Contents 1 Introduction 2 Fats Defined 3 Dietary Fats Are Modified in the Rumen by Bacteria 4 Fat Supplementation and Conception Rates 4.1 Focus on Flaxseeds 5 Amount of Fat to Feed 6 When to Initiate Fat Supplementation 7 How Might Fat Supplementation Help Improve Conception Rates? 7.1 Improving Energy Status? 7.2 Meeting an Essential Fatty Acid Requirement? 7.3 Healthier Ovarian Follicles? 7.4 Better Quality Embryos Produced? 7.5 Less Embryonic Loss? 8 Summary 9 References 10 Author Information

Introduction

The proportion of lactating dairy cows on commercial farms that become pregnant at the first insemination has decreased over the last 25 to 30 years. Data from New York herds indicate that the first service conception rates decreased from ~55% in 1975 to just under 40% in 2001 (Butler et al., 2005). The number of services per conception increased from 1.62 in 1970 to 2.91 in 1999 in Kentucky herds (Silvia, 1998). The DHIA records from Raleigh, North Carolina, show that the 143 herds on DHIA for 29 consecutive years (1970 to 1999) experienced a decrease in services per conception from ~1.75 to just under 3.0 (Lucy, 2001). Conception rates measured for cows managed under controlled experimental conditions as reported in scientific journals also have decreased. Rates dropped from ~55% to ~45% (breeding at spontaneous estrus) to ~35% (timed AI) over a 50-year period (Lucy, 2001). Poor reproductive performance was the first or second reason for culling dairy cows from the herd in 10 states based on DHI records (Hadley et al., 2006). On average, about 19% of the culling was due to poor reproductive performance. Many reasons for this decline in reproductive efficiency have been offered, including an increase in postpartum disease (ketosis, mastitis, retained fetal membranes, cystic ovaries, fatty liver, etc.), an increase in herd size resulting in increased management challenges, an increase in the proportion of milking heifers in the herd which cycle later, an increase in genetic inbreeding, and an increase in milk production (Lucy, 2001). Average milk production per lactation has increased by 57% from ~12,000 to ~19,000 pounds per cow in the last 25 years (Eastridge, 2006).

The dollar value assigned to pregnancy for dairy cows varies because it depends on several factors, such as how many days she has been milking, her lactation number, her milk yield, the replacement costs of a pregnant heifer, milk price, etc. A modeling program developed at the University of Florida was used to predict pregnancy value. Some key input values were a milk price of $14.09 per 100 pounds of milk, 305-day milk production of 23,144 pounds for young animals and 25,994 pounds for third-lactation cows, and a replacement heifer cost of $1,600 per head. The value of a new pregnancy at about 100 days in milk was calculated to be ~$200 for a milking heifer and ~$300 for a cow in her second lactation (de Vries, 2006). Even when a cow conceives, the pregnancy does not go to term about 50% of the time. If an average-producing cow in the herd conceived at 61 days in milk but was declared open 30 days later, the calculated loss ranged from $110 for heifers to $336 for cows in their third lactation (de Vries, 2006). Efforts to reduce this loss are certainly justified.

The influence of nutrition on reproductive performance is a growing field of study, including the effect of feeding supplemental fat. If fat supplementation can improve pregnancy rates, then cow longevity is improved. The purpose of this paper is to review some of the effects of fat supplementation on reproductive tissues and pregnancy.

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Fats Defined

Many different types of supplemental fat have been fed to lactating cows. Some fat sources fed are listed in Table 1. Each fat source is composed of a different mix of individual fatty acids. Rendered fats include animal tallow and yellow grease (recycled restaurant grease) and are composed mainly of oleic acid (~43%). Granular fats are dry fats and are composed mainly of palmitic acid (36-50%). Examples include Energy Booster 100, EnerG-II, and Megalac-R. Canola oil is high in oleic acid. Cottonseed, safflower, sunflower, and soybean oils are high in linoleic acid. Flaxseed is high in linolenic acid. Fish oil contains eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), fatty acids found in fish tissue due to their consumption of marine plants. Fresh temperate grasses contain 1 to 3% fatty acids of which 55 to 65% is linolenic acid (Chilliard et al., 2001). Corn silage lipid contains much more linoleic acid (49%) than linolenic acid (4%) due to the presence of corn grain (Petit et al., 2004b).

