Introduction

Livestock industry is facing many challenges of economic, environmental, and social sustainability worldwide. According to the requirements of ruminant species, the formulation of balanced diet is necessary because ruminants are facing many nutritional problems particularly feed formulation according to animal stage, performance, and production. The major factors which affect ruminants’ nutrition are protein quality, fiber ratio, feed type, rumen microbiota population, feed digestion, animal health, animal type, age, weather condition, and nutrient source (National Research Council (NRC) 2001). Protein is an essential nutrient and performs an important function in ruminants. In dairy cow, increasing the crude protein (CP) level in the diet not only increases milk production (Armentano et al. 1993; Wu and Sattar 2000) but also increases concentration of ruminal ammonia (NH3), blood urea nitrogen (N), and huge urinary N excretion (Castillo et al. 2001). Nowadays, researchers are paying more attention to feed lower CP rations with rumen-protected limiting amino acid (AA) supplementation to ruminant diets. In dairy rations, the benefits in reducing CP contents not only decrease N excretion and feed costs but also reduce greenhouse gases emission (GHG), ammonia pollution, and water and soil contamination (Wu and Satter 2000). The ruminal N losses play a major role in the metabolic losses as indigestible microbial N also reduces the absorption of AA in milk protein (approximately 72% of urinary N excretion was recorded in dairy cow, Tamminga 1992). Different sources and types of CP levels were examined like rumen-degradable protein (RDP), rumen-undegradable protein (RUP), and metabolizable protein (MP) to reduce N losses in ruminants. However, it was difficult to note the individual contribution of RDP, RUP, or MP in urinary N losses by animals (Lobley et al. 1995; Tufarelli et al. 2009a). The RDP is used for microbial protein synthesis (MPS), which provides energy in some cases when limiting conditions of energy in dairy animals. If RDP is not used for MPS, it will be converted into NH3 and absorbed via ruminal wall into the liver where it will be detoxified into urea and lost in the urine (Lobley et al. 1995). However, RDP is a major source which increases duodenal AA and peptide flow in animals (Choi et al. 2002), but sometimes, RDP acts as a key factor which affects the ruminants’ production, economic cost, and environmental pollution (Polan et al. 1991; Ali et al. 2009; Tufarelli et al. 2009b). The rumen-undegradable protein intake is a source of AA which is absorbed from the small intestine and is a vital source for milk production and growth of ruminants. Protein of the formulated diet is broken down by ruminal microbes into peptides, AA, and NH3, which are reutilized by microbes for synthesis of microbial protein, but the synthesis rate depends on passage rate of feed in rumen. The microbial protein cannot provide enough quantity of AA to maintain the AA balance of high milk-yielding cows (Polan et al. 1991; Ali et al. 2009). The main goal of feeding RDP is to provide a sufficient quantity of microbial protein to increase AA absorption in the small intestine of animals, but rumen-undegradable AA must be supplemented to high-producing cow rations in order to increase the flow of AA in the small intestine to maintain balance of animal performance (Leonardi et al. 2003). In ruminant ration, the balance of AA is the most significant factor compared with RDP in order to maintain milk production, milk protein yield, and N utilization in dairy cow (Noftsger and St-Pierre 2003; Laudadio and Tufarelli 2010). Feed plays a key role in the cost of animal products because it constitutes about 70% of production cost (Luiting 1990). Feeding 18% CP diets to sheep have the same results of dry matter intake, feed conversion ratio, and body weight performance as 16% CP, but economically, 18% CP diet was found to be significantly high (Abbasi et al. 2014). So, supplementation of limiting AA with low CP diets decreases the spilling of N in the environment and also reduces the environmental pollution (Archibeque et al. 2002).

