) also found increased weight gain and improved feed efficiency by feeding phytogenic compounds. The effects of essential oils and other phytogenic feed additives in pigs have been variable because of different study conditions (Janczyk et al., 2009), differences in the type of essential oils used, and variation in the dose provided (Windisch et al., 2008). The growth promotion effects on swine performance have been concentrated primarily in young pigs and the effect of phytogenic feed additives on reproductive performance in swine is very limited.

Two preliminary studies were conducted at the University of Arkansas to evaluate the effect of a commercial sensory additive which is a blend of volatile and non-volatile components (Luctamax® SowVive, Lucta S.A., Barcelona, Spain) on sow feed intake, body weight loss and reproductive performance. Experimental lactation diets with and without 0.075% sensory additive were offered upon entry to the lactation room and fed through weaning at 21 days. The initial study was conducted during summer months (n = 52 sows; Tsai et al., 2021) and the second study was from October through December (n = 61 sows; Davis et al., 2021). During lactation in the initial study, sows fed the sensory additive diet showed an increase in feed intake during days 13 to 15 and 19 to 21 (P < 0.05) and a trend for overall increased intake from day 10 to 21 (P < 0.10) when compared to control-fed sows. Although not significantly different, sows fed the sensory additive had numerically heavier weaning body weight (239.4 vs 236.2 kg, P = 0.42) and increased litter weight gain (51.96 vs 48.69 kg, P = 0.24) compared to control-fed sows. In the second study, no differences were observed in sow feed intake (P > 0.4), 10th rib backfat thickness change (P = 0.632) or body weight change from d 110 of gestation to weaning (P = 0.65). However, sows fed the sensory additive had a reduction of 4.9% in preweaning piglet mortality (P = 0.05) and weaned an additional 1.42 piglets (P = 0.026) with similar piglet weaning weights (P = 0.86) when compared to controls. These preliminary studies indicate that lactation diets supplemented with a sensory additive has potential to stimulate sow appetite during the lactation period, particularly during the summer months, which may enhance sow reproductive performance. Zhang et al. (2016) observed that supplementation of two sources of phytogenic compounds, alone or in combination, were beneficial in improving feed intake and body condition in lactating sows (P < 0.05) and the combination also resulted in the least (P < 0.05) reduction of backfat thickness during lactation among the groups. Interestingly, while Hall et al. (2021) observed no significant differences in sow reproductive performance to phytogenic compounds added to the sow diet, they did observe an enhancement in offspring performance in the pigs weaned from those sows.

Although results are inconsistent, preliminary evidence suggest that feeding phytogenic feed additives and sensory additives to sows during late gestation and lactation may enhance intake, reduce excessive weight loss during lactation and enhance subsequent progeny health and growth performance. More studies from varied environments are warranted and the concept of multistate participation in a common protocol offers an ideal environment for this important technology evaluation.

Accomplishments of the Previous Project: Due to other research commitments at the various participating stations (i.e., an externally funded project or a graduate student project) exclusive use of the sows at all participating stations to contribute to the multi-state project was not possible. However, progress was made in all objectives, and there have been refereed publications as well as abstracts resulting from the project in all objective areas. With regard to the specific objectives for the S-1081 project (2018-2023), the following has occurred:

  1. Boron in sow diets: Currently, only one station (University of Kentucky) has contributed data, but the total contribution was significant. More than 200 litters were collected on the objective with all associated sow and piglet performance numbers. Progeny from the sows were continued and the same sow supplementation levels through to market weight with slaughter of subsets of pigs at different time points. When pigs were slaughtered, carcass data as well as tissue data was collected. Tissue mineral concentrations as well as bone mineral content and breaking strength were measured. Two abstracts of the progeny data have been presented and a PhD dissertation associated with the project will be defended in mid-2023. It is anticipated that there will be three refereed publications eventually resulting from the research. The timing of the publications will depend on whether additional stations decide to further pursue the objective.

