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Lipid Nutrition
Dairy Nutrition Management
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Lipid Nutrition Roy S. Emery, PhD, * and Thomas H. Herdt, DVM, MSt
Efficient production of milk requires efficient transfer of energy to milk. Fatty acids are a concentrated energy source. In the fed, lactating cow about 32% of the metabolizable energy intake may be metabolized through fatty acids. 2 Lipid metabolism is definitely important. Energy can come temporarily from adipose tissue. A cow in early lactation may typically mobilize 50 kg of body fat containing 365 Mcal of metabolizable energy. 18 This is enough energy to make about 31 7 kg of milk but the fat must be replaced before the process can be repeated. When this cow achieves positive energy balance, much of the milk fat must come from dietary fat. Dietary fat is the focus of this article. About one half of the fatty acids in milk, those with 16 carbons or less, are made in the mammary gland and the remainder are transferred to milk from blood. I9 Fatty acids of 16 to 18 carbons are made from acetate and 3-hydroxybutyric acid in adipose tissue, but these acids mostly remain in that tissue when cows are in positive energy balance. For this reason, nearly one half of the milk fat must come from dietary fat. The traditional ruminant diet contains about 3.5% crude fat or ether extract. Fatty acids, however, are the energy-rich portion of fat and the precursor for milk fat. Crude fat may be only 50% to as much as 90% fatty acids, depending on the source. High-producing cows frequently lack the fatty acids required to produce normal milk. NATURE OF FAT Different fatty acids have different characteristics. Stearic acid (CI8: 0) is a fully saturated or hydrogenated fat that contains 18 carbons, is solid or hard at room temperature, and is unique to animal fats. Oleic acid (CI8: 1) lacks hydrogen at carbon 9, which makes it liquid at room temperature. Palmitic a~id (CI6: 0) contains 16 carbons. All three of these acids are made in ruminant adipose tissue. Polyunsaturated fats, with unsaturation at more than one carbon, are found in vegetable oils. Crude fat in plants may contain as much as 48% nonsaponifiable lipids. 47 From Michigan State University College of Veterinary Medicine, East Lansing, Michigan *Professor, Department of Animal Science fDiplomate, American College of Veterinary Nutrition; Associate Professor, Department of Large Animal Clinical Sciences Veterinary Clinics of North America: Food Animal Practice-Vol. 7, No.2, July 1991
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EMERY AND THOMAS H. HERDT
N onsaponifiable lipids include such nonlipid components as galactose and glycerol, which are fermented in the rumen after hydrolysis (Fig. 1). Galactolipids comprise 70 to 80% of the crude fat in alfalfa and corn plants. 23 Nonsaponifiable lipids also contain pigments and steroids that are not digested and provide no energy. 43 The remaining fat in plants and the predominant fat in animals is triacylglycerol (triglyceride), which contains three fatty acids esterified to the Fat Fat sources supplement Forages Fat
type
FA's or TG
Cereal grains
Oil seeds
TG
TG
GlyL
RUMEN hydrogenation
hydrolysis Glycerol
TG
-E ---f
GlyL
O-FA O-FA O-FA
-f
OH FA OH + OH FA ...
>G fr-.
I
FA FA's-c::J o-suga1: OH acids FA - - - O-FA OH + FA=_ _ _ _ _ __ O-FA OH FA ~ \.. Microblal-r Microbial t-PL I ~I cells lEhospholipid~
syar
Traps
t
To intestine
08(!)
VFA's Inert fats
INTESTINE Pancreas
Liver
From { rumen
Pancreatic juice Bile ---.. Bile salts (BS)
Free fatty acids (FA's) Microbial phospholipids
Micelle Lumen of small intestine
Lecithin Phospholipases • Lysolecithin phospholipids (Lyso-L)
Mucosal tissue
Acetate
t
Lysolecithin .....--1r----- Bile Salts - - - - '
J--
Fatty acids
PhosphOlipids Cholesterol I Fatty racias Cholesterol-esters ----~
---"'J-- a-glycerol-P
t
Triglycerides
Glucose
t
Glucose (from blood)
Protein Phospholipids Cholesterol To lymph
Figure 1. Fat digestion and absorption. GlyL = glycolipids; TG = triglycerides; FA's = mixture of fatty acids; FA = saturated fatty acids; F A= = unsaturated fatty acids; VFA' s = volatile fatty acids; PL = phospholipids; Trans acids = intermediates in the hydrogenation process; Cl FA = fatty acids attached to food particles.
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three hydroxyl groups of glycerol. When one of the fatty acids is replaced by a phosphate moiety such as phosphorylcholine, the resultant molecule is a phospholipid. Phospholipids are an essential part of all living membranes. FAT IN THE RUMEN Fatty acids are not an energy source for rumen microbes. 23 Fatty acids are taken into the microbes, particularly those microbes that are adherent to particles of feed. 4,32 These anaerobic microbes hydrogenate 60 to 90% of the unsaturated bonds, forming fully saturated fatty acids or monounsaturated fatty acids with trans configuration. 36 These trans fatty acids inhibit synthesis of milk fat in the mammary gland. Microbes in the rumen also synthesize 0.1 to 0.2 kg fatty acid per day.15,60 This may seem like a trivial amount of fat, but fatty acid intake by cows fed typical diets may be only about twice this amount. Typical diets refers to diets that do not contain oilseeds or supplemental fat. Fat can inhibit digestion and fermentation in the rumen and may inhibit growth of the microbes themselves. 25 Unsaturated fats and free oils give more problems in this respect than oilseeds or saturated fats like tallow. There are fewer problems if fat intake is well distributed over time and within feed as occurs when total mixed rations are fed. Feed intake will decline if fat seriously inhibits rumen fermentation. This reduction of intake frequently offsets the increased dietary energy density achieved by adding fat to the diet. Increased postruminal digestion frequently compensates for decreased ruminal digestion, but this may not compensate for reduced feed intake. Fatty acids also favor production of propionic acid at the expense of acetic acid in the rumen and this reduces production of milk fat. 19,49 Feeding extra calcium reduces these harmful effects of fat in the rumen. In contrast to its repression of fermentation, fat also has several beneficial effects on rumen fermentation. Some microbes require certain dietary fatty acids for growth. 4 Fat seems to inhibit degradation of protein in the rumen and, thus, provide more dietary amino acids for postruminal absorption. 24 The yield of microbial protein per unit of carbohydrate fermented in the rumen is also increased by feeding fat. These benefits are offset by the decreased intake of ruminal fermentable carbohydrate and a possible decrease in fermentation. The net effect is that extra rumen undegradable protein should be fed when supplemental fat is included in the diet. FAT IN THE INTESTINE There is a variable but continuous entry into the intestine of fatty acids, microbial lipid, and rumen-inert fat as shown in Figure 1. The amount of fat entering the intestine due to microbial synthesis and biliary secretion exceeds the dietary supply. 41,43 Bile and pancreatic Huid are added proportionally to the ingesta How. Enterohepatic circulation is essential to the maintenance of bile How, formation of a colloidal solution of micelles, lipolysis, and absorption of fatty acids into the intestinal mucosa. Of the lipid in bile, 90% is phospholipid and the remainder is cholesterol, triglyceride, and fatty acid. The concentration of bile acids is twice that of lipids. 13 Pancreatic lipase is most active at pH 7 to 8 and requires the presence of phospholipids. In ruminants, pH of duodenal ingesta is more acid than pH 7 and remains relatively low until the jejunum is reached, which delays absorption of fatty acids; however, fatty acids are still absorbed before the ileum. 43
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The activity of pancreatic triglyceride lipase is somewhat lower in ruminants than in nonruminants. Phospholipase activity is present in excess of needs, which assures conversion of phospholipids to the better wetting agent, lysophospholipids. Lysophospholipids, bile acids, and fat reform into micelles, thereby facilitating lipase action. Taurine conjugates of bile acids, which predominate in ruminant bile, are unique in that they act at acid as well as alkaline pH.43,47 True digestibility of well-dispersed fatty acids is 83 to 92%.58 Stearic acid is the predominate acid present in the intestinal lumen due to hydrogenation of unsaturated acids in the rumen. Digestibility of palmitic acid and the monounsaturated acids may be somewhat greater than that of stearic acid. Digestibility of fatty acids can be much lower if the physical or chemical form inhibits dispersion. In one trial, flakes of stearic and palmitic acid had an apparent digestibility of only 47%.25 Fatty acid digestion may decrease as the concentration in the diet becomes excessive. Excessive is more than 2% added fat in the dry matter. 42 In another trial, digestibility of fat was 81 % when the diet contained 5.1 % crude fat, but only 56% when the diet contained 10.7% crude fat.48 It seems that the ruminant's ability to handle supplemental fat is limited at the level of the intestine as well as in the rumen. Supplemental fatty acids decrease calcium absorption by 25 to 40% and magnesium absorption by, perhaps, 15%.58 This indicates adding extra calcium and magnesium to diets that are supplemented with fat. Adding extra calcium and magnesium to diets for mice, and presumably for ruminant animals, enhanced fatty acid absorption. 