Gas production from livestock farms and impact on the environment

Livestock industry plays an important role in producing GHG at approximately 18% including methane (CH4) and nitrous oxide (N2O) and has a significant impact on air quality and health of care takers and animals (FAO 2006). Emission of CH4 in China recorded 11% of total GHG accounts; practically, livestock industry produces 21% of CH4 due to enteric fermentation in ruminants (Zhang and Chen 2010). The livestock industry produces different gases into atmosphere which cause vast global warming and contributes 37% CH4, 64% NH3 emissions, and 65% N2O synthesis from manure, and these emissions of gases can damage the eco-system and act as a source of acid rain (United States Environmental Protection Agency (USEPA) 2004; Erisman et al. 2008). Emission of CH4 depends on the type of feed, population of microbes in the rumen, and fermentation rate of animals (Steinfeld et al. 2006). Regarding environmental pollution, cows contribute through excretion of more N in manure and produce CH4 which is more effective than carbon dioxide (CO2) emissions (FAO 2013). Methane production did not only cause environmental pollution but also energy loss from 2 to 12% of gross energy intake (Johnson and Johnson 1995). Globally, livestock contributes about 14.5% emission of anthropogenic GHG and 25% of enteric CH4 emission (Steinfeld et al. 2006; FAO 2013). Inadequate amount of RDP in the diet not only decreases the animal performance but also enhances CH4 emission as waste material in environment due to excess carbohydrate in ration (Johnson and Johnson 1995). Rumen methanogen and protozoal species population play a significant role in CH4 production (Clark et al. 2011); manipulating the population of both may alter the CH4 production in ruminant (Johnson and Johnson 1995). The process of enteric CH4 production in cow depends on microbe’s population, methanogen in the rumen, and feed intake diet factors (van der Maas et al. 2010) because CH4 is produced by anaerobic fermentation in the rumen forming methanogenic archaea for disposing metabolic hydrogen (H2) and CO2 formed by metabolic activity (Bhatta et al. 2014). Figure 1 illustrates the CH4 production pathway. The figure shows that hydrogen is produced by breakdown of feed stuffs into various fermentation indexes in the rumen and then utilized by methanogens for CH4 production in the rumen and N is excreted in manure and urine (Janssen 2010). The produced CH4 by rumen methanogen significantly contributes to greenhouse gas pollution in the ecosystem (Hristov et al. 2013).

Fig. 1
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Feed fermentation in ruminant’s body and emission of H2, CO2, N2O, and CH4 gases

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High utilization of crude protein in the diet and nitrogen loss from ruminant industry

Nitrogen utilization efficiency of dairy cows

Excreted N by ruminants is reacted in the environment and converted into several forms, i.e., nitrous oxide, di-nitrogen, nitrate, nitric oxide, and ammonia having significant harmful effects on human health as well as on water, soil, and air (Fig. 2). Farmers are facing several environmental disturbances due to excess N excretion. The N excretion quantities were noted to be low when animals were fed ration with low CP% approximately 12 to 21% and N volatilization were recorded 15 to 33% (Erickson and Klopfenstein 2010). Also, N excretion was significantly low when feeding diets are with low CP% during production round of dairy cow, decreasing 1% CP in the diet and reducing 8 to 10% N excretion, and when the CP level decreased up to 3 to 4% with supplementation of limiting AA, giving up good developmental performance which reduces N excretion up to 30% (Cole et al. 2005; Vasconcelos et al. 2007). Formulated feed with low CP% allows the animal to excrete minor N in manure that affects N mineralization rate which releases less plant-accessible N (Powell and Broderick 2011). Finishing diets of beef cattle with about 10 to 20% consumed N were utilized in animal tissues, 30 to 50% fed N excreted in feces, and the remaining 40 to 70% N were excreted in urine (Cole and Todd 2009). The efficiency of feed N utilization by animal in production was noted to be approximately 30%; the other 70% N were excreted from the animal body to the environment (30% excretion in feces and 40% in urine) (VandeHarr and St-Pierre 2006).

Fig. 2
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Illustrated modified diagram showing soil, water and environment pollution by excreted nitrogen, nitrous /nitric acid and ammonia (Adapted from: Ajinomoto Group Environmental Report 2009)

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Nitrogen excretion by ruminants as a source of water and soil pollution

Feed based on alfalfa and corn silage increases the excretion of urinary N and NH3 production from manure (Weiss et al. 2009). Moreover, feeding a ration based on lower CP is a good approach to deal with NH3 and N2O in manure (Dijkstra et al. 2011). Excessive N loss causes water pollution and acts as a significant factor for eutrophication of coastal and marine ecosystems (Howarth and Marino 2006). Excretion of excessive N in the environment causes eutrophication which increases the growth of algae, acidification, excessive breakdown of organic material, as well as decreasing oxygen in environment (Wright et al. 2001; FAO 2006). High excreted N concentration increases the hazards of aquatic system as animal manure is transformed into nitrate by soil microbes and increases the pollution of ground water when nitrate of ground water ends up drinking water and caused many health problems like methanoglobinaemia (Majumdar and Gupta 2000; Nosengo 2003). Losses of N cause water pollution and also affect production of animal farms (Bowmans et al. 2013). Protection from excessive amount of N in the soil is necessary to reduce the N from ground water. Waste water from production of feed or processing of feed is also a primary contributor of N in water bodies (Losso 1999; Bashan and Bashan 2004). In some conditions, the use of low-cost diet in animals to meet the nutrient balance and reduce the water and air pollution decreases farms’ gate cost of animal productions and reduces deforestation (Makkar 2013).

