Scientific progress in ruminant production in the 1 st decade of the XXI century

Scientific advances in nutrition of dairy cows in the first decade of the XXI century have occurred. This paper will review the most common additives fed to transition cows to decrease the incidence of metabolic disorders, which will be discussed separately with emphasis on their mechanisms of action, utilization and efficiency. Some changes on protein in the 2001 updated version of the Nutrient Requirements of Dairy Cattle to reach better precision of the nitrogen and amino acid requirements of lactating cows also are presented. Many of the advances in nutritional manipulation of milk fat concentration are related to fat supplementation, then the relationship between the action of rumen microbes on biohydrogenation of dietary polyunsaturated fatty acids and milk fatty acid profile are discussed as well as the main factors identified as being responsible for milk fat depression.


Introduction
This paper will review the major scientific advances in nutrition of dairy cows in the first decade of the XXI century.Although many factors influence the performance of cows, the focus of this paper is to present the main research areas studied over the last ten years.Moreover, new areas of research in cow nutrition that have attracted attention over the last few years also are presented.The most common additives fed to transition cows to decrease the incidence of metabolic disorders are presented and discussed separately with emphasis on their mechanisms of action, utilization and efficiency.Changes were brought by the 2001 updated version of the Nutrient Requirements of Dairy Cattle (NRC, 2001).Better precision of the nitrogen (N) and amino acid (AA) requirements of lactating cows has provided improved guidelines for establishing concentration of crude protein (CP) in the diet of lactating cows.A mechanistic model has been developed from in situ data for calculating the rumen undegradable protein (RUP) content of feedstuffs, which is based on individual characteristics of each feed ingredient.As result, RUP concentration is now better defined for individual feed ingredients.Moreover, more emphasis is placed on the relationship between CP concentration of the diet and N excretion to avoid environmental pollution.Manipulation of milk fatty acid (FA) profile also is discussed with emphasis on the sources of fat required to meet the consumer's demand for R. Bras.Zootec., v.39, p.478-490, 2010 (supl.especial) products resulting in better human health.Under the action of rumen microbes, biohydrogenation of polyunsaturated FA originating from the diet is very high.Therefore, the most common treatments that have been studied to prevent FA biohydrogenation are discussed.Many of the advances in nutritional manipulation of milk fat concentration are related to fat supplementation.Therefore, the main factors identified as being responsible for milk fat depression are presented.

Additives for dairy cows
In the last decade, several studies on the effects of additives on metabolism of the transition dairy cow and milk yield in early lactation have been published (Grünberg et al., 2009;Moreira et al., 2009;Ramos-Nieves et al., 2009).Ionophores and anionic salts are the most common additives used on dairy farms and the most evaluated in scientific trials.This paper will review the main additives that have been fed in the transition period (from three weeks prepartum until three weeks postpartum) in early lactation, and briefly, in mid and late lactation of dairy cows.

Antibiotics (Ionophores)
The main ionophores used in the nutrition of bovine are monensin (Rumensin ® , Rumensin Controlled Release Capsule ® (CRC), and Monensin ® ), lasalocid, salinomycin, and laidlomycin propionate.However, there are more than 120 ionophores available, but not all have license to be used in animal nutrition.Monensin is a carboxylic polyester antibiotic ionophore produced by natural fermentation of Streptomyces cinnamonensis and its utilization in dairy cow nutrition is approved in many countries such as Argentina, Australia, Brazil, Canada, New Zealand, South Africa, and USA (Odongo et al., 2007a).Administration of monensin is done through the diet, mixed in the minerals or the concentrate, or through a CRC (Mutsvangwa et al., 2002;Petersson-Wolfe et al., 2007).