7 Fatty acids infused into the abomasum appear in the lymph in 1 to 2 hours. 41 ,43 Some of this time is spent in the mucosal cell. Fatty acid transport into enterocytes is facilitated by binding proteins. 41 Lysophospholipids are transported by choline phosphotransferase. Fatty acids are esterified stepwise to glycerol-3-phosphate or to 2-monoacylglycerol. The supply of 2-monoacylglycerol in ruminant intestinal ingesta is limited due to lipolysis of triacylglycerol in the rumen. Lysophospholipids are also re-acylated in the enterocyte. The intestinal mucosa is also the primary site for cholesterol synthesis in cows, as confirmed by Z. Chen (unpublished data) in our laboratory, and cholesterol synthesis is increased as dietary fat increases. 41 Triglyceride, phospholipid, and cholesterol are added to apolipoproteins in the Golgi of enterocytes to form lipoproteins. The apolipoproteins are synthesized in the endoplasmic reticulum and moved to the Golgi. Vesicles containing these lipoproteins migrate to the baso-Iateral membrane and are secreted into the lymph by reverse pinocytosis. These newly secreted lipoproteins have a continuous spectrum of density from less than 0.93 to more than 1.006. 41 A small continuous supply of fatty acids, as would normally be presented from the ruminant intestine, results in production of lipoproteins with less triglyceride and higher density than typical chylomicrons. The smaller size and greater diversity of ruminant intestinal lipoproteins differentiate them from the classical chylomicrons found in nonruminant intestinal lymph. Lymph flows to blood via intestinal and thoracic lymph ducts and its flow is proportional to the lipid load. 43 This means that dietary fat is directed to all tissues, in proportion to blood flow, in contrast to nutrients absorbed into the portal vein that are directed to the liver. POSTABSORPTIVE FAT METABOLISM Newly secreted intestinal lipoproteins mature in blood by exchange of lipids and proteins among circulating lipoproteins. 17,41 Hepatic uptake of tri-
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glyceride from these nascent lipoproteins is hindered by the lack of appropriate enzymes. Dietary fat is therefore directed to extrahepatic tissues. 33 Typical lipid composition values for cow blood plasma are given in Table 1. These values vary widely with diet and with the different analytical methods used among laboratories, but several trends are consistent. Circulating triglyceride declines abruptly with the initiation of lactation, possibly due to transfer of blood fat to milk by the mammary gland. Cholesterol declines in late gestation and slowly increases after calving. This may be due to decreased feed intake in late gestation and early lactation. N onesterified fatty acids increase sharply during the negative energy balance accompanying early lactation. Although it is not indicated on the table, circulating triglyceride, cholesterol, and nonesterified fatty acids increase with increasing dietary fat. Lipoproteins react with lipoprotein lipase in the capillary endothelium of target tissues. 17,41 Fatty acids are released by lipolysis, which raises their local concentration after which they enter the cells by apparent mass action. Some fatty acids escape and move on to other tissues including the liver. Lipoprotein lipase activity is sensitive to energy balance, physiologic state, and hormones. Adipose lipoprotein lipase, for example, is stimulated by insulin while mammary lipoprotein lipase is stimulated by prolactin. These hormonal regulations and changes in blood How shift delivery of circulating triglyceride from adipose tissue to the mammary gland at parturition. Liver is not an important source of circulating triglyceride in cows, although secretion of small amounts of lipoprotein triglyceride can be detected. 30,52,53 Liver may even remove small amounts of triglyceride under some circumstances. 5 Excessive circulating fatty acids tend to accumulate in the liver as triglyceride and this can cause a number of problems resulting in increased culling and death. 22 Mazur et aP7 found abnormal lipoproteins with an increased ratio of triglyceride to protein in cows with hepatic lipidosis but it is not clear whether these lipoproteins were of intestinal or hepatic origin. Hepatic uptake of nonesterified fatty acids is proportional to the plasma concentration. Fatty acid concentration in the hepatic vein is 10 to 25% of the arterial concentration. 5,5o This measurement, however, ignores the large contribution of fatty acids by the portal vein. Hepatic blood How is 52% greater in fed, lactating cows than in dry cows, which means more fatty acids are presented to the liver during lactation. Under these conditions, the liver extracted 7% of the nonesterified fatty acids presented to it. 35 Fatty acids partially equilibrate between liver and plasma with an exchange of some palmitic for oleic and stearic acid. 10 Fatty acids are oxidized in the liver to release acetic acid and 3-hydroxybutyric acid to blood. Oxidation of palmitic acid accounted for 2% of the circulating acetic acid and 5% of the circulating 3-hydroxybutyric acid in the Table 1. Typical Lipid Composition (mg/l00 mL) of Blood Plasma Relative to Calving When Cows Are Not Fed Supplemental Fat STAGE OF LACTATION
UPID
Triglyceride Phospholipid Cholesterol N onesterified fatty acid
Dry
Calving
30-60 days in milk
18
4 185 115 18
210 190 10
210 168 6
16
(Data from many sources. Values vary with method of analysis and diet. Sources include references 48, 57, and 62.)
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EMERY AND THOMAS H. HERDT
fed state and increased to 16 and 29% during fasting. 45 ,5o These figures can be approximately doubled to reflect the total contribution of nonesterified fatty acid to circulating 3-hydroxybutyric acid, considering palmitic acid accounts for one half of the nonesterified fatty acids in blood. Oxidation is regulated by metabolites such as propionic acid and by carnitine acyltransferase, which is the enzyme responsible for transferring fatty acids into the mitochondria where they are oxidized. Malonyl-CoA inhibits carnitine acyltransferase. 9,27 This is particularly true when linoleic acid is the substrate. 55 Hydrogenation in the rumen limits the supply of polyunsaturated fatty acids such as linoleic acid, which makes this protection against loss through oxidation very important. The mammary gland takes up fatty acids from circulating triglyceride through the action of lipoprotein lipase. 19,62 Mammary lipoprotein lipase activity is increased by prolactin and by parturition itself.34 The activation due to parturition is thought to be due to removal of progesterone inhibition. Uptake is linear, amounting to 30 to 50% of the arterial concentration over the usual range of plasma triglyceride of 10 to 30 mg/l00 mL but decreases at greater concentrations. 1 This efficient uptake coupled with the large mammary blood flow causes a transfer to milk of between 30 and 76% of the plasma triglyceride. 46 Most of this plasma triglyceride must originate as dietary fat because only 7 to 13% of blood triglyceride comes from circulating nonesterified fatty acids. 54 The transfer of dietary fat to milk fat is energetically more efficient than synthesis of milk fat from acetic acid. Thus, dietary fat is essential for synthesis of part of the milk fat and is more efficient than synthesis of new fatty acids. Dietary fat decreases the mammary synthesis of short and medium chain fatty acids, and to some extent, replaces them with C 18 : 0 and C 18 : 1 fatty acids. 3 Trans-C 118 : 1 is a particularly potent inhibitor of fatty acid synthesis in both cow and mouse mammary tissue. The variation in concentration of trans fatty acids in cow milk accounted for about 25% of the variation in milk fat percentage among COWS. 61 The mammary gland has a net uptake of nonesterified fatty acids when the concentration exceeds 0.3 mmol/L (about 8 mg/l00 mL).31,54 At lower concentrations there is considerable exchange of fatty acid between plasma and mammary cells, which must be considered when interpreting isotope studies. Labeled nonesterified fatty acids may appear to contribute much more to milk fat than their actual contribution. The net effect is that fatty acids can transfer directly from adipose tissue to milk in early lactation when cows are in negative energy balance, but this contribution will be very small during positive energy balance. Adipose tissue is important to the cow because, other than the mammary gland, it is the primary site of fatty acid synthesis as well as an energy store. 63 Adipose tissue can also take up fatty acid from plasma triglyceride as described for mammary tissue. In this case, however, lipoprotein lipase is inhibited by lactation, negative energy balance, and growth hormone but stimulated by insulin. 17,38,64 Mobilization of fatty acid from adipose tissue is stimulated by parturition and growth hormone but inhibited by insulin. 39 To some extent, glycerides are hydrolyzed and reformed continually within adipose tissue. It is the release of fatty acids that increases with negative energy balance. 16,17 Cows frequently mobilize 30 to 50 kg of fat during the first few weeks of lactation. 6,18 This amount of fat can supply enough energy to make an extra 10 to 12 Ib milk per day for 60 to 90 days. The mobilization of body protein in early lactation is much smaller than that of energy and the protein is replaced much sooner. 20 It is important to supply enough ruminally undegraded dietary protein to balance the energy mobilized from adipose tissue.