Table 1. Major fatty acid composition of select dietary fat sources.

Fat source	Fatty acid
	C14:0 Myristic	C16:0 Palmitic	C16:1 Palmitoleic	C18:0 Stearic	C18:1 Oleic	C18:2 Linoleic	C18:3 Linolenic
Tallow	3	25	3	18	43	3.8	<1
Yellow grease	2	21	4	11	44	14	<1
Energy Booster 1001	3	40	1	41	10	2	<1
Megalac; EnerG-II1	1	50	<1	4	36	8	<1
Megalac-R1	1	36	<1	4	26	29	3
Canola oil	<1	4	<1	2	63	19	9
Cottonseed oil	1	23	1	3	18	54	1
Flaxseed oil	<1	5	<1	3	20	16	55
Rapeseed oil	<1	5	<1	2	54	22	11
Safflower oil	<1	7	<1	2	12	78	<1
Soybean oil	<1	11	<1	4	23	54	8
Sunflower oil	<1	7	<1	5	19	68	1
Menhaden fish oil2	7	16	8	3	12	1	2
1Commercial preparations considered partially inert in the rumen. 2Also contains 14% C20:5 and 9% C22:6.

The shorthand notation for identifying fatty acids is to give the number of carbons and double bonds in the molecule. For example, a designation of 18:2 indicates a fatty acid chain of 18 carbons having two double bonds. Fats that have double bonds are classified as unsaturated fats. The term “omega” refers to the location of the double bond in the carbon chain. An omega-6 fatty acid has its first double bond located between the sixth and seventh carbon counting from the methyl end of the chain. An omega-3 fatty acid has its first double bond located between the third and fourth carbon counting from the methyl end of the carbon chain. Linoleic acid, abbreviated C18:2, is an essential omega-6 fatty acid. Linolenic acid, abbreviated C18:3, is an essential omega-3 fatty acid. Two additional omega-3 fatty acids are EPA (C20:5) and DHA (C22:6).

Dietary Fats Are Modified in the Rumen by Bacteria

The ruminal microbes will convert unsaturated fats to saturated fats by replacing the double bonds with single bonds between the carbons (called biohydrogenation). Some scientists have speculated that this act of biohydrogenation by bacteria is an attempt to protect the bacteria, as unsaturated fats can be toxic especially to fiber digesters. The majority of the consumed unsaturated essential fatty acids, C18:2 and C18:3, are converted by the bacteria to C18:0. Whereas approximately 20 g of C18:0, 110 g of C18:1, 280 g of C18:2, and 40 g of C18:3 are consumed daily by cows fed typical totally mixed rations, approximately 370 g of C18:0, 25 g of C18:1, 40 g of C18:2, and 4 g of C18:3 leave the rumen daily because of biohydrogenation. Several intermediate forms of fatty acids, called trans fatty acids, also are formed during biohydrogenation. Some of the trans fatty acids, such as the trans-10, cis-12 conjugated linoleic acid (CLA) and the trans-10 C18:1, can influence the cow’s metabolism, such as depressing milk fat synthesis. This intervention by ruminal bacteria to change essential fatty acids in the diet to other fatty acids has made the study of dietary fat effects on reproduction quite challenging.

Fat Supplementation and Conception Rates

According to the scientific literature, a variety of fat supplements have benefited conception rates of lactating dairy cows (Table 2). The cited conception rates are sometimes reported for first insemination or for accumulated inseminations. Feedstuffs that have improved conception rates included calcium salts of palm oil distillate, tallow, Energy Booster (prilled fatty acids), flaxseed, MegaPro Gold (a calcium salt of palm oil plus rapeseed meal and whey permeate) fed to grazing cows, calcium salt of a mixture of soy oil and monounsaturated trans fatty acids, Megalac-R (calcium salt of fatty acids enriched in C18:2), calcium salt of CLA, and fish meal. The average improvement in conception rate was 21 percentage units. This is not to imply that the feeding of one of these feedstuffs to cows on a commercial dairy farm will increase herd conception rate by 21 percentage units. Any benefit experienced on a commercial dairy farm will likely be less than 10 percentage units because management is usually not as tight as that exercised on an experiment. Other studies have reported no positive pregnancy benefit to fat-supplementation (Table 3). The average response was 44.6 vs. 41.8% for control and test-fat treatment groups. Some reasons for the lack of consistent effects of dietary fat on conception rate may include differences in body condition, in the fatty acid profile of the control diets, in the extent of biohydrogenation of fatty acids by ruminal bacteria, and in the types of fatty acids stored in adipose tissue prior to the start of the study.