Excess nitrogen loss and ammonia emission by livestock industry

All ruminant farming systems defecate excessive N which contributes significantly to the increase in NH3 in the environment (Tamminga 1996). The N excretion in feces and urine depends on N% in ration, digestibility rate, and animal species (Devendra and Imaizumi 1989). Consumption of excessive feed N and their degradability in rumen is proportional to NH3 emissions from cattle manure and is caused by its high reactive property; NH3 is reacted in the atmosphere and converted into different forms, for example, in gas form like NH3 in fine particle (PM 2.5), ammonium sulfate, ammonium nitrate, ammonium bisulfate, sulfuric acid, nitric acid, and in liquid as cloud fog like ammonium hydroxide (USEPA 2004). These fine particles cause significant air pollution worldwide and is estimated to cause two million premature deaths annually (WHO 2005). The most number of quantities of dietary protein and non-protein compounds degraded by ruminal microorganisms into peptides, AA and NH3, and excessive urinary N at about 60 to 80% is excreted in the environment (Hristov and Jouany 2005; Reynal and Broderick 2005; Vander Pol et al. 2006). Decreasing the policy of dietary protein in the animal ration may decline the NH3 volatilization from manure (Krober et al. 2000). The volatilization of NH3 is the main reason of surface water pollution, soil eutrophication, and acidification in which approximately 50% of animal farming and 50% of animal waste emit NH3 in ecosystem, causing anthropogenic GHG into atmosphere (NRC 2001). Due to air pollution from nitrogen oxides, sulfur dioxide and NH3 increased acid rain, significantly threatening biodiversity and causing human health problems (McGinn et al. 2003; FAO 2006; Erisman et al. 2008). Further NH3 pollution in the atmosphere impacts animals’ health and performance. Odors also disturbs the daily life routine of peoples who are neighboring to ruminants farms or work at farms (Arogo et al. 2003; McGinn et al. 2003). Also, monogastric species performance is highly sensitive in excessive NH3 conditions (Wathes et al. 2004). Some of the major factors which affect NH3 production are soil conditions, soil fertilization, feed nitrogen concentration, management manure from animals, and environmental conditions (Arogo et al. 2003; Sommer et al. 2004; Gay and Knowlton 2009). However, the significant factor is dietary CP levels in the ration which affects NH3 emissions from animal manure (Paul et al. 1998). Approximately 79% of urinary N is excreted in the environment, and volatilization losses about 64 to 124% of N were recorded (Cole and Todd 2009). It was observed that urea is not a volatile, but when it meets with feces/manure of animals, it is quickly hydrolyzed into NH3 and CO2 by the urease action in manure material and also it was clearly recorded that urinary N is the main cause of ammonia nitrogen volatilization (Bussink and Oenema 1998).

Low dietary CP in the diet and effects on nitrogen loss

Protein categorized as the most significant nutrient in ruminants is gradually degraded by microbes in the rumen into peptide-bound amino acids (PBAA), free amino acids (FAA), and ammonia. However, PBAA, FAA source of AA, and ammonia are utilized for the synthesis of microbial protein by ruminal microbes. Further, some dietary-soluble proteins cannot be degraded in the rumen and escaped to increase the source and absorption of AA in the small intestine and increase milk production and milk contents of dairy cows (Choi et al. 2002). Feeding high concentration of N to dairy cow did not only increase microbial protein production but also raised rumen NH3 quantity and inevitably increase losses of N through urinary N excretion (VandeHaar and St-Pieere 2006). Trend of increasing CP% in dairy cow diets not only increase milk production but also increase surge of ruminal NH3 production with blood urea N and subsequently increasing the losses of urinary N (Castillo et al. 2001; Wu and Satter 2000). More research was done to decline N excretion by reducing CP% in ruminants ration; feeding dietary CP concentration from 18.4 to 15.1% is found to decrease the urinary N linearly with significant factor (Broderick et al. 2008). Microbes play a major role in the rumen as they ferment feed and produce volatile fatty acids (VFA), CO2, H2, CH4 gases, and play a key role in degrading feed N for production of microbial crude protein (MCP) as a major source of AA for ruminants (Cottle 1991). The production amount of MCP contributes to supply AA in the small intestine at 40 to 90% of total absorbable protein (Koenig et al. 2000). Approximately 25 to 35% of dietary protein is utilized and secreted in milk, but the residual portion of N is excreted in urine and feces (Broderick 2003). Furthermore, very positive and strong association was found among manure N output and intake of dietary protein concentration, decreasing the CP in the ruminant ration which reduces excretion of N to the environment (Yan et al. 2010). Also, feeding diet with low CP concentration has been recognized as an effective approach to decrease N excretion without effect on energy balance of ruminants (Agle et al. 2010).