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PRACTICAL ASPECTS OF FEEDING FAT Feed-grade fat is readily available in three forms: oilseeds, commodity fats, and specialty fats. Some characteristics of these sources of fats are listed in Table 2. Cottonseed and soybeans contain 18 to 20% fat. They are usually the first choice for supplemental fat because of price, acceptability by cows, convenience of handling, and relative inertness in the rumen. Cottonseed has the advantage over soybeans of supplying effective fiber to the ration. Cows can eat about 7lb of whole cottonseed per day (17% of dry matter intake) without contracting problems such as reduced feed intake or decreased percentage of milk fat. Gossypol toxicosis may occur if cottonseed intake exceeds 7 lb per day.28 Signs of this toxicosis are poor performance, anorexia, decreased packed cell volume, and red cell fragility. Toxicity is cumulative and may take several months to appear. Raw soybeans are less acceptable to cows than cottonseed and intake should be limited to 5 lb per day. Heating or roasting soybeans increases the cost as well as the acceptability to cows and permits intake of 7 lb per day. Heating increases the inertness of both fat and protein in the rumen. Cracking soybeans may increase their digestibility slightly but it also decreases ruminal inertness and may be contraindicated. Cows accept oilseeds better in a total mixed ration than if top dressed or fed in a grain mix. They may slow the rate of grain intake if fed in the milking parlor. Lupines with low alkaloid content have been developed as a feed in climates with shorter growing seasons. They contain about 10% fat and have been fed successfully as 18% of the dry matter intake. Lupines contain more fiber than soybeans but less than cottonseed. Comments on feeding soybeans apply to lupines. Rapeseed or the improved variety called canola contains about 40% fat, but feeding should be restricted to about 2 lb per cow per day. Sunflower seeds contain 35 to 40% oil and the whole seed can be fed as 5% of the dry matter intake. Commodity fats refer to animal fats such as tallow or yellow grease and
Table 2. Characteristics of Commonly Used Sources of Fat *
FAT
PROTEIN
(%)
(%)
(%)
Oilseeds Cottonseed
18-20
20-25
28-34
Soybeans
18-20
38-42
10-11
95-100 95-100
none none
none none
solid at room temperature semi-soft at room temperature
95-100
none
none
usually liquid
80
none
none
Energy booster
99
none
none
Booster fat
90
none
none
Carolac
98
none
none
calcium salts of palm oil fatty acids prilled, relatively saturated fatty acids tallow and protein in an alginate matrix hydrogenated tallow, prilled
FEED
Commodity fats Tallow Yellow grease Animal/vegetable blends Specialty fats Megalac
* = acid detergent fiber.
FIBER
COMMENTS
with lint or "fuzzy," may bridge in equipment heat treating adds ruminal protection
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EMERY AND THOMAS H. HERDT
blends of animal and vegetable fats. Their acceptability varies greatly with source. These fats are subject to rancidity and oxidation and must be fresh or well protected. Yellow grease seems to be less acceptable than tallow. Response to blends is often better than with pure tallow. Tallow is a solid and must be melted before adding to the ration. The price of these fats varies greatly with locality and the quantity ordered. Handling difficulties plus price generally limit use of commodity fats to larger herds. Intake should be limited to 1.5 to 2 lb per cow per day and this should be part of a total mixed ration. Commodity fats are not ruminally inert and will decrease digestibility, intake, and milk fat percentage if too much is fed or unsaturation is excessive. Specialty fats have been treated to make them more ruminally inert. Treatments include forming fine particles of solid fatty acids, encapsulation, or formation of calcium soaps. These treatments are expensive but do form products that can be fed after the limits of feeding oilseeds and commodity fats have been attained. Feeding is restricted to about 2% of dry matter intake. These dry products are easily handled and mixed with either grain or the total ration. They may delay rate of eating, which can be a problem when fed with grain. Feeding fat in early lactation is controversial. Some trials have shown no beneficial effect of supplemental fat until the fifth week of lactation. 26 It may be that the increased concentration of nonesterified fatty acids in blood saturates the cow's ability to use fat. This explanation does not make sense when we consider that dietary fat is directed to the mammary gland and other extrahepatic tissues, while nonesterified fatty acids are directed to liver and other tissues according to blood How. Dietary fat tends to depress dry matter intake until cows become adapted to it for several weeks and intake is already depressed in early lactation. Some people start feeding fat shortly before calving in an attempt to avoid depression of intake by fat but experimental data supporting this practice are lacking. Ostergaard et al44 summarized a number of feeding trials and concluded that supplemental fat improved production more during the first 12 weeks of lactation than during midlactation. Early lactation is exactly the time when cows need more energy than they can usually consume. On the farm, it is often impractical to begin feeding fat only after the fifth week of lactation. We conclude that fat should be introduced into the diet in early lactation but with caution and with close monitoring of feed intake. Delayed reproduction is one of the consequences of energy deficit in early lactation. Most of the feeding trials with fat have not involved enough cows to detect changes in reproductive performance. Conception rate has been improved by addition of fat to the diet in some of the larger trials. 21 ,57 Luteal function was better with diets containing 8% as opposed to 2.8% crude fat.65 Improved reproduction is a strong incentive to include supplemental fat in the diet during early lactation. Feeding fat in later lactation may not be economical. Addition of a specialty fat to a diet, to increase the crude fat concentration from 3.8% to 5.8%, increased milk production 7 lb per day during the first 60 days of lactation but this response declined to 3 lb per day during the next 60 days.21 These cows were producing more than 80 lb of milk per day at lactation day 120. In another trial, which began at lactation day 120 with cows producing the equivalent of 51lb of milk with 3.5% fat, the increase in milk production was 3.3 lb per pound of specialty fat consumed. 29 Numerous trials have failed to show a response to added fat when cows were producing less than 60 lb of milk per day. Season or temperature may provide an explanation for some of the variation in results among trials. Unfortunately, these conditions are not usually reported. In a Wisconsin trial using cows during their first 105 days of lacta-
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tion, addition of a specialty fat as 5% of the ration dry matter increased production of 3.5% fat-corrected milk 15.6 lb per day during the warm season but there was no effect during the cool season. 56 Feed intake was increased by supplemental fat in the warm season but it was decreased during the cool season. Maintaining feed intake seems to be important to obtaining a response from supplemental fat. In field trials with a specialty fat in Pennsylvania and Israel, supplemental fat increased milk production much more in the warm climate of Israel than in the mild climate ofPennsylvania. 21 Climate seems to be important to predicting the response to fat supplementation but trials designed to study climate are needed. Several precautions and special considerations are required for successful feeding of fat. Diets supplemented with fat need to have the calcium content increased to 1% and the magnesium increased to 0.35% of the dry matter. 46 Fatty acids form soaps with these minerals, which may explain their decreased absorption. Extra calcium also increases dry matter digestibility in the presence of added fat. 59 Calcium soaps of fatty acids dissociate at pH 5 or less, suggesting that a higher pH should be maintained in the rumen to enhance rumen inertness of fat. Use of rumen buffers should be considered when fat is added to the ration. Feeding fat usually decreases the protein concentration in milk. 12,46 This decrease is small but may be important in some markets. The decrease in milk protein may be due to a lower extraction of blood amino acids by the mammary gland but it is not clear whether this lower extraction is cause or effect.ll Dietary fat can reduce the supply of microbial protein to the cow and certainly does not provide extra protein to balance the extra energy. Theoretical calculations suggest that 0.3 lb of extra ruminally undegraded protein should be fed for each 1 lb of fat added to the ration. Although several trials have failed to show a benefit from extra ruminally degraded protein with extra fat, ruminally protected methionine and lysine did alleviate the decreased percentage of milk protein in one trial. 12 The amino acid composition of the ruminally undegraded protein should also be considered, and this may explain the lack of a positive response in several trials. Supplemental niacin also has increased the protein concentration of milk from cows fed fat-supplemented diets (see the article by Hutjens in this volume). Dietary fat decreases the concentration of growth hormone and increases that of insulin in blood plasma. 8 ,14 These two hormonal changes are associated with decreased production of milk, which argues against feeding fat unless it is definitely needed. On the other hand, supplemental fat increased the number of hepatic growth hormone receptors and mammary prolactin receptors. 51 Mammary growth was increased in prepubertal lambs. These changes in receptors may counteract the hormonal changes in serum. As a final caution, one study showed that feeding fat to cows that were fed a diet with only 10% concentrate during early lactation caused a ketosis incidence of 62%.51 In another trial in which the diet contained 14% concentrate fed with ensiled immature alfalfa, no problems were encountered. SUMMARY
Cows in early lactation or producing more than 80 lb of milk per day need supplemental fat and can benefit from it. Fat should be added to the diet over a period of several weeks to allow the cows to become accustomed to it. Feed intake should be monitored because additional fat may decrease feed intake and offset the benefit of the fat. Supplemental fat should not exceed 4 to 5% of
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the dry matter intake. The first 2% of added fat should be supplied by oilseeds under most circumstances. The next 1 or 2% can come from commodity fat if availability and handling ability permits its use. If the last increment of fat is needed, it should be supplied by specialty fats that have been processed to improve ruminal inertness. Extra calcium, magnesium, and ruminally undegraded protein should be added to the diet when fat is added. Fat is a more expensive source of energy than feed grains in most of the world and should not be used beyond needs.