Mechanism of action
Ionophores are liposolubles and their basic function is to create a transport flow of ions across cells.They depress or inhibit rumen microorganism growth, specially Gram (+) bacteria, and stimulate the development of bacteria that produce propionic acid, thus increasing glucose concentration in blood (Juchem et al., 2004;Zahra et al., 2006).Moreover, ionophores such as Rumensin CRC (300 mg/cow/day) may alter volatile FA (VFA) production in the rumen (Conti et al., 2008).The effects of ionophores on the metabolism of beef and dairy cattle are similar (McGuffey et al., 2001).
Apparent benefits of ionophores during the transition period of dairy cows are linked to lower loss of body condition as shown by lower concentration of non-esterified FA (NEFA) in blood and, consequently, decreased concentration of b-hydroxytutyrate (BHBA) in blood (McGuffey et al., 2001).Monensin also improves body condition (Melendez et al., 2004), energy status and decreases the incidence of some metabolic disorders such as ketosis, abomasum displacement, and placenta retention (Duffield et al., 2002;Duffield et al., 2003) when fed at doses ranging from 9 to 23 mg/kg of DM/day.Moreover, monensin supplementation decreases the incidence of milk fever as shown in Table 1 from data obtained from more than 2.300 cows in Québec, Ontario, and Prince Edward Island (Canada).
In a meta-analysis of the impact of monensin on cow performance, Duffield et al. (2008a) reported that monensin supplemented from day 21 to 60 before calving as topdress or CRC with doses ranging from 254 and 320 mg/d decreases the incidence of ketosis, abomasum displacement and mastitis although there was no effect on the incidence of milk fever, laminitis, dystocia, placenta retention and metritis.Monensin has no effect on dry matter intake (DMI) of dairy cows during the transition period and in early lactation (Ruiz et al., 2001;Eifert et al., 2005;Fairfield, et al., 2007).However, Duffield et al. (2008b) observed that monensin depressed DMI (0.3 kg/d) and increased milk production (0.7 kg/d), thus improving milk production efficiency (2.5%).Martineau (2004) also observed a slight decline in DMI (2.5%) and an improvement in milk production (2.8%) of dairy cows in mid lactation fed ionophores but milk percentages of fat and protein decreased by 0.13% and 0.03%, respectively (Table 2).Nevertheless, the effects of ionophore supplementation on milk yield are inconsistent.Some studies have shown positive effects of ionophores on milk yield and milk protein yield (McGuffey et al., 2001) but others reported no significant effects (Juchem et al., 2004;Zahra et al., 2006;Fairfield et al., 2007;Chung et al., 2008;Conti et al., 2008).
A function also attributed to ionophores is the reduction of methane production as shown for cows fed monensin at a dose of 24 mg/kg of DM (Martineau et al., 2007;Odongo et al., 2007a).Methane may contribute to ozone layer destruction, and a reduction in methane production may be good for the environment.However, studies with dairy cows on pasture receiving a monensin CRC did not show any effect on methane production when fed at doses of 240 mg/day/cow (Grainger et al., 2008).
Some byproducts from feed processing containing oil or pectin, such as cottonseed hulls, citric pulp, and soybean hulls are incorporated in the diet of cows during the transition period, which could be a good strategy to improve farm profitability and reduce methane emission.Bacteria that degrade pectin are Gram (-) and resistant to monensin (Melendez et al., 2004).Pectin degradation does not produce lactic acid as does starch degradation.Therefore, monensin may affect pH, and the molar proportions of VFA and concentrations of lactate in the rumen, and concentrations of blood metabolites of cows during the transition period (Melendez et al., 2004).
There is an agreement in the literature that supplementation of monensin and other ionophores in the diet of dairy cows during the transition period may decrease the incidence of ketosis and abomasum displacement and milk fat percentage, and increase LCFA concentration in milk fat.However, other effects attributed to monensin are inconsistent.

Utilization and efficiency
Ionophores are recommended to minimize problems during the transition period of dairy cows.Utilization of ionophores has shown cost benefits for farmers although some studies have reported inconsistent results.

Anionic salts
Anionic salts are minerals with a negative charge supplied mainly by chlorine and sulfur used for the prevention of milk fever occurring early after calving of dairy cows.They are characterized by a bitter taste which may affect ingestion of anionic diets by cows.However, it is possible to mask the bitter taste of the anionic diets through the addition of molasses.
to maintain calcium homeostasis around parturition.They observed that timothy hay with low compared to high DCAD decreased urinary pH and blood bicarbonate concentration at 2 mM.They concluded that timothy hay with low cation-anion difference was effective to improve calcium homeostasis with no interference on DMI.Therefore, utilization of grass hay with low DCAD is another alternative to diminish the severity of hypocalcemia in dairy cows (Charbonneau et al., 2008;Penner et al., 2008).Urinary pH monitoring is another way to observe the efficiency of anionic salts administration.Adequate amounts of anionic salts should result in urinary pH values between 6.0 and 7.0 (6 h postfeeding) for most breed of cows and values between 5.5 and 6.0 for Jersey cows as they are more vulnerable to hypocalcemia (Herdt, 2000).A great problem with anionic salts utilization is their low palatability, mainly for ammonium chloride, which may decrease DMI and then augment the risk of fatty liver and ketosis in the postpartum period (Ortolani, 2002).