Table 2. Studies reporting improved conception rates (first service or cumulative services) of lactating dairy cows fed supplemental fatty acids (P < 0.10). Unless otherwise indicated with a footnote, the control diet did not contain a fat supplement.

Reference	Fat source and concentration or amount in diet	Number of cows in trial	Control treatment	Fat treatment
			——–%——–
Ferguson et al., 1990	2% Ca-palm oil	253	43	591
Sklan et al., 1991	2.6% Ca-palm oil	99	62	82
Scott et al., 1995	1 lb/d Ca-palm oil	443	93	98
Garcia-Bojalil et al., 1998	2.2% Ca-palm oil	43	52	86
Son et al., 1996	3% tallow	68	44	62
Frajblat and Butler, 2003	1.7% Energy Booster	81	582	86
Petit et al., 2001	17% formaldehyde-treated flaxseed	30	503	871
Petit et al., 2004a	Whole unprocessed flaxseed	30	29	593
Ambrose et al., 2006b	9% rolled flaxseed	121	324	481
McNamara et al., 2003	3.3 lb/d MegaPro Gold	129	35	54
Juchem et al., 2004	1.5% (Soy + Trans C18:1)	397	263	341
Cullens, 2004	2% Megalac-R	42	27	581
Castaneda-Gutierrez et al., 2005	0.3 lb/d Ca-CLA	32	443	81
Bruckental et al., 1989	7.3% fish meal	132	52	72
Armstrong et al., 1990	1.8 lb/d fish meal	80	44	64
Carrol et al., 1994	3.5% fish meal	44	68	891
Burke et al., 1997	2.8% fish meal	300	32	41
Average			49.0	70.6
1First insemination. 2Control diet contained equal energy to fat-supplemented diet. Fat was fed prepartum only. 3Control diet contained Ca salt of palm oil distillate. 4Control diet contained rolled sunflower seeds.

Table 3. Studies reporting a negative effect or no improvement in conception rates (first service or cumulative services) of lactating dairy cows fed supplemental fatty acids. Unless otherwise indicated with a footnote, the control diet did not contain a fat supplement.

Reference	Fat source and concentration or amount in diet	Number of cows in trial	Control treatment	Fat treatment
			——–%——–
Schneider et al., 1988	1.1 lb/d Ca-palm oil	108	43	601
Sklan et al., 1989	1.1 lb/d Ca-palm oil	108	28	441
Carroll et al., 1990	5% prilled fat	46	33	751
Holter et al., 1992	1.2 lb/d Ca-palm oil	38	502	441
Lucy et al., 1992	3% Ca-palm oil	40	44	12a
Sklan et al., 1994	2.5% Ca-palm oil primiparous cows	40	74	331,a
Sklan et al., 1994	2.5% Ca-palm oil multiparous cows	62	42	331
Salfer et al., 1995	2% partially hydrogenated tallow	32	32	331
Juchem et al., 2002	1.6% (Ca-palm + fish oils)	500	413	431
Bernal-Santos et al., 2003	0.3 lb/d Ca-CLA	30	274	42
Bruno et al., 2004	1.5% (Ca-palm + fish oils)	331	263	271
Petit and Twagiramungu, 2006	10.6% whole flaxseed	70	585	64
Ambrose et al., 2006a	9% rolled flaxseed	309	376	261
Ambrose et al., 2006 personal comm.	8% rolled flaxseed	266	427	43
Fuentes et al., 2007	5.5% extruded flaxseed	356	398	391
Carroll et al., 1994	3.5% fish meal	18	67	331,a
Burke et al., 1997	2.7% fish meal	341	65	60
Average			44.6	41.8
1First insemination. 2Control diet contained whole cottonseed at 15% of dietary dry matter. 3Control diet contained tallow. 4Control diet contained Ca salt of palm oil distillate. 5Control diet contained micronized soybeans. 6Control diet contained Ca salt of palm oil distillate and High Fat Product from ADM. 7Control diet contained Ca salt of palm oil distillate and tallow. 8Control diet contained extruded soybeans and Ca salt of palm oil distillate. aSignificant dietary effect, P < 0.05.