REFERENCES 1. Baldwin RL, Smith NE, Taylor J, et al: Manipulating metabolic parameters to improve growth rate and milk secretion. Comp Biochem Physiol 94B:411, 1989 2. Baldwin RL, Yang IT, Crist K, et al: Theoretical model of ruminant adipose tissue metabolism in relation to the whole animal. Fed Proc 35:2314, 1976 3. Banks W, Clapperton JL, Girdler AK: Effect of dietary unsaturated fatty acids in various forms on the de novo synthesis of fatty acids in the bovine mammary gland. J Dairy Res 57:179, 1990 4. Bauchart D, Legay-Carmier F, Doreau M, et al: Lipid metabolism of liquid-associated and solid-adherent bacteria in rumen contents of dairy cows offered lipid supplemented diets. Br J Nutr 63:563, 1990 5. Bell AW: Lipid metabolism in liver and selected tissues and in the whole body of ruminant animals. Prog Lipid Res 18:117, 1980 6. Belyea RL, Adams MW: Energy and nitrogen utilization of high versus low producing dairy cows. J Dairy Sci 73:1023, 1990 7. Bernard A, Fleith M, Carlier H, et al: Effect of calcium and magnesium ions on the intestinal absorption of oleic acid in vitro. Reprod Nutr Dev 29:63, 1989 8. Blum JW, Jans F, Moses W, et al: Twenty four-hour pattern of blood hormones and metabolite concentrations in high-yielding dairy cows: Effects of feeding low or high amounts of starch, or crystalline fat. Zbl Vet Med A32:401, 1985 9. Brindle NPJ, Zammit VA, Pogson CI: Regulation of carnitine palmitoyltransferase activity by malonyl-CoA in mitochondria from sheep liver, a tissue with a low capacity for fatty acid synthesis. Biochem J 232:177, 1985 10. Butler SM, Faulkner A, Zammit VA, et al: Fatty acid metabolism of the perfused caudate lobe from livers of fed and fasted non-pregnant and fasted late pregnant ewes. Comp Biochem Physiol 91B:25, 1988 11. Casper DP, Schingoethe D J: Model to describe and alleviate milk protein depression in early lactation dairy cows fed a high fat diet. J Dairy Sci 72:3327, 1989 12. Chow JM, DePeeters EJ, Baldwin RL: Effects of rumen-protected methionine and lysine on casein in milk when diets high in fat or concentrate are fed. J Dairy Sci 73:1051, 1990 13. Christie WW: The composition, structure and function of lipids in the tissues of ruminant animals. Prog Lipid Res 17:111, 1978 14. Cummins KA, Sartin JL: Response of insulin, glucagon, and growth hormone to intravenous glucose challenge in cows fed high fat diets. J Dairy Sci 70:277, 1987 15. Czerkawski JW, Clapperton JL: Fats as energy yielding compounds in the ruminant diet. In Wiseman J (ed): Fats in Animal Nutrition. London, Butterworths, 1984, p 249 16. Dunshea FR, Bell AW, Trigg TE: Non-esterified fatty acid and glycerol kinetics and fatty acid re-esterification in goats during early lactation. Br J Nutr 64:133, 1990 17. Emery RS: Deposition, secretion and oxidation of fats in ruminants. J Anim Sci 48:1530, 1979 18. Emery RS: Feed intake and change in body composition of lactating mammals. lSI Atlas of Science: Animal and Plant Sciences 1:51, 1988 19. Emery RS: Milk fat depression and the influence of diet on milk composition. Vet Clin North Am [Food Anim Pract] 4:289, 1988 20. Ferguson JD, Otto KA: Managing body condition in dairy cows. In Proceedings of the Cornell Nutrition Conference, Ithaca, NY, 1989, p 75
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21. Ferguson JD, Sklan D, Chalupa WV, et al: Effects of hard fats on in vitro and in vivo rumen fermentation, milk production and reproduction in dairy cows. J Dairy Sci 73:2864, 1990 22. Gerloff BJ, Herdt TH, Emery RS: Relationship of hepatic lipidosis to health and performance in dairy cattle. J Am Vet Med Assoc 188:845, 1986 23. Harfoot CG: Lipid metabolism in the rumen. Prog Lipid Res 17:21, 1978 24. Jenkins TC, Fatouhi N: Effects of lecithin and com oil on site of digestion, ruminal fermentation and microbial protein synthesis in sheep. J Anim Sci 68:460, 1990 25. Jenkins TC, Jenny BF: Effect of hydrogenated fat on feed intake, nutrient digestion, and lactation performance of dairy cows. J Dairy Sci 72:2316, 1989 26. Jerred MJ, Carroll DJ, Combs DK, et al: Effects of fat supplementation and immature alfalfa to concentrate ratio on lactation performance of dairy cattle. J Dairy Sci 73:2842, 1990 27. Jesse BW, Emery RS, Thomas JW: Aspects of the regulation of long-chain fatty acid oxidation in bovine liver. J Dairy Sci 69:2298, 1986 28. Kerr LA: Gossypol toxicosis in cattle. Compend Contin Educ Pract Vet 11:1139, 1989 29. King KR, Stockdale CR, Trigg TE: The effect of a blend of dietary unesterified and saturated long-chain fatty acids on the performance of dairy cows in mid-lactation. Aust J Agric Res 41:129, 1990 30. Kleppe BB, Aiello RJ, Grummer HR, et al: Triglyceride accumulation and very low density lipoprotein secretion by rat and goat hepatocytes in vitro. J Dairy Sci 71:1813, 1988 31. Kronfeld DS: Plasma non-esterified fatty acid concentrations in the dairy cow: Responses to nutritional and hormonal stimuli, and significance in ketosis. Vet Rec 77:30, 1965 32. Legay-Carmier F, Bauchart D: Distribution of bacteria in the rumen contents of dairy cows given a diet supplemented with soya-bean oil. Br J Nutr 61:725, 1989 33. Liesman J, Emery RS, Gerloff B, et al: Salt-resistant triglyceride lipase related to postpartum disease. Can J Anim Sci 64(Suppl):248, 1984 34. Liesman JS, Emery RS, Akers RM, et al: Mammary lipoprotein lipase in plasma of cows after parturition or prolactin infusion. Lipids 23:504, 1988 35. Lomax MA, Baird GD: Blood flow and nutrient exchange across liver and gut of the dairy cow. Br J Nutr 49:481, 1983 36. Mattos W, Palmquist DL: Biohydrogenation and availability of linoleic acid in lactating cows. J Nutr 107:1755, 1977 37. Mazur A, Marcos E, Rayssiguier Y: Plasma lipoproteins in dairy cows with naturally occurring severe fatty liver: Evidence of alteration in the distribution of Apo A-I containing lipoproteins. Lipids 24:805, 1989 38. McNamara JP, Hillers JK: Regulation of bovine adipose tissue metabolism during lactation. 1. Lipid synthesis in response to increased milk production and decreased energy intake. J Dairy Sci 69:3032, 1986 39. McNamara JP, Hillers K: Regulation of bovine adipose tissue metabolism during lactation. 2. Lipolysis response to milk production and energy intake. J Dairy Sci 69:3042, 1986 40. McFadden TB, Daniel TE, Akers RM: Effects of plane of nutrition, growth hormone, and unsaturated fat on growth hormone, insulin and prolactin receptors in prepubertallambs. J Anim Sci 68:3180, 1990 41. Moore JH, Christie WW: Digestion, absorption and transport of fats in ruminant animals. In Wiseman J(ed): Fats in Animal Nutrition. London, Butterworths, 1984, p 123 42. Ngidi ME, Loerch SC, Fluharty FL, et al: Effects of calcium soaps oflong-chain fatty acids on feed lot performance, carcass characteristics and ruminal metabolism of steers. J Anim Sci 68:2555, 1990 43. Noble RC: Digestion, absorption and transport of lipids in ruminant animals. Prog Lipid Res 17:55, 1978 44. Ostergaard V, Danfaer A, Daugaard J, et al: The effects of dietary lipids on milk production in dairy cows. Beretning fra Statens Husdybrugs Forsog No. 508, 1981 45. Palmquist DL: Palmitic acid as a source of endogenous acetate and beta-hydroxybutyrate in fed and fasted ruminants. J Nutr 102:1401, 1972