  2. Phytogenic feed additives and reproductive performance of sows: Two stations (University of Kentucky and Virginia Tech) participated in this objective with a commercial feed additive that contained some phytogenic properties. Data from more than 150 litters were collected for the sows and their progeny. In addition, more than 60 gilts were bred and used for a mid-gestation (about day 45) slaughter for the collection of numerous ovarian, uterine, and fetal measures. The data has been pooled and analyzed and a manuscript is being prepared for submission. A numerical, though not statistically significant, increase in sow daily feed intake, that was associated with a statistically significant increase in litter weaning weight was observed. This observation along with the previously described preliminary studies at the University of Arkansas serve as the impetus to further pursue phytogenic/sensory additives for sow diets in the current proposal.

  3. Phase feeding of gestational sows: One station (South Dakota State University) participated in this objective with one study examining SID Lys:energy ratios throughout gestation using 50 sows and their offspring from birth to market. The data has been pooled, analyzed and a manuscript is in preparation. There were minor differences in sow weight and reproductive performance; however, pigs from sows with greater SID Lys:energy in late gestation were heavier at weaning and had lower days to market weight. In addition, an exploratory data analysis was conducted to discover patterns, identify anomalies, and check assumptions of previously published amino acid requirement models for gestating sows. The amino acid requirements estimated based on maximum growth and protein retention do not appear sufficient to optimize key physiological responses associated with survival and reproduction. The manuscript is currently in preparation.



  1. 1. To examine supplemental histidine on the lactation performance of sows.
  2. 2. To evaluate potential biomarkers related to sow metabolic status, piglet vigor, and reproductive potential.
  3. 3. To determine the effect of a sensory additive during lactation on sow feed intake and reproductive performance.


Measurement of Progress and Results


  • For all objectives, evaluation of reproductive performance will be based on changes in sow body weight in gestation and/or lactation, litter size at birth and weaning, offspring growth (suckling and post-wean), days to estrus in subsequent parities, and lactation feed consumption.
  • For Objective 1 (histidine supplementation to sow diets): the impact of supplementation on milk analysis (total solids, fat, lactose, and protein) will also be assessed by at least two stations.
  • For Objective 2 (potential biomarkers): the impact of sow Hb status on colostrum and milk iron content will also be assessed.
  • For Objective 3 (sensory additive): the impact of phytogenic feed additives on fecal consistency (as an indicator of constipation) will be evaluated. The influence of dietary treatment on milk nutrient composition as a contributor to potentially improved offspring performance will be assessed by continuing a portion of the weaned pigs on a nursery study with/without the sow additive.

Outcomes or Projected Impacts

  • Across all objectives, we will be able to determine if sow reproductive efficiency is affected when sows are administered one of the respective feeding strategies or alterations. Where results are positive, increases in sow productivity will increase producer income. An agricultural economist will be engaged to provide an assessment of the economic feasibility of the application of this research.
  • Each project has potential to not only improve sow reproductive performance but also enhance offspring performance. Additionally, this project will contribute a greater understanding of the impact of sow feeding strategies over multiple parities - knowledge that is critical to sow wellbeing and which will reduce sow culling, thereby improving long-term success and sustainability of pork production.
  • While it is difficult to predict an economic impact of these strategies, a 1% reduction in sow mortality equates to 62,000 sows annually, and a 1% reduction of pre-wean mortality would result in > 1 million additional market hogs without an increase in litter size or size of the sow herd, both of these representing a substantial economic impact.
  • Additionally, it should be noted that increases in sow productivity will result in the apportionment of her waste output over more pigs, thus decreasing improving the environmental impact of the sow production phase of the swine industry.



Projected Participation

View Appendix E: Participation

Outreach Plan

Data from each objective will be published first in abstract form by the coordinator of the objective, and he/she will present the data orally or in poster form at a national or regional scientific conference. Next, a manuscript will be prepared and submitted to a refereed journal, probably the Journal of Animal Science. Each participant on the objectives may choose to publish their contribution to the objective in producer field days, university publications, or in a graduate student's thesis or dissertation. Also, some members of the committee hold extension appointments, and all members of the committee interact with their extension colleagues or participate in extension functions. Results will also be published in trade magazine Research Reports with the specific note that the results are from a multi-state research project.