From the studies listed in Tables 2 and 3, it is very difficult to determine which fat supplements or which fatty acid(s) may be most efficacious. When cows fed fat sources containing mainly palmitic and oleic acids (tallow, Energy Booster, and Ca salts of palm oil distillate) were compared against cows fed no supplemental fat, the fat-supplemented cows had better conception rates. In four head-to-head comparisons of fat supplements, cows fed calcium salts of palm oil distillate did not deliver as many calves as those fed formaldehyde-treated flaxseed (Petit et al., 2001), unprocessed whole flaxseed (Petit et al., 2004a), a calcium salt mixture of soybean oil and monounsaturated trans fatty acids (Juchem et al., 2004), or CLA (Castaneda-Gutierrez et al., 2005; Table 2). Therefore, fats containing mainly palmitic and oleic acids may not be as effective as polyunsaturated fats. Research discussed on the following pages also suggests that the polyunsaturated fats may be most effective. Studies utilizing three diets are needed (e.g., no fat, fat source 1, and fat source 2) in order to better assess the effect of fat and fat source on conception rate.

Focus on Flaxseeds

Although the fatty acids in fresh grass can contain a high proportion of linolenic acid, flaxseeds are the only concentrated source of linolenic acid (~20% of DM as C18:3) available. Flaxseeds have been evaluated as a promotant of reproductive performance of lactating dairy cows with mixed results. First service conception rate was increased from 50 to 87% when lactating cows in the United Kingdom were fed formaldehyde-treated flaxseed at 17% of a ryegrass silage-based diet between 9 and 19 weeks postpartum (Petit et al., 2001). Control cows were fed a calcium salt of palm oil (5.6% of diet) and flaxseed meal. Cows had been on their diets for 6 weeks prior to insemination. Production of uncorrected milk (41.0 vs. 43.7 lb/day) and 4% fat-corrected milk (44.5 vs. 50.5 lb/day) was less for cows fed flaxseed, but DM intake was not changed. In a Canadian study involving 121 Holstein cows (Ambrose et al., 2006b), cows fed coarsely rolled flaxseed at 9% of the diet had a better first service conception rate (P < 0.07) compared to the control cows fed rolled sunflower seeds at 8.7% of dietary DM (48.4 vs. 32.2%). Although the overall pregnancy rates were not different between the two groups (67.7 vs. 59.3%), the proportion of pregnant cows that delivered a calf favored those fed flaxseed (90.2 vs. 72.7%), indicating that early and late pregnancy loss was less for cows fed flaxseed. Diets were fed for 28 days prior to insemination using a timed AI protocol and continued for 32 days after AI. Dry matter intake (49.6 vs. 47.0 lb/day) but not milk yield (80.9 vs. 79.4 lb/day) tended to be greater by cows fed flaxseed. In a second Canadian study conducted on two commercial dairy farms, conception rate was not different between cows fed whole flaxseed at 10.6% of the diet and those fed micronized soybeans starting at calving (Petit and Twagiramungu, 2006). However, cows fed flaxseed experienced less (P < 0.07) embryonic loss. From the same researchers, embryos collected from cows fed whole unprocessed flaxseed had a better gestation rate when transferred to heifers than embryos collected from cows fed Ca salt of palm oil distillate (58.8 vs. 29.3%; Petit et al., 2004a). Three recent studies involving a greater number of dairy cows did not report any pregnancy advantage to cows fed flaxseed. Holstein cows (n = 356) on a commercial dairy in Spain were fed diets of either 5.5% extruded whole flaxseed or 4.9% extruded soybeans plus 1% calcium salts of palm oil between 4 to 20 weeks postpartum (Fuentes et al., 2007). Cows were detected in estrus using visual observation and the Afimilk system. First service (39 vs. 39%) and overall conception rates (40 vs. 34%) did not differ between soybean and flaxseed groups, respectively. Yield of 4% fat-corrected milk was less for cows fed flaxseed (83.1 vs. 78.0 lb/day) due to a lower milk fat concentration (2.65 vs. 2.86%). A commercial dairy in Oregon (n = 303 cows) was used to evaluate rolled flaxseed, fed from about 32 days postpartum through 31 days after timed AI (Ambrose et al., 2006b). Cows were on diets at least 28 days prior to AI. Conception rates at 94 days after AI were not different, being 36.7% for controls and 25.6% for cows fed flaxseed when all cows were considered. When only cows that responded to synchronization were included in the data set (n = 169), conception rate was lower for cows fed flaxseed at 31 days post AI (51.2 vs. 35.3%). Loss of embryos between 31 and 94 days post AI was not affected by diet, but nine control cows lost their embryos, whereas four flaxseed-fed cows lost their embryos. Lastly, lactating dairy cows fed rolled flaxseed (8% of diet DM) had a similar conception rate (43.3%; n=141) to those fed a mixture of tallow and Ca salt of palm oil distillate (41.6%; n = 125) at 35 days post AI (Ambrose et al., 2006, personal communication). Although not different, embryo loss was 8% vs. 16% for cows fed flaxseed vs. control fat. Although the evidence is not strong, it appears that feeding flaxseed may not improve initial pregnancy rates but may reduce embryonic loss.