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46. Palmquist DL: Use of fats in diets for lactating diary cows. In Wiseman J{ed): Fat in Animal Nutrition. London, Butterworths, 1984, p 357 47. Palmquist DL: The feeding value of fat. In Orskov ER{ed): Feed Science. Amsterdam, Elsevier, 1988 48. Palmquist DL, Conrad HR: High fat rations for dairy cows. Effects on feed intake, milk and fat production and plasma metabolites. J Dairy Sci 61:890, 1978 49. Palmquist DL, Jenkins TC, Joyner AE Jr: Effect of dietary fat and calcium source on insoluble soap formation in the rumen. J Dairy Sci 69:1020, 1986 50. Pethick DW, Bell AW, Annison EF: Fats as energy sources in animal tissues. In Wiseman J(ed): Fats in Animal Nutrition. London, Butterworths, 1984, p 225 51. Phipps RH, Weller RF, Elliot RJ, et al: The effect of level and type of conserved forage on dry matter intake and milk production oflactating dairy cows. J Agric Sci 111:179, 1988 52. Pullen DL, Emery RS, Ames NK: Turnover of hepatic and plasma triacylglycerol in sheep. J Anim Sci 66:1538, 1988 53. Pullen DL, Liesman JS, Emery RS: A species comparison of liver slice synthesis and secretion of triacylglycerols from non-esterified fatty acids in media. J Anim Sci 68:1395, 1990 54. Pullen DL, Palmquist DL, Emery RS: Effect of days of lactation and methionine hydroxy analog on incorporation of plasma fatty acids into plasma triglycerides. J Dairy Sci 72:49, 1989 55. Reid CW, Husbands DR: Oxidative metabolism oflong-chain fatty acids in mitochondria from sheep and rat liver-evidence that sheep conserve linoleate by limiting its oxidation. Biochem J 225:233, 1985 56. Skaar TC, Grummer RR, Dentine MR, et al: Seasonal effects of prepartum and postpartum fat and niacin feeding on lactation performance and lipid metabolism. J Dairy Sci 72:2028, 1989 57. Sklan D, Bogin E, Avidar Y, et al: Feeding calcium soaps of fatty acids to lactating cows: Effect on production, body condition and blood lipids. J Dairy Res 56:675, 1989 58. Steele W: Intestinal absorption of fatty acids and blood lipid composition in sheep. J Dairy Sci 66:520, 1983 59. Sukhija PS, Palmquist DL: Dissociation of calcium soaps of long-chain fatty acids in rumen Huid. J Dairy Sci 73:1784, 1990 60. Sutton JD, Storry JE, Brumby PE: Rumen microbial synthesis of long chain fatty acids. Proc Nutr Soc 38:4A, 1978 61. Teter BB, SampugnaJ, Keeney M: Milk fat depression in C57B1/6J mice consuming partially hydrogenated fat. J Nutr 120:818, 1990 62. Thompson GE: Mammary extraction of plasma triglycerides in the cow during lactogenesis. Comp Biochem Physiol 94B:411, 1989 63. Vernon RG: Lipid metabolism in adipose tissue of ruminant animals. Prog Lipid Res 19:23, 1980 64. Vernon RG: Endocrine control of metabolic adaptation during lactation. Proc Nutr Soc 48:23, 1989 65. Williams GL: Modulation ofluteal activity in postpartum beef cows through changes in dietary lipid. J Anim Sci 67 :785, 1989
Address reprint requests to Roy S. Emery, PhD Room 221, Anthony Hall Department of Animal Science Michigan State University East Lansing, MI 48824
0749-0720/91 $0.00
+ .20
Lipid Nutrition Roy S. Emery, PhD, * and Thomas H. Herdt, DVM, MSt
Efficient production of milk requires efficient transfer of energy to milk. Fatty acids are a concentrated energy source. In the fed, lactating cow about 32% of the metabolizable energy intake may be metabolized through fatty acids. 2 Lipid metabolism is definitely important. Energy can come temporarily from adipose tissue. A cow in early lactation may typically mobilize 50 kg of body fat containing 365 Mcal of metabolizable energy. 18 This is enough energy to make about 31 7 kg of milk but the fat must be replaced before the process can be repeated. When this cow achieves positive energy balance, much of the milk fat must come from dietary fat. Dietary fat is the focus of this article. About one half of the fatty acids in milk, those with 16 carbons or less, are made in the mammary gland and the remainder are transferred to milk from blood. I9 Fatty acids of 16 to 18 carbons are made from acetate and 3-hydroxybutyric acid in adipose tissue, but these acids mostly remain in that tissue when cows are in positive energy balance. For this reason, nearly one half of the milk fat must come from dietary fat. The traditional ruminant diet contains about 3.5% crude fat or ether extract. Fatty acids, however, are the energy-rich portion of fat and the precursor for milk fat. Crude fat may be only 50% to as much as 90% fatty acids, depending on the source. High-producing cows frequently lack the fatty acids required to produce normal milk. NATURE OF FAT Different fatty acids have different characteristics. Stearic acid (CI8: 0) is a fully saturated or hydrogenated fat that contains 18 carbons, is solid or hard at room temperature, and is unique to animal fats. Oleic acid (CI8: 1) lacks hydrogen at carbon 9, which makes it liquid at room temperature. Palmitic a~id (CI6: 0) contains 16 carbons. All three of these acids are made in ruminant adipose tissue. Polyunsaturated fats, with unsaturation at more than one carbon, are found in vegetable oils. Crude fat in plants may contain as much as 48% nonsaponifiable lipids. 47 From Michigan State University College of Veterinary Medicine, East Lansing, Michigan *Professor, Department of Animal Science fDiplomate, American College of Veterinary Nutrition; Associate Professor, Department of Large Animal Clinical Sciences Veterinary Clinics of North America: Food Animal Practice-Vol. 7, No.2, July 1991
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EMERY AND THOMAS H. HERDT
N onsaponifiable lipids include such nonlipid components as galactose and glycerol, which are fermented in the rumen after hydrolysis (Fig. 1). Galactolipids comprise 70 to 80% of the crude fat in alfalfa and corn plants. 23 Nonsaponifiable lipids also contain pigments and steroids that are not digested and provide no energy. 43 The remaining fat in plants and the predominant fat in animals is triacylglycerol (triglyceride), which contains three fatty acids esterified to the Fat Fat sources supplement Forages Fat
type
FA's or TG
Cereal grains
Oil seeds
TG
TG
GlyL
RUMEN hydrogenation
hydrolysis Glycerol
TG
-E ---f
GlyL
O-FA O-FA O-FA
-f
OH FA OH + OH FA ...