Methionine as a limiting amino acid in ruminants

Methionine is categorized as a top limiting AA in ruminants (Schwab et al. 1992); naturally, it cannot be synthesized in humans and animals but their requirements balancing through supplementations or ingesting of methionine containing protein. Methionine biosynthesis in plants and microorganisms is related to aspartate family (Ferla and Patrick 2014). Many proteins are deficient in essential AA, rations which formulated form those protein sources for medium or high yielding dairy cow are very low in Met quantity and did not maintain the requirements of milk production and lean tissue growth. Bacterially synthesized CP is a major source of AA, but unfortunately, their amount is not satisfactory for the requirements and maintenance of lean tissue and milk performance of dairy cow (NRC 2001; Trinacty et al. 2006). To support these requirements for medium- or high-producing dairy cow, supplementation of this limiting AA in ration is a good opportunity. However, the addition of methionine is not an easy task in ruminant rations because of the rumen microbial environment breakdown of these AA and utilize for their own needs. To reduce this condition, animal nutritionists must add or supplement rumen-undegraded or rumen-protected forms of AA to increase flow of AA in the small intestine for absorption. Furthermore, Met works as a methyl donor source for S-adenosyl methionine plus de novo synthesis of choline AA from phosphatidyl ethanolamine (Berthiaume et al. 2006). After more assessment, the ratio of Lys:Met suggested in MP for maximum MPS is 3:1. It decreased CP% in ration from 18 to 19% to only 15–16% and increased reproduction of dairy cows in addition to declining quantity of excreted N in feces and urine (NRC 2001; Schwab et al. 2003).

Methionine catabolism and its role as a precursor of other amino acids

Methionine categorized as an essential and top-limiting AA in ruminant diet and incorporated into polypeptide chains plays a major role in the production of cysteine and α-ketobutyrate and is involved in the synthesis pathway of S-adenosyl methionine (SAM). In the transulfuration reactions, cysteine synthesis from homocysteine and α-ketobutyrate from serine. After that, in catabolism of Met α-ketobutyrate transformed into propionyl-CoA, then propionyle-CoA will be converted by mitochondrially ATP-dependent pathways a three-step develop procedure into succinyl-CoA. After conversion, succinyl-CoA will go to TCA for further oxidation under propionyl-CoA carboxylase, methylmalonyl-CoA epimerase, and methylmalonyl-CoA mutase enzymes, respectively. Methionine metabolic pathway regulation depends on the present amount of Met and cystine; the quantity of both AA must be satisfactory for SAM synthesis and have a positive role for cystathionine β-synthase. If Met is rare during the metabolic duration, it affects the synthesis of SAM quantity and will be obstructive for cystathionine β-synthase. If this situation happened, homo cysteine will be remethylated into Met by using N 5-methyl tetrahydrofolate (N 5-methyl-THF) which works as a methyl donor (Ferla and Patrick 2014). After that, the conversion of S-adenosyl-homocysteine will start by adenosyl-homocysteine hydrolase to produce homocysteine and adenosine. Vitamin B12 plays a major role for conversion of homocysteine into Met under homocysteine methyltransferase. Genetically, synthesis of Met encoded by the 5-methyltetrahydrofolate-homocysteine methyltransferase gene (MTR gene) which is placed on Iq43 chromosome and created 34 exons, producing three different spliced mRNAs (Ikeuchi et al. 2010). In the synthesis process of cysteine, homocysteine associated with serine to yield cystathionine which is catalyzed by the action of cystathionine β-synthase (CBS). After catalyzing, cystathionine breakdown γ-lyase into cysteine and α-ketobutyrate by cystathionine. During the synthesis period of cystine, many vitamins are needed for metabolic pathway (i.e., B12, B6, folate, pyridoxal phosphate); deficiency of these causes homocysteinemia or macrocytic anemias. Furthermore, sometimes cystine impedes the expression of cystathionine β-synthase gene which causes genetic disease methionineurea. After synthesis, cysteine plays a major role for different protein syntheses and is utilized for different body requirements. However, α-ketobutyrate works for TCA cycle as an intermediate succinyl-CoA; all reactions are presented in Figs. 3 and 4 (Ferla and Patrick 2014).