Anion function
To better understand the utilization of anionic salts in the diet of the transition dairy cow, one must understand some metabolic conditions that regulate calcemia in the bovine.Three elements act on the maintenance of calcemia.A first hormone is calcitonin that suppresses bone resorption through diminution of osteoclastic activity, and a second one is parathyroid hormone (PTH) that has the opposite effect (hypercalcemia).A third element is 1,25dihydrocholecalciferol, an active metabolite of vitamin D with similar functions to PTH (Blood and Radostits, 1991).Diets with anionic salts or acidifier agents promote metabolic acidosis that may improve action of PTH and 1,25dihydrocolecalciferol (Penner et al., 2008).The DCAD may be inadequate if cows have acidosis or metabolic alkalosis, and the balance may be calculated using the following equation proposed by Block (1984) Forages rich in cations (potassium) such as grass silage and hay will favor metabolic alkalosis, thus contributing to higher milk fever incidence (Charbonneau et al., 2008).Some researchers have fed forages with low DCAD to reduce urinary pH and blood concentration of bicarbonate (Charbonneau et al., 2008), which facilitates administration of anion sources that are normally refused by cows.A negative DCAD is desirable for cows during the dry period to increase bone calcium mobilization and blood calcium concentration, thus preventing clinical and subclinical signs of milk fever.Chlorine based products are more efficient than sulfur based products to meet desirable negative charges in the diet (-100 to -150 meq/kg DM) (Cavalieri & Santos, 2001) but as chlorine based products are expensive, nutritionists have been using a mixture of different sources.

Utilization and efficiency
Recommended dosage may vary according to the product being used.First, it is necessary to analyze mineral composition (potassium, sodium, chlorine, and sulfur) of forages and concentrates fed to dairy cows during the transition period and then to calculate the DCAD of diets for transition cows.Urinary pH monitoring of transition cows allows verification of the dietary DCAD, with normal values ranging from 6.0 to 6.5.
Milk fever is costly to farmers because of lower milk yield and higher incidence of health problems.Low levels of calcium in blood in the postpartum period will affect placenta expulsion due to low uterine contractility.In the same way, lower contraction of sphincter smooth muscle may contribute to higher incidence of mastitis.Therefore, it is highly recommended to prevent milk fever during the transition period, especially by administration of anionic diets to cows producing more than 28 kg of milk at lactation peak and to cows from the Jersey breed.At the same time, it is important to care about administration of calcium in the diet.It is recommended to provide around 150 g of calcium per cow daily (Cavalieri & Santos, 2001).