Other oil seeds have not been well evaluated for their ability to improve conception. Although the oil in many oil seeds contains more than 50% C18:2 (Table 1), the delivery of C18:2 past the rumen to the small intestine is not the same for all oil seeds. Based on the C18:2 content of milk fat, soybeans appear to be most effective and cottonseeds seem to be ineffective to deliver C18:2 to the tissues (Table 4). Sunflower seeds and safflower seeds also can increase the C18:2 of milk fat. The processing of whole seeds also can influence their ability to deliver unsaturated fat past the rumen. Roasting of soybeans and rolling of sunflowers seemed to increase delivery of C18:2. Although whole flaxseed fed at about 10% of the diet can deliver some C18:3 to the tissues, grinding the flaxseed may deliver even more C18:3 (Table 4). Obviously, more research needs to be done to better identify the most effective fat sources, whether from seeds, oils, or calcium salts, to increase essential fatty acid concentration of milk.

Table 4. Effect of feeding various oilseeds on the essential fatty acid concentration of milk fat from dairy cows.

Reference	Seed type	Diet
		Control	+ Oil Seed
		% of milk fatty acids
C18:2
Dhiman et al., 1995	0% vs. 16% soybeans	3.2%	6.2%*
Holter et al., 1992	0% vs. 15% whole cottonseeds	4.0%	4.2%*
Markus et al., 1996	0% vs. 7.1% whole sunflower seeds	2.3%	2.8%*
Petit et al., 2004b	0% or 9.6% whole sunflower seeds	3.2%	3.8%*
Stegeman et al., 1992	0% or 10% rolled sunflower seeds	2.2%	3.3%*
Tice et al., 1994	19.7% raw vs. roasted whole soybeans	5.5%	6.7%*
Stegeman et al., 1992	0% or 10% rolled safflower seeds	2.2%	3.1%*
C18:3
Petit et al., 2004	0% vs. 9.7% whole flaxseed	0.6%	1.1%*
Gonthier et al., 2005	0% vs. 12.5% ground flaxseed	0.4%	1.3%*
*Values under the oilseed column having an asterisk were significantly different from the control values.