>G fr-.
I
FA FA's-c::J o-suga1: OH acids FA - - - O-FA OH + FA=_ _ _ _ _ __ O-FA OH FA ~ \.. Microblal-r Microbial t-PL I ~I cells lEhospholipid~
syar
Traps
t
To intestine
08(!)
VFA's Inert fats
INTESTINE Pancreas
Liver
From { rumen
Pancreatic juice Bile ---.. Bile salts (BS)
Free fatty acids (FA's) Microbial phospholipids
Micelle Lumen of small intestine
Lecithin Phospholipases • Lysolecithin phospholipids (Lyso-L)
Mucosal tissue
Acetate
t
Lysolecithin .....--1r----- Bile Salts - - - - '
J--
Fatty acids
PhosphOlipids Cholesterol I Fatty racias Cholesterol-esters ----~
---"'J-- a-glycerol-P
t
Triglycerides
Glucose
t
Glucose (from blood)
Protein Phospholipids Cholesterol To lymph
Figure 1. Fat digestion and absorption. GlyL = glycolipids; TG = triglycerides; FA's = mixture of fatty acids; FA = saturated fatty acids; F A= = unsaturated fatty acids; VFA' s = volatile fatty acids; PL = phospholipids; Trans acids = intermediates in the hydrogenation process; Cl FA = fatty acids attached to food particles.
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three hydroxyl groups of glycerol. When one of the fatty acids is replaced by a phosphate moiety such as phosphorylcholine, the resultant molecule is a phospholipid. Phospholipids are an essential part of all living membranes. FAT IN THE RUMEN Fatty acids are not an energy source for rumen microbes. 23 Fatty acids are taken into the microbes, particularly those microbes that are adherent to particles of feed. 4,32 These anaerobic microbes hydrogenate 60 to 90% of the unsaturated bonds, forming fully saturated fatty acids or monounsaturated fatty acids with trans configuration. 36 These trans fatty acids inhibit synthesis of milk fat in the mammary gland. Microbes in the rumen also synthesize 0.1 to 0.2 kg fatty acid per day.15,60 This may seem like a trivial amount of fat, but fatty acid intake by cows fed typical diets may be only about twice this amount. Typical diets refers to diets that do not contain oilseeds or supplemental fat. Fat can inhibit digestion and fermentation in the rumen and may inhibit growth of the microbes themselves. 25 Unsaturated fats and free oils give more problems in this respect than oilseeds or saturated fats like tallow. There are fewer problems if fat intake is well distributed over time and within feed as occurs when total mixed rations are fed. Feed intake will decline if fat seriously inhibits rumen fermentation. This reduction of intake frequently offsets the increased dietary energy density achieved by adding fat to the diet. Increased postruminal digestion frequently compensates for decreased ruminal digestion, but this may not compensate for reduced feed intake. Fatty acids also favor production of propionic acid at the expense of acetic acid in the rumen and this reduces production of milk fat. 19,49 Feeding extra calcium reduces these harmful effects of fat in the rumen. In contrast to its repression of fermentation, fat also has several beneficial effects on rumen fermentation. Some microbes require certain dietary fatty acids for growth. 4 Fat seems to inhibit degradation of protein in the rumen and, thus, provide more dietary amino acids for postruminal absorption. 24 The yield of microbial protein per unit of carbohydrate fermented in the rumen is also increased by feeding fat. These benefits are offset by the decreased intake of ruminal fermentable carbohydrate and a possible decrease in fermentation. The net effect is that extra rumen undegradable protein should be fed when supplemental fat is included in the diet. FAT IN THE INTESTINE There is a variable but continuous entry into the intestine of fatty acids, microbial lipid, and rumen-inert fat as shown in Figure 1. The amount of fat entering the intestine due to microbial synthesis and biliary secretion exceeds the dietary supply. 41,43 Bile and pancreatic Huid are added proportionally to the ingesta How. Enterohepatic circulation is essential to the maintenance of bile How, formation of a colloidal solution of micelles, lipolysis, and absorption of fatty acids into the intestinal mucosa. Of the lipid in bile, 90% is phospholipid and the remainder is cholesterol, triglyceride, and fatty acid. The concentration of bile acids is twice that of lipids. 13 Pancreatic lipase is most active at pH 7 to 8 and requires the presence of phospholipids. In ruminants, pH of duodenal ingesta is more acid than pH 7 and remains relatively low until the jejunum is reached, which delays absorption of fatty acids; however, fatty acids are still absorbed before the ileum. 43
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The activity of pancreatic triglyceride lipase is somewhat lower in ruminants than in nonruminants. Phospholipase activity is present in excess of needs, which assures conversion of phospholipids to the better wetting agent, lysophospholipids. Lysophospholipids, bile acids, and fat reform into micelles, thereby facilitating lipase action. Taurine conjugates of bile acids, which predominate in ruminant bile, are unique in that they act at acid as well as alkaline pH.43,47 True digestibility of well-dispersed fatty acids is 83 to 92%.58 Stearic acid is the predominate acid present in the intestinal lumen due to hydrogenation of unsaturated acids in the rumen. Digestibility of palmitic acid and the monounsaturated acids may be somewhat greater than that of stearic acid. Digestibility of fatty acids can be much lower if the physical or chemical form inhibits dispersion. In one trial, flakes of stearic and palmitic acid had an apparent digestibility of only 47%.25 Fatty acid digestion may decrease as the concentration in the diet becomes excessive. Excessive is more than 2% added fat in the dry matter. 42 In another trial, digestibility of fat was 81 % when the diet contained 5.1 % crude fat, but only 56% when the diet contained 10.7% crude fat.48 It seems that the ruminant's ability to handle supplemental fat is limited at the level of the intestine as well as in the rumen. Supplemental fatty acids decrease calcium absorption by 25 to 40% and magnesium absorption by, perhaps, 15%.58 This indicates adding extra calcium and magnesium to diets that are supplemented with fat. Adding extra calcium and magnesium to diets for mice, and presumably for ruminant animals, enhanced fatty acid absorption. 7 Fatty acids infused into the abomasum appear in the lymph in 1 to 2 hours. 41 ,43 Some of this time is spent in the mucosal cell. Fatty acid transport into enterocytes is facilitated by binding proteins. 41 Lysophospholipids are transported by choline phosphotransferase. Fatty acids are esterified stepwise to glycerol-3-phosphate or to 2-monoacylglycerol. The supply of 2-monoacylglycerol in ruminant intestinal ingesta is limited due to lipolysis of triacylglycerol in the rumen. Lysophospholipids are also re-acylated in the enterocyte. The intestinal mucosa is also the primary site for cholesterol synthesis in cows, as confirmed by Z. Chen (unpublished data) in our laboratory, and cholesterol synthesis is increased as dietary fat increases. 41 Triglyceride, phospholipid, and cholesterol are added to apolipoproteins in the Golgi of enterocytes to form lipoproteins. The apolipoproteins are synthesized in the endoplasmic reticulum and moved to the Golgi. Vesicles containing these lipoproteins migrate to the baso-Iateral membrane and are secreted into the lymph by reverse pinocytosis. These newly secreted lipoproteins have a continuous spectrum of density from less than 0.93 to more than 1.006. 41 A small continuous supply of fatty acids, as would normally be presented from the ruminant intestine, results in production of lipoproteins with less triglyceride and higher density than typical chylomicrons. The smaller size and greater diversity of ruminant intestinal lipoproteins differentiate them from the classical chylomicrons found in nonruminant intestinal lymph. Lymph flows to blood via intestinal and thoracic lymph ducts and its flow is proportional to the lipid load. 43 This means that dietary fat is directed to all tissues, in proportion to blood flow, in contrast to nutrients absorbed into the portal vein that are directed to the liver. POSTABSORPTIVE FAT METABOLISM Newly secreted intestinal lipoproteins mature in blood by exchange of lipids and proteins among circulating lipoproteins. 17,41 Hepatic uptake of tri-