Fig. 3
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Role of methionine for synthesis of cysteine

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Fig. 4
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Pathway of synthesis of S-adenosyl methionine (SAM) from methionine

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Meta-analysis on low CP diet plus rumen-protected AA and effect on dairy cows

It is a good approach to reduce CP concentration with supplementation of rumen-protected AA without decline of milk performance and milk contents of dairy cows and decreasing the environmental pollution burden by ruminant industry (Leonardi et al. 2003). Cow during lactation period could fight to maintain the balance between energy and protein in the body. In this situation, most of the absorbed AA by the cow contributes to protein and large quantity of that protein deaminated in the liver and utilized by animal to get energy (NRC 2001). In pregnant cows, more than 50% of the absorbed AA is utilized by the growing fetus in order to balance the glucose concentration (Bell et al. 1995). Cows obtain AA by two main sources, the protein which is made by microbes in the rumen and that which bypassed the rumen. It is essential for these sources to be absorbed in the small intestine in the form of free AA to maintain animal performance. In some situation, when blood concentration decreased in the mammary gland, the animal has good capability to change the blood flow and extraction coefficients to get more AA to balance milk production and characteristics (Vanhatalo et al. 2003). During early lactation, animals fed ration with low CP and supplemented with rumen-protected Met showed good performance (Krober et al. 2000). Addition of Met or Lys in the ration combined or alone generally has good effect on early lactation period of cow (Patton 2010).

Nowadays, the ruminant industry researchers have more interest to increase the supplementation of rumen-protected Met and lysine (Lys) as the two top-limiting AA in ruminants (Lapierre et al. 2006). The most effective method is to increase the concentration of rumen-protected Met for dairy cows at the early lactation stage in order to increase milk and milk protein yield (Davidson et al. 2008) and also milk fat production (Overton et al. 1996). Supplementation of l-lysine-Hcl with ration containing steam flakes corn accelerated the synthesis of MCP and the flow of AA at duodenum (Bernard et al. 2004). Furthermore, supplementation of Met plus Lys to dairy cows increased the milk production, milk protein yield (Armentano et al.1997; Xu et al. 1998), and milk fat content (Xu et al. 1998). Analysis of the effects of supplementation of rumen-protected AA in milk production or contents depends on the availability of concentration of Met and Lys in the small intestine (Socha and Schwab 1994), and addition of these AA in ruminant ration increases the flow of AA at the small intestine and raise the percentage of milk production and milk contents (Schwab et al. 1992; Trinacty et al. 2009). Globally, nowadays, many animal nutritionists work on screening broadly the relationship between protein and supplementing AA on ruminant performance. Feeding ration containing 16.1% CP with supplementation of rumen-protected Met recorded the same milk quantity as 17.3% CP without supplementation (Broderick et al. 2008). Furthermore, milk yield was noted to be higher with supplementation of rumen-protected Met during early or mid-lactation to the basal ration (Armentano et al. 1993). Animals fed the basal diet formulated with corn silage, ground corn, and corn gluten meal significantly increased the milk yield and milk contents at mid- and total lactation period (Polan et al. 1991). Ration containing 14% CP supplemented with Met and Lys yields more milk and protein nitrogen than 18% CP-containing ration (Piepenbrink et al. 1996). Diets were supplemented at pre- and post-partum stages with rumen-protected Met and Lys increased milk performance of dairy cows (Socha et al. 2005). Formulated ration for dairy cow contained 15% CP or lower percent with addition of limiting AA increases N utilization efficiency to 30% or more than 30%, respectively (Haque et al. 2012). Furthermore, ration formulated at 14% CP supplemented with 25 g/head/day rumen-undegradable Met significantly increased milk production, fat%, total solid concentration, and yield of casein protein as compared with results of 16% CP control ration (Hosam et al. 2013). The data and the results of supplemented AA and performance of cows are presented in Table 1.

Table 1 Supplemented levels of rumen protected amino acids methionine and lysine their effects on milk production, fat, and milk protein
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Conclusion

Globally, the utilization of high CP concentration in ruminants’ diets is high to N excretion. To overcome over N or NH3 pollution, decrease pollution of water and soil, reduce feed cost, and improve herd performance and reproduction of ruminant species, supplementing diets with top-limiting AA is a key solution. The results of meta-analysis study proved that reducing the amount of surplus protein in ruminant ration with supplementation of limiting AA maintained the balance of MP, increasing milk yield. There is more opportunities for further research on other rumen-protected essential AA, rumen microbial manipulation, manure application, bedding, and feed additives to reduce nitrification, volatilization losses, and emission of greenhouse gases.