Although the main nutrient in fish meal is protein and not fat, it is included here because the oils unique to fish may play a role in establishing pregnancy. The inclusion of fish meal in the diet (2.7 to 7.3% of dietary DM) has improved either first service or overall pregnancy rate in four studies. In some of these studies, fish meal partially replaced soybean meal resulting in a reduction of an excessive intake of ruminally degradable protein. Therefore, the improved conception rates may have been due to the elimination of the negative effect of excessive intake of ruminally degradable protein on conception. However, in a field study in which the concentration of ruminally undegradable protein was kept constant between dietary treatments, cows fed fish meal had a better conception rate (Burke et al., 1997), suggesting that the positive response was due to something other than a reduction in intake of ruminally degradable protein.

Amount of Fat to Feed

A frequently asked question is “How much fat or a specific fatty acid should be fed in order to try to improve reproduction?” In the studies listed in Table 2, the fat sources were fed at a minimum of 1.5% of dietary DM. We know that feeding these amounts were effective. We do not know if feeding a smaller amount of fat would be effective as well. It is certainly possible that feeding supplemental fat at a lower rate such as 0.25 or 0.5 pounds per day could be effective. The key fatty acids (whether it is linoleic, linolenic, trans fatty acids, EPA, DHA, or something else) that do reach the small intestine of the cow are absorbed into the bloodstream and deposited into tissues, including her reproductive tissues. Some of these can accumulate over time. In a Florida study, hepatic fat concentration of EPA increased from approximately 0.05 to 0.5 to 0.9% in liver samples collected at 2, 14, and 28 days in milk from cows fed linseed oil starting 5 weeks prepartum. A small but steady supply of these key fatty acids streaming to the tissues can allow the tissues to accumulate the fatty acids and have them ready at the proper time for reproductive purposes. Therefore, even a fat-feeding rate smaller than the 1.5% could prove beneficial but studies are lacking to support a recommendation.

When to Initiate Fat Supplementation

Fat feeding must be initiated far enough ahead of time before the fats are needed for restoring the reproductive tissues to a new fertile state. This would involve the involution of the uterus, the return of the ovaries to growing and ovulating new follicles, and the uterus to receiving and maintaining a new embryo successfully. As will be discussed later, cows fed selected fat sources have responded with larger (still of acceptable size) ovarian follicles. Since ovarian activity usually returns within the first 4 weeks of calving, initiating fat feeding prepartum would allow the absorbed fatty acids to influence early ovarian activity. Feeding supplemental fat for at least 21 days, preferably for 40 days, prior to the desired physiological response is our recommendation. We have initiated fat supplementation in the close-up nonlactating period (3 to 5 weeks before the calculated due date). This allows the tissues to begin storing the key fatty acids prior to when they will be most needed. We conducted an experiment to test whether the initiation of fat supplementation (Megalac-R at 2% of dietary DM) should begin at 5 weeks prepartum, at calving, or at 28 days postcalving (Cullens, 2005). Cows fed fat starting in the prepartum period had fewer health problems in the first 10 days after calving than cows in the other groups. Much more data need to be collected on this topic of health and fat feeding, but if some fat sources provide a benefit to the cow’s immune system, then the fat feeding should begin during the transition period.

How Might Fat Supplementation Help Improve Conception Rates?

Improving Energy Status?

Those lactating dairy cows that experience a prolonged and intense negative energy state have a delayed resumption of estrous cycles after parturition which can increase the number of days open. If fat supplementation can help increase energy intake, then possibly the negative energy state can be lessened and estrous cycles start sooner and conception occur sooner. While adding an energy-dense nutrient such as fat to the diet will usually increase the cow’s energy intake, the energy status of the cow is usually not improved because of a slight to moderate depression in feed intake and/or an increase in milk production. Dairy cows fed tallow at 3% of dietary DM tended to have a greater pregnancy rate (62 vs. 44%; Son et al., 1996) despite having a less positive net energy status from weeks 2 to 12 postpartum compared to cows not fed tallow. Likewise, cows fed calcium salts of CLA (Castaneda-Gutierrez, 2005) or palm oil distillate (Garcia-Bojalil et al., 1998; Sklan et al., 1991) had better conception rates without an improvement in energy balance. Although there is evidence that the feeding of fat can improve the energy status of lactating dairy cows, an improvement in reproductive performance occurred in several instances apart from an improving energy status of the experimental animals. Therefore, fat supplementation likely is improving reproductive performance by other means.

Meeting an Essential Fatty Acid Requirement?