Protein
The XXI century began with publication of an updated edition NRC (2001) that intended to approach the cow's requirements for desired yields of milk and milk protein with minimum amounts of CP in the diet.Thus, it can be assumed that the goals of ruminant protein nutrition are to optimize utilization efficiency of dietary CP which depends on protein and non-protein nitrogen (NPN) concentrations of the diet.These two fractions of N will provide the types and amounts of rumen degradable protein (RDP) for synthesis of microbial crude protein (MCP) in the rumen, and of RUP that will provide AA for absorption in the small intestine by the host.
Some differences between the updated edition (NRC, 2001) and the previous one (NRC, 1989) are found in terminology.To avoid the implication that all proteins are absorbed, the term metabolizable protein (MP) replaces absorbed protein, and also, the terms degradable intake of CP (DIP) and undegradable intake of CP (UIP) are replaced with RDP and RUP, respectively.A constant digestibility of 80% was used for RUP of all feeds by NRC (1989) while RUP Scientific progress in ruminant production in the 1 st decade of the XXI century digestibility is considered to be dependent on feed type in NRC (2001).
There is an effort to calculate more accurate values of RUP in the current edition instead of using constant values of RUP in the former one.A mechanistic model developed from in situ data is used for calculating the RUP content of feedstuffs, which is based on how rapidly the protein degrades in the rumen, related on rate of digestion (Kd) of feedstuff and its properties, and how rapidly the feed passes through the rumen, related on rate of passage (Kp) according to DMI and percentages of concentrate and neutral detergent fiber (NDF) in the DM (Figure 1).
The effect of RDP from NPN sources such as urea and from true protein sources on N efficiency and excretion by dairy cows has been evaluated.An adequate diet formulation to not exceed RDP content will contribute to optimize N efficiency utilization by dairy cows and to reduce N excretion and environmental pollution.For example, high producing Holstein cows fed a diet with 16.5% CP and 1.9% (DM basis) urea had higher concentration of urinary N, milk urea nitrogen (MUN), blood urea N (BUN) and lower N efficiency than those fed true protein sources, which would increase the potential for N pollution (Brito & Broderick, 2007).Moreover, Broderick & Reynal (2009) offered increasing levels of urea (0, 0.41, 0.84, and 1.31% of DM) to dairy cows fed a diet containing 16.1% CP (DM basis) and they observed a linear increase in excretion of urinary and fecal N, concentrations of MUN, BUN, and ruminal ammoniaN.
In the first decade of the XXI century, Journal of Dairy Science has published more than 176 articles on protein for dairy cow nutrition.Authors have evaluated the efficacy of N and AA utilization by dairy cows in a large number of published articles as demonstrated in Figure 2.Moreover, other studies have been looking at synchronization of RDP with that of soluble carbohydrates to improve synthesis of MCP, milk urea nitrogen (MUN) concentration, and the need to reduce N excretion by cows to decrease environmental pollution and feeding costs.
According to Broderick (2003), concentration of dietary CP above 16.7% (DM basis) has no benefit in terms of milk yield or milk components although 18% CP (DM basis) is commonly reported in the diet of high producing dairy cows.Law et al. (2009) assumed that in early lactation, dietary protein concentration above 17.3% has beneficial effects on milk yield and that better N efficiency can be achieved by feeding diets with lesser dietary CP levels (14.4-17.3%)without realizing detrimental effects on milk production of late lactation cows.Briefly, some studies indicate that improved N efficiency is reached when dairy cows producing 35-40 kg/day of milk are fed diets with 16.6 to 17% CP (% DM) (Frank & Swensson, 2002;Olmos Colmenero & Broderick, 2006).
Attempts to diminish CP concentration in the diet of dairy cows is desirable for the purpose of enhancing N utilization as showed by Kalscheur et al. (2006) in Figure 3.These authors related that N efficiency declined as RDP in the diet of dairy cows increased (6.8, 8.2, 9.6, and 11.0 % of RDP in DM basis) and that N efficiency decreased as CP intake increased within treatments with the lowest dietary RDP concentration.Moreover, MUN concentration increased linearly from cows fed the lowest concentration of RDP to cows fed the highest one.
Assessment of MUN concentration analyzed in bulk tank or individual milk samples has been associated to urinary N excretion as shown in Figure 4 by Nousiainen et al. (2004).According to Kohn et al. (2002), urinary N excretion may be estimated by the following mathematical equation: 0.026 * BW (kg) * MUN (mg/dL).Moreover, MUN