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LIPID NUTRITION
glyceride from these nascent lipoproteins is hindered by the lack of appropriate enzymes. Dietary fat is therefore directed to extrahepatic tissues. 33 Typical lipid composition values for cow blood plasma are given in Table 1. These values vary widely with diet and with the different analytical methods used among laboratories, but several trends are consistent. Circulating triglyceride declines abruptly with the initiation of lactation, possibly due to transfer of blood fat to milk by the mammary gland. Cholesterol declines in late gestation and slowly increases after calving. This may be due to decreased feed intake in late gestation and early lactation. N onesterified fatty acids increase sharply during the negative energy balance accompanying early lactation. Although it is not indicated on the table, circulating triglyceride, cholesterol, and nonesterified fatty acids increase with increasing dietary fat. Lipoproteins react with lipoprotein lipase in the capillary endothelium of target tissues. 17,41 Fatty acids are released by lipolysis, which raises their local concentration after which they enter the cells by apparent mass action. Some fatty acids escape and move on to other tissues including the liver. Lipoprotein lipase activity is sensitive to energy balance, physiologic state, and hormones. Adipose lipoprotein lipase, for example, is stimulated by insulin while mammary lipoprotein lipase is stimulated by prolactin. These hormonal regulations and changes in blood How shift delivery of circulating triglyceride from adipose tissue to the mammary gland at parturition. Liver is not an important source of circulating triglyceride in cows, although secretion of small amounts of lipoprotein triglyceride can be detected. 30,52,53 Liver may even remove small amounts of triglyceride under some circumstances. 5 Excessive circulating fatty acids tend to accumulate in the liver as triglyceride and this can cause a number of problems resulting in increased culling and death. 22 Mazur et aP7 found abnormal lipoproteins with an increased ratio of triglyceride to protein in cows with hepatic lipidosis but it is not clear whether these lipoproteins were of intestinal or hepatic origin. Hepatic uptake of nonesterified fatty acids is proportional to the plasma concentration. Fatty acid concentration in the hepatic vein is 10 to 25% of the arterial concentration. 5,5o This measurement, however, ignores the large contribution of fatty acids by the portal vein. Hepatic blood How is 52% greater in fed, lactating cows than in dry cows, which means more fatty acids are presented to the liver during lactation. Under these conditions, the liver extracted 7% of the nonesterified fatty acids presented to it. 35 Fatty acids partially equilibrate between liver and plasma with an exchange of some palmitic for oleic and stearic acid. 10 Fatty acids are oxidized in the liver to release acetic acid and 3-hydroxybutyric acid to blood. Oxidation of palmitic acid accounted for 2% of the circulating acetic acid and 5% of the circulating 3-hydroxybutyric acid in the Table 1. Typical Lipid Composition (mg/l00 mL) of Blood Plasma Relative to Calving When Cows Are Not Fed Supplemental Fat STAGE OF LACTATION
UPID
Triglyceride Phospholipid Cholesterol N onesterified fatty acid
Dry
Calving
30-60 days in milk
18
4 185 115 18
210 190 10
210 168 6
16
(Data from many sources. Values vary with method of analysis and diet. Sources include references 48, 57, and 62.)
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EMERY AND THOMAS H. HERDT
fed state and increased to 16 and 29% during fasting. 45 ,5o These figures can be approximately doubled to reflect the total contribution of nonesterified fatty acid to circulating 3-hydroxybutyric acid, considering palmitic acid accounts for one half of the nonesterified fatty acids in blood. Oxidation is regulated by metabolites such as propionic acid and by carnitine acyltransferase, which is the enzyme responsible for transferring fatty acids into the mitochondria where they are oxidized. Malonyl-CoA inhibits carnitine acyltransferase. 9,27 This is particularly true when linoleic acid is the substrate. 55 Hydrogenation in the rumen limits the supply of polyunsaturated fatty acids such as linoleic acid, which makes this protection against loss through oxidation very important. The mammary gland takes up fatty acids from circulating triglyceride through the action of lipoprotein lipase. 19,62 Mammary lipoprotein lipase activity is increased by prolactin and by parturition itself.34 The activation due to parturition is thought to be due to removal of progesterone inhibition. Uptake is linear, amounting to 30 to 50% of the arterial concentration over the usual range of plasma triglyceride of 10 to 30 mg/l00 mL but decreases at greater concentrations. 1 This efficient uptake coupled with the large mammary blood flow causes a transfer to milk of between 30 and 76% of the plasma triglyceride. 46 Most of this plasma triglyceride must originate as dietary fat because only 7 to 13% of blood triglyceride comes from circulating nonesterified fatty acids. 54 The transfer of dietary fat to milk fat is energetically more efficient than synthesis of milk fat from acetic acid. Thus, dietary fat is essential for synthesis of part of the milk fat and is more efficient than synthesis of new fatty acids. Dietary fat decreases the mammary synthesis of short and medium chain fatty acids, and to some extent, replaces them with C 18 : 0 and C 18 : 1 fatty acids. 3 Trans-C 118 : 1 is a particularly potent inhibitor of fatty acid synthesis in both cow and mouse mammary tissue. The variation in concentration of trans fatty acids in cow milk accounted for about 25% of the variation in milk fat percentage among COWS. 61 The mammary gland has a net uptake of nonesterified fatty acids when the concentration exceeds 0.3 mmol/L (about 8 mg/l00 mL).31,54 At lower concentrations there is considerable exchange of fatty acid between plasma and mammary cells, which must be considered when interpreting isotope studies. Labeled nonesterified fatty acids may appear to contribute much more to milk fat than their actual contribution. The net effect is that fatty acids can transfer directly from adipose tissue to milk in early lactation when cows are in negative energy balance, but this contribution will be very small during positive energy balance. Adipose tissue is important to the cow because, other than the mammary gland, it is the primary site of fatty acid synthesis as well as an energy store. 63 Adipose tissue can also take up fatty acid from plasma triglyceride as described for mammary tissue. In this case, however, lipoprotein lipase is inhibited by lactation, negative energy balance, and growth hormone but stimulated by insulin. 17,38,64 Mobilization of fatty acid from adipose tissue is stimulated by parturition and growth hormone but inhibited by insulin. 39 To some extent, glycerides are hydrolyzed and reformed continually within adipose tissue. It is the release of fatty acids that increases with negative energy balance. 16,17 Cows frequently mobilize 30 to 50 kg of fat during the first few weeks of lactation. 6,18 This amount of fat can supply enough energy to make an extra 10 to 12 Ib milk per day for 60 to 90 days. The mobilization of body protein in early lactation is much smaller than that of energy and the protein is replaced much sooner. 20 It is important to supply enough ruminally undegraded dietary protein to balance the energy mobilized from adipose tissue.