Linoleic acid and linolenic acid are essential fatty acids for the cow because neither her body nor her ruminal microorganisms can synthesize them. Both linoleic and linolenic acid in forages can decrease during storage. As we have moved our dairy cows from pastures to barns and fed them stored forage, their intake of linolenic acid and possibly linoleic acid has likely decreased. Although current wisdom in the dairy industry is that the dietary intakes of linoleic and linolenic fatty acids are sufficient for meeting the lactating cow’s requirements, the recently developed fat submodel of the Cornell-Penn-Miner (CPM) Institute Dairy Ration Analyzer v3.0.7a (Moate et al., 2004) indicates that the modern cow is exporting more linoleic acid in her milk than she is absorbing from her diet; that is, she is in a negative linoleic acid balance. For example, using data from a recent study at the University of Florida, the model calculated that the diet supplied 33 grams of linoleic acid, but the milk exported 53 grams of linoleic acid, a 20 gram/day deficiency. Several studies using multi-canulated cows have reported less linoleic acid absorbed in the small intestine than found in the milk secreted. The difference is likely supplied from adipose tissue. The pools of C18:2 in adipose tissue are likely very dynamic. Feeding fat sources rich in linoleic acid that can reach the small intestine may reduce the negative balance of linoleic acid and improve performance. The reproductive performance of nonruminant animals, such as pigs and poultry, was greatly improved when an essential fatty acid deficiency was solved. Certainly the lactating cow does not show obvious signs of fatty acid deficiency such as scaly skin and dandruff, so if a deficiency does exist, it is not overtly obvious.

Healthier Ovarian Follicles?

In the initial days of the estrous cycle, a group of small follicles grow up on each ovary. From this group, one follicle (called the dominant follicle) continues to grow while the others regress. This will usually happen two or three times during a single estrous cycle. These dominant follicles increase in diameter from a detectable size of 3 mm up to about 15 to 18 mm before regressing or ovulating. After the dominant follicle releases its egg into the oviduct, the ruptured follicle forms a yellow structure called a corpus luteum, which produces the very important hormone called progesterone. Progesterone not only prepares the uterus for implantation of the embryo but helps coordinate the nutrients for development of the embryo and also maintains pregnancy until parturition. Cows that have a greater concentration of progesterone in their blood after insemination (during days 4 to 15) also have a better chance of becoming pregnant. What leads to greater progesterone in the blood? One factor can be a large corpus luteum formed from a large dominant follicle that ovulated. Therefore, larger dominant follicles (up to about 20 mm in size) are often beneficial. Ovulation of smaller follicles is associated with a lower conception rate.

The size of the dominant follicle is often larger in lactating dairy cows receiving supplemental fat. On average, the size of the dominant follicle was 3.2 mm larger (a 23% increase) in fat-supplemented cows compared to control cows (Table 5). As shown in Table 5, a variety of dietary fat sources have had this effect on cow ovaries. Yet, are certain fats more effective? Some studies did compare fat sources head-to-head. In two studies, it was the feeding of fats enriched in omega-6 (linoleic acid) or omega-3 fatty acids (linolenic or EPA and DHA) (Staples et al., 2000; Bilby et al., 2006) that stimulated larger dominant follicles compared to fats enriched in oleic acid. Thus, the polyunsaturated fats were most effective in increasing follicle size. Just the ovulation of larger follicles has improved fertility apart from elevated progesterone (Peters and Pursley, 2003), suggesting a more viable oocyte.

Table 5. Diameter of the dominant ovarian follicle of lactating dairy cows fed fat supplements was greater than that of cows fed the control diet (P < 0.10).

Reference	Fat source	Experimental diets
		Control	Fat
		——mm——
Lucy et al., 1991	Ca salt of palm oil	12.4	18.2
Lucy et al., 1993	Ca salt of palm oil	16.0	18.6
Oldick et al., 1997	Yellow grease	16.9	20.9
Beam and Butler, 1997	Tallow – yellow grease	11.0	13.5
Staples et al., 2000	Soybean oil, fish oil	14.3	17.1
Robinson et al., 2002	Protected soybeans	13.3	16.9
Bilby et al., 2006	Megalac-R or Flaxseed oil	15.0	16.5
Ambrose et al., 2006b	Rolled flaxseed	14.1	16.9
Average		14.1	17.3

Better Quality Embryos Produced?