The Multi-state research committee includes the regional Administrative Advisor (nonvoting); a technical representative of each cooperating experiment station, appointed by the respective Director; and a non-voting, consulting member representing CSREES. The Committee will elect a Chair, Vice-Chair, and Recorder. The Recorder position will be elected each year; the Vice-Chair will move into the position of Chair, the Recorder will move to the position of Vice-Chair. Administrative guidance will be provided by an assigned Administrative Advisor and a CSREES Representative.

Literature Cited

Citations for Issues and Justification section NEW PROPOSAL


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Aaron, D. K., and V. W. Hays. 2004. How many pigs? Statistical power considerations in swine nutrition experiments. J. Anim. Sci. 82(E. Suppl.):E245-E254.

Bhattarai S., T. Framstad, J. P. Nielsen. 2018. Stillbirths in relation to sow hematological parameters at farrowing: A cohort study. J Swine Health Prod. 26(4):215-222.

Bhattarai S., T. Framstad, J. P. Nielsen. 2019. Association between sow and piglet blood hemoglobin concentrations and stillbirth risk. Acta Vet Stand 61, 61.

Cho J.H., Y.J. Chen, B.J. Min, H.J. Kim, O.S. Kwon, K.S. Shon, I.H. Kim, S.J. Kim, and A.  Asamer. 2006. Effects of essential oils supplementation on growth performance, IgG concentration and fecal noxious gas concentration of weaned pigs. Asian-Aust J Anim. Sci. 19:80–85.

Clowes, E. J., F. X. Aherne, A. L. Schaefer, G. R. Foxcroft, and V. E. Baracos. 2003. Parturition body size and body protein loss during lactation influence performance during lactation and ovarian function at weaning in first-parity sows. J. Anim. Sci.  81:1517–1528. doi:10.2527/2003.8161517x

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Eckberg, B. 2022. 2021 Sow mortality analysis. National Hog Farmer, February 3, 2022. https://www.nationalhogfarmer.com/animal-health/2021-sow-mortality-analysis

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Giallongo, F., A. N. Hristov, J. Oh, T. Frederick, H. Weeks, J. Werner, H. Lapierre, R. A. Patton, A. Gehman, and C. Parys. 2015. Effects of slow-release urea and rumen-protected methionine and histidine on performance of dairy cows.  J. Dairy Science, 98:3292-3308.  DOI: http://dx.doi.org/10.3168/jds.2014-8791

Giallongo, F., M. T. Harper, J. Oh, C. Parys, I. Shinzato, and A. N. Hristov. 2017. Histidine deficiency has a negative effect on lactational performance of dairy cows. J. Dairy Science, 100:2784-2800. DOI: https://doi.org/10.3168/jds.2016-11992.

Hall, H. N., D. J. Wilkinson, and M. Le Bon. 2021. Oregano essential oil improves piglet health and performance through maternal feeding and is associated with changes in the gut microbiota. Anim. Microbiome 3:2. https://doi.org/10.1186/s42523-020-00064-2

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Hu L, L. Che, C. Wu, M.V. Curtasu, F, Wu, Z. Fang , Y. Lin, S. Xu, B. Feng, J. Li, Y. Zhuo, P. K. Theil, and D. Wu. 2019.  Metabolomic profiling reveals the difference on reproductive performance between high and low lactational weight loss sows. Metabolites 4;9:295. doi: 10.3390/metabo9120295.

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Kiefer, Z. E., J. M. Studer, A. L. Chipman, M. K. Adur, C. Mainquist-Whigham, N. K. Gabler, A. R. Keating, and J. W. Ross. 2021. Circulating biomarkers associated with pelvic organ prolapse risk in late gestation sows. J. Anim. Sci. 99: 1-8.

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