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LIPID NUTRITION
PRACTICAL ASPECTS OF FEEDING FAT Feed-grade fat is readily available in three forms: oilseeds, commodity fats, and specialty fats. Some characteristics of these sources of fats are listed in Table 2. Cottonseed and soybeans contain 18 to 20% fat. They are usually the first choice for supplemental fat because of price, acceptability by cows, convenience of handling, and relative inertness in the rumen. Cottonseed has the advantage over soybeans of supplying effective fiber to the ration. Cows can eat about 7lb of whole cottonseed per day (17% of dry matter intake) without contracting problems such as reduced feed intake or decreased percentage of milk fat. Gossypol toxicosis may occur if cottonseed intake exceeds 7 lb per day.28 Signs of this toxicosis are poor performance, anorexia, decreased packed cell volume, and red cell fragility. Toxicity is cumulative and may take several months to appear. Raw soybeans are less acceptable to cows than cottonseed and intake should be limited to 5 lb per day. Heating or roasting soybeans increases the cost as well as the acceptability to cows and permits intake of 7 lb per day. Heating increases the inertness of both fat and protein in the rumen. Cracking soybeans may increase their digestibility slightly but it also decreases ruminal inertness and may be contraindicated. Cows accept oilseeds better in a total mixed ration than if top dressed or fed in a grain mix. They may slow the rate of grain intake if fed in the milking parlor. Lupines with low alkaloid content have been developed as a feed in climates with shorter growing seasons. They contain about 10% fat and have been fed successfully as 18% of the dry matter intake. Lupines contain more fiber than soybeans but less than cottonseed. Comments on feeding soybeans apply to lupines. Rapeseed or the improved variety called canola contains about 40% fat, but feeding should be restricted to about 2 lb per cow per day. Sunflower seeds contain 35 to 40% oil and the whole seed can be fed as 5% of the dry matter intake. Commodity fats refer to animal fats such as tallow or yellow grease and
Table 2. Characteristics of Commonly Used Sources of Fat *
FAT
PROTEIN
(%)
(%)
(%)
Oilseeds Cottonseed
18-20
20-25
28-34
Soybeans
18-20
38-42
10-11
95-100 95-100
none none
none none
solid at room temperature semi-soft at room temperature
95-100
none
none
usually liquid
80
none
none
Energy booster
99
none
none
Booster fat
90
none
none
Carolac
98
none
none
calcium salts of palm oil fatty acids prilled, relatively saturated fatty acids tallow and protein in an alginate matrix hydrogenated tallow, prilled
FEED
Commodity fats Tallow Yellow grease Animal/vegetable blends Specialty fats Megalac
* = acid detergent fiber.
FIBER
COMMENTS
with lint or "fuzzy," may bridge in equipment heat treating adds ruminal protection
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blends of animal and vegetable fats. Their acceptability varies greatly with source. These fats are subject to rancidity and oxidation and must be fresh or well protected. Yellow grease seems to be less acceptable than tallow. Response to blends is often better than with pure tallow. Tallow is a solid and must be melted before adding to the ration. The price of these fats varies greatly with locality and the quantity ordered. Handling difficulties plus price generally limit use of commodity fats to larger herds. Intake should be limited to 1.5 to 2 lb per cow per day and this should be part of a total mixed ration. Commodity fats are not ruminally inert and will decrease digestibility, intake, and milk fat percentage if too much is fed or unsaturation is excessive. Specialty fats have been treated to make them more ruminally inert. Treatments include forming fine particles of solid fatty acids, encapsulation, or formation of calcium soaps. These treatments are expensive but do form products that can be fed after the limits of feeding oilseeds and commodity fats have been attained. Feeding is restricted to about 2% of dry matter intake. These dry products are easily handled and mixed with either grain or the total ration. They may delay rate of eating, which can be a problem when fed with grain. Feeding fat in early lactation is controversial. Some trials have shown no beneficial effect of supplemental fat until the fifth week of lactation. 26 It may be that the increased concentration of nonesterified fatty acids in blood saturates the cow's ability to use fat. This explanation does not make sense when we consider that dietary fat is directed to the mammary gland and other extrahepatic tissues, while nonesterified fatty acids are directed to liver and other tissues according to blood How. Dietary fat tends to depress dry matter intake until cows become adapted to it for several weeks and intake is already depressed in early lactation. Some people start feeding fat shortly before calving in an attempt to avoid depression of intake by fat but experimental data supporting this practice are lacking. Ostergaard et al44 summarized a number of feeding trials and concluded that supplemental fat improved production more during the first 12 weeks of lactation than during midlactation. Early lactation is exactly the time when cows need more energy than they can usually consume. On the farm, it is often impractical to begin feeding fat only after the fifth week of lactation. We conclude that fat should be introduced into the diet in early lactation but with caution and with close monitoring of feed intake. Delayed reproduction is one of the consequences of energy deficit in early lactation. Most of the feeding trials with fat have not involved enough cows to detect changes in reproductive performance. Conception rate has been improved by addition of fat to the diet in some of the larger trials. 21 ,57 Luteal function was better with diets containing 8% as opposed to 2.8% crude fat.65 Improved reproduction is a strong incentive to include supplemental fat in the diet during early lactation. Feeding fat in later lactation may not be economical. Addition of a specialty fat to a diet, to increase the crude fat concentration from 3.8% to 5.8%, increased milk production 7 lb per day during the first 60 days of lactation but this response declined to 3 lb per day during the next 60 days.21 These cows were producing more than 80 lb of milk per day at lactation day 120. In another trial, which began at lactation day 120 with cows producing the equivalent of 51lb of milk with 3.5% fat, the increase in milk production was 3.3 lb per pound of specialty fat consumed. 29 Numerous trials have failed to show a response to added fat when cows were producing less than 60 lb of milk per day. Season or temperature may provide an explanation for some of the variation in results among trials. Unfortunately, these conditions are not usually reported. In a Wisconsin trial using cows during their first 105 days of lacta-
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tion, addition of a specialty fat as 5% of the ration dry matter increased production of 3.5% fat-corrected milk 15.6 lb per day during the warm season but there was no effect during the cool season. 56 Feed intake was increased by supplemental fat in the warm season but it was decreased during the cool season. Maintaining feed intake seems to be important to obtaining a response from supplemental fat. In field trials with a specialty fat in Pennsylvania and Israel, supplemental fat increased milk production much more in the warm climate of Israel than in the mild climate ofPennsylvania. 21 Climate seems to be important to predicting the response to fat supplementation but trials designed to study climate are needed. Several precautions and special considerations are required for successful feeding of fat. Diets supplemented with fat need to have the calcium content increased to 1% and the magnesium increased to 0.35% of the dry matter. 46 Fatty acids form soaps with these minerals, which may explain their decreased absorption. Extra calcium also increases dry matter digestibility in the presence of added fat. 59 Calcium soaps of fatty acids dissociate at pH 5 or less, suggesting that a higher pH should be maintained in the rumen to enhance rumen inertness of fat. Use of rumen buffers should be considered when fat is added to the ration. Feeding fat usually decreases the protein concentration in milk. 12,46 This decrease is small but may be important in some markets. The decrease in milk protein may be due to a lower extraction of blood amino acids by the mammary gland but it is not clear whether this lower extraction is cause or effect.ll Dietary fat can reduce the supply of microbial protein to the cow and certainly does not provide extra protein to balance the extra energy. Theoretical calculations suggest that 0.3 lb of extra ruminally undegraded protein should be fed for each 1 lb of fat added to the ration. Although several trials have failed to show a benefit from extra ruminally degraded protein with extra fat, ruminally protected methionine and lysine did alleviate the decreased percentage of milk protein in one trial. 12 The amino acid composition of the ruminally undegraded protein should also be considered, and this may explain the lack of a positive response in several trials. Supplemental niacin also has increased the protein concentration of milk from cows fed fat-supplemented diets (see the article by Hutjens in this volume). Dietary fat decreases the concentration of growth hormone and increases that of insulin in blood plasma. 8 ,14 These two hormonal changes are associated with decreased production of milk, which argues against feeding fat unless it is definitely needed. On the other hand, supplemental fat increased the number of hepatic growth hormone receptors and mammary prolactin receptors. 51 Mammary growth was increased in prepubertal lambs. These changes in receptors may counteract the hormonal changes in serum. As a final caution, one study showed that feeding fat to cows that were fed a diet with only 10% concentrate during early lactation caused a ketosis incidence of 62%.51 In another trial in which the diet contained 14% concentrate fed with ensiled immature alfalfa, no problems were encountered. SUMMARY
Cows in early lactation or producing more than 80 lb of milk per day need supplemental fat and can benefit from it. Fat should be added to the diet over a period of several weeks to allow the cows to become accustomed to it. Feed intake should be monitored because additional fat may decrease feed intake and offset the benefit of the fat. Supplemental fat should not exceed 4 to 5% of
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the dry matter intake. The first 2% of added fat should be supplied by oilseeds under most circumstances. The next 1 or 2% can come from commodity fat if availability and handling ability permits its use. If the last increment of fat is needed, it should be supplied by specialty fats that have been processed to improve ruminal inertness. Extra calcium, magnesium, and ruminally undegraded protein should be added to the diet when fat is added. Fat is a more expensive source of energy than feed grains in most of the world and should not be used beyond needs.
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Address reprint requests to Roy S. Emery, PhD Room 221, Anthony Hall Department of Animal Science Michigan State University East Lansing, MI 48824