All embryos are not created equal. Embryos are classified as high quality when they have a symmetrical and spherical mass with individual cells that are uniform in size, color, and density. These are most likely to become established and result in a diagnosed pregnancy. In an experiment in California, 154 dairy cows were supplemented with either a calcium salt blend of linoleic acid and trans C18:1 (EnerG I Transition Formula) or a calcium salt of palm oil distillate (EnerG II) (Virtus Nutrition) from 25 days before calving through 60 days postpartum at which time the cows underwent timed AI. Five days after AI, the uterus was flushed out to recover and evaluate the fertilized structures (Cerri et al., 2004). A greater proportion of the cows fed the mixture of linoleic acid and trans fatty acids tended to have fertilized structures compared to those fed the other fat source (87 vs. 73%); they had more sperm attached to each structure collected (34 vs. 21), and they tended to have more of their embryos classified as high quality (73 vs. 51%). In a larger set of cows numbering 397, conception rate at first AI was greater for cows fed the linoleic and trans acid mixture (33.5 vs. 25.6%) (Juchem et al., 2004). It is not clear if linoleic acid or the trans fatty acid in this mixture was most responsible for this benefit. The fatty acids in the supplement likely changed the fatty acid makeup of the cell membranes of these structures flushed from the cow’s uterus, improving their quality. In a second study, the embryos collected from superovulated Holstein cows (n = 30) fed whole unprocessed flaxseed and transferred to Holstein heifers resulted in a better gestation rate than embryos coming from cows fed Ca salts of palm oil distillate (58.8 vs. 29.3%) (Petit et al., 2004a). The diet of the donor animal was more important than the diet of the recipient animal in this study, suggesting that the dietary fat helps the cow develop a robust embryo. Embryos recovered from superovulated cows fed whole flaxseed (10% of diet) or sunflower seeds had greater cell numbers than embryos coming from superovulated cows fed tallow (Thangavelu et al., 2006). Intake of supplemental fat was about 1.65 lb/day. The feeding of polyunsaturated fats appears to have a more positive impact on embryo development than do monounsaturated or saturated fat supplements.

Less Embryonic Loss?

Here too, progesterone plays an important role. The embryo must signal to the uterus that it is present, so that the uterus does not release prostaglandin F_2α. If prostaglandin F_2α is released by the uterus, the corpus luteum will disappear, progesterone synthesis will drop, the embryo will die for lack of support, and the cow will start a new estrous cycle. About 50% of embryos die (~40% during the first 28 days after AI and ~14% between 28 and 45 days after AI). Embryonic loss is a significant problem in the dairy industry.

It is postulated that omega-3 fatty acids stored in the uterus from the diet can aid the process of embryo preservation by helping to reduce the synthesis of prostaglandin F_2α. In demonstration of this suppressing effect, cows fed omega-3 fatty acids in the form of fish meal or flaxseed had lower concentrations of prostaglandin F_2α in their blood when the uterus was artificially stimulated by an oxytocin injection (Mattos et al., 2004; Robinson et al., 2002; Petit et al., 2002). Can omega-6 fatty acids have a similar beneficial effect? Not likely, because omega-6 fatty acids are used to synthesize prostaglandin F_2α. As proof, lactating dairy cows fed soybeans or sunflower seeds (both good sources of linoleic acid, the omega-6 fatty acid) had increased concentrations of prostaglandin F_2α in their blood when the uterus was artificially stimulated with an oxytocin injection (Robinson et al., 2002; Petit et al., 2004b). Cows that are fed omega-3 fatty acids partially replace the omega-6 fatty acids stored in the uterus so that there is less omega-6 inventory for the cow to draw from for synthesis of prostaglandin F_2α.

If dietary omega-3 fatty acids are exerting a suppressing effect on PGF_2α around the time of embryo recognition, then embryo loss should be reduced. Holstein cows (n = 121) were allotted to one of two dietary treatments initiated at 55