Source: Data from authors (2010) Source: Kalscheur et al. (2006) concentration is considered a great tool to identify overfeeding or underfeeding of CP in diet of dairy cows.Then, monitoring MUN could help to reduce excessive N in manure of dairy cows, monitor protein status of dairy cow diets and improve farm profitability.
Considerable effort has been extended to evaluate the synergism of rumen protein degradability with different sources of carbohydrates (CHO) for optimization of MCP synthesis.High-energy diets stimulate MCP synthesis, which increases the supply of MP for the animal (Cadorniga & Satter, 1993) but may increase also RDP requirements.However, attention must be considered to not over feed protein, which may result in excessive urinary N excretion and increase environmental pollution (urine and feces).
Studies on AA have increased since 2001 to bring more information about its usage in dairy diets for the purpose to augment N efficiency, milk and milk protein yield, and also to diminish N excretion.Among 20 AA, 10 AA are considered essentials i.e., that they are not synthesized by the animal.The only exceptions are arginine and histidine (HIS) that are synthesized by the animal but not in sufficient levels for high production.The two AA that have been mainly considered in research trials are lysine (LYS) and methionine (MET).According to NRC (2001), MET is the first limiting AA for milk production and its requirement increases when cows are fed small amounts of corn, high forage diets or RUP provided by soybean products, animal-derived proteins or associations of both.The second limiting AA is LYS and its requirement increases when corn and feeds of corn origin provide most RUP in diets of dairy cows.
Considerations about these two AA are related by NRC ( 2001): (1) content of protein in milk is more responsive than milk yield to supplemental LYS and MET, particularly in post-peak lactation cows; (2) increases in milk protein percentage are independent of milk yield; (3) casein is the most influenced milk protein fraction; (4) increases in milk protein production to increased supplies of either LYS or MET in MP are the most predictable when the resulting predicted supply of the other AA in MP is near or at estimated requirements; (5) milk yield responses to LYS and MET are more common in cows during early lactation than in mid or late lactation cows; (6) production responses to increased supplies of LYS and MET in MP typically are greater when CP in diet DM approximates normal levels (14 to 18%) than when it is lower or higher.
Research trials have related that increasing amounts of CP in the diet significantly increased MUN and urinary and fecal N excretion, and lower N efficiency (Leonardi et al., 2003;Broderick et al., 2008;Broderick et al., 2009).Leonardi et al. (2003) reported an increase in milk protein concentration (3.17 to 3.26%) when cows received rumen protected MET (RPM, 0.07 g/100g of DM) in a soybean-based diet independent of CP level (16.1% or 18.8%, DM basis).Broderick et al. (2009) observed a trend for higher milk true protein concentration when cows received a supplement of RPM (9 g/cow/day) in diets with 15.8% and 17.1% of CP with or without supplemental RUP from expeller soybean meal.
Changes in milk protein yield in several studies have been extensively related to supplemental AA (mainly MET) or considerable changes in amount or source of protein in the diet of dairy cows although as shown in a review from Jenkins & McGuire (2006), only modest changes in protein content of milk were reported (Figure 5).A small range of milk protein concentration from 2.85 to 3.27% occurred when protein content in the diet varied from 15.0 to 19.5% with many sources of dietary protein and rumen-protected AA.
According to Broderick et al. (2009), dairy cows fed a diet with 15.8% CP (% DM) with supplemental RPM showed similar milk yield and milk components to those fed 17.1% CP (%DM) without RPM.Moreover, a diet with RPM decreased CP content of the diet and consequently, decreased MUN and urinary N excretion by dairy cows, thus reducing potential environment pollution.
Additional research also is needed to obtain more accurate predictive models to determine AA requirements for high-producing dairy cows and to further investigate other essential AA that may limit or co-limit milk yield and N efficiency.According to Lapierre et al. (2006), determining and recognizing the influence of gut metabolism on each measurement (duodenal flow, apparent small intestinal disappearance, portal absorption) of available AA to peripheral tissues should provide additional information to create more accurate mathematical models for dairy cows AA requirements.

Lipids
The use of fat in ruminant nutrition, mainly for lactating animals, has the objective to increase energy density of the diet.Beyond this goal, fat has been used to change composition of the product, e.g., milk, in order to increase the incorporation of beneficial compounds for better human health.In the last decade, many new concepts about fat utilization in ruminant feeding have arisen.The release of an updated version of NRC (2001) brought important information about biohydrogenation of FA by rumen microbes, which can reach up to 90% for polyunsaturated FA that are not protected or 40% for those that are fed as calcium salts.
From 2001 to 2009, a total of 220 papers have been published in the main Brazilian national and international journals on the topic of "fat" for ruminant production, and emphasis was mostly on dairy cattle.The Journal of Dairy Science is the major source of these papers with a number of publications exceeding the number of the sum of all papers published in Animal Feed Science and Technology, Journal of Animal Science, Brazilian Journal of Animal Science (Revista Brasileira de Zootecnia).The emphasis on feeding fat to animals was to change composition of the product (e.g., milk and meat) for better human health.Because biohydrogenation of polyunsaturated FA by rumen microbes is high, treatment of fat sources to protect them was the target of many papers.Extensive work on treatments such as extrusion, micronization, grinding, and protection as rumen-inert calcium salts, and supplementation with products that alter ruminal fermentation (as ionophores) occurred.Moreover, sources of oilseeds, fish oil, tallow, and synthetic fat also were compared with the expectation to improve composition of the final product.Other research also has been carried out on modification of milk composition to increase the proportion of CLA in milk fat with comparison between cows on pasture and those fed a total mixed ration (TMR).
Many studies to increase the proportion of CLA in milk fat occurred during the last 10 years.In the early 2000s, Chouinard et al. (2001) tested many fat treatments (Figure 6) and concluded that calcium salts of soybean oil and linseed oil were more efficient to increase c9t11 CLA in milk fat than a control diet or a diet with calcium salts of canola oil.Moreover, extruded soybeans were better than micronized, roasted or ground soybeans to increase c9t11 CLA in milk fat.Extruded soybeans result in higher concentration of c9t11 CLA than a control diet (Chouinard et al., 2001;Neves et al., 2007) and temperature of extrusion (120ºC until 140ºC) has no effect on c9t11 CLA concentration of milk fat.Moreover, fish oil supplementation from 0 to 200 ml/d increased concentration of CLA in milk fat linearly and there was a quadratic effect when providing 400 mL/d of fish oil.
Pasture also has an important role on milk fat composition.According to White et al. (2001), the isomer c9t11 CLA is 83% higher when animals are fed on pasture compared to when they are fed a TMR.Moreover, many papers reported that pasture and fresh grass are more efficient to increase CLA and polyunsaturated FA proportions in milk fat than TMR or silage (Elgersma et al., 2004;Dewhurst et al., 2006).Access to a TMR and a mixed grass/clover pasture for 8 h daily also increases concentrations of 18:3n-3, t11c15-18:2, c9t11 CLA, and t11-18:1 in milk fat (Loor et al., 2003).On the other hand, Source: Jenkins & McGuire (2006) changes in herbage mass or daily herbage allowance has no effect on milk FA composition of grazing dairy cows (Palladino et al., 2009).
The choice of dietary fat to supplement in order to improve milk FA profile depends on the type of FA to increase in milk fat (e.g., increase CLA or polyunsaturated FA concentrations or decrease the omega 6:3 ratio).For example, fish oil is more efficient to increase CLA in milk fat than soybean oil (Qiu et al., 2004).However, c9t11 and t10c12 CLA concentrations are greater when feeding sunflower oil than when feeding linseed oil or fish oil (Loor et al., 2004).On the other hand, linseed oil is better than soybean oil to decrease the omega 6:3 ratio (Petit, 2002).In general, oils rich in linoleic acid are more effective in enhancing contents and yield of c18:1 (trans vaccenic -VA) and CLA in milk fat than oils containing linolenic acid (Bu et al., 2007).Or-Rashid et al. (2008) concluded that algae meal could be used to increase concentration of trans-18:1 isomers in rumen contents which serve as precursors for CLA synthesis in the mammary gland of dairy cows.
Many protection methods were studied and proved to be efficient to protect FA against ruminal biohydrogenation.Protection methods like calcium salts (Lundy et al., 2004), extrusion (Neves et al., 2007), andgrinding (Da Silva et al., 2007) of fat sources improve milk FA profile for better human health.Furthemore, monensin supplementation was successful in decreasing ruminal biohydrogenation of FA (Bell et al., 2006;Eifert et al., 2006;Da Silva et al., 2007).However, the use of essential oils such as cinnamaldehyde, condensed tannins, and saponins have a low potential to decrease ruminal biohydrogenation and modify milk FA profile when these extracts are used at practical feeding rates in dairy cow diets (Benchaar & Chouinard, 2009).Some studies were conducted to investigate the effect of supplemental fat on DMI.There was a negative effect of fat supplementation on DMI and the decrease was more important with diets rich in concentrate than with diets rich in forage (Ueda et al., 2003).Moreover, high ether extract concentration in the diet (7.5% of DM) had no effect on DMI (Gonthier et al., 2004).The detrimental effect of fat on DMI seems to be related to the source of fat.For example, unsaturated free FA more potently suppresses DMI than an equivalent amount of unsaturated triglycerides (Litherland et al., 2005).Moreover, linoleic acid, linolenic acid, and oleic acid have all suppressive effects on DMI of dairy cows (Drackley et al., 2007).
One of the most cited papers in Journal of Dairy Science is entitled "trans-10, cis-12 Conjugated Linoleic Acid Decreases Lipogenic Rates and Expression of Genes Involved in Milk Lipid Synthesis in Dairy Cows" (Baumgard et al., 2002), which relates the role of CLA in milk fat depression and is one of the major breakthroughs on the theories of milk fat depression in dairy cattle.Baumgard et al (2002) related that t10c12 and not c9t11 is the FA responsible for milk fat depression.Moreover, they observed that the depression was a result of a decrease in mRNA production of key-enzymes involved in fat production, i.e., a decrease rate of acetate incorporation into FA which decreases de novo synthesis and triglycerides production in mammary gland.This led to the publication of many papers relating the effect of t10c12 on milk fat depression (Giesy et al., 2002;Mackle et al., 2003;Gervais et al., 2009).Moore et al. (2004) related that the depression in milk fat concentration was linearly correlated with the level of t10c12 supplementation (Figure 7).Supplementation with amide and encapsulated t10c12 CLA to provide 10 g/cow/day of t10c12 CLA depressed milk fat concentration by 21 and 22%, respectively (Perfield et al., 2004).However, Piperova et al. (2004) reported that both t10c12 and t10-18:1 were involved in milk fat depression.According to Loor et al. (2005), increases in ruminal t11-18:1 may represent an effective strategy to maintain c9t11 CLA synthesis in the mammary gland.Furthermore, they hypothesized that besides t10c12 CLA, other biohydrogenation intermediates may be involved in milk fat depression.In a review paper on milk fat depression, Kadegowda et al. (2008) related that among trans-18:1 isomers, t10-18:1 was the most negatively correlated to milk fat percentage and that not only t10c12, but also t7c9 CLA may be potentially involved in milk fat depression.On the other hand, it was discovered recently that the addition of vitamin E to the diet partially prevented the depression in milk fat associated with the addition of oils to the dairy cow diet (Bell et al., 2006;Pottier et al., 2006).
Offer (2002) showed that a depression in milk fat proportion of c9t11 CLA could be achieved if the grass was simply cut and fed after a short period of wilting.This led Lee et al. (2007) to investigate the effects of green odor FA oxidation products (FAOP) from cut grass on lipid metabolism and microbial ecology using in vitro incubations of rumen microorganisms.They related that FAOP can alter FA biohydrogenation in the rumen, increasing it with changes in the microbial population that were detected through DNA and branched-and odd-chain FA profiling approaches.
In conclusion, there has been extensive research on feeding fat to dairy cows to improve composition of the final product although the source of fat to feed depends on the expected aim: increase concentrations of CLA, polyunsaturated FA, or omega 3 FA, or decrease the omega 6:3 FA ratio.Many sources of fat and methods have been employed to achieve these purposes.

Future Directions
Manipulation of product composition from animal origin will continue to be highly regarded in the future.Consumers' demand for high quality products resulting in better human health will remain of great interest.However, this will have to be reached by looking at ways that are compatible with preserving the environment and animal welfare.This means that emphasis will be placed on the study and development of innovative models of sustainable industry capable of optimizing production, processing and distribution of milk and dairy products and meeting the needs of lucrative domestic and export market segments.Another issue that is central to concerns is the establishment of directives supporting environmental protection through sustainable development throughout the entire dairy chain from farm to plate.Animal health and welfare are important aspects for sustainable development of the dairy industry.In fact, North American consumers have been increasingly calling for animal welfare regulations.Several American states now have adopted animal welfare laws through public referendum votes.Recent research covering all types of food shows

Figure 2 -
Figure 2 -Relative distribution of crude protein related articles published in the Journal of Dairy Science from 2001 through 2009, subdivided in amino acids, rumen degraded and undegraded protein (RDP and RUP), sources and levels of protein feeds, and nitrogen (N) excretion themes.

Figure 5 -
Figure 5 -Samples of data from published studies showing the extent to which milk protein percentage varies with amount and type of dietary protein.Diet protein percentage is shown in parenthesis following source of protein, which includes the amino acids methionine and lysine, soybean meal (SBM), corn gluten feed (CGF), heated soybean meal (HSBM), ground shelled corn (GSC), and extended soybean meal (ESBM)

Figure 7 -
Figure 7 -Temporal pattern of milk fat yield (A) and temporal appearance (B) of trans-10, cis-12 CLA (mg/g) in milk fat from cows fed increasing amounts of a rumen-protected conjugated linoleic acid (CLA) supplement during early lactation.

Table 1 -
Effect of monensin (Rumensin CRC ® containing 32 g of monensin) supplementation between week 2 and 4 precalving on some metabolic disorders in periparturient cows (number of occurrence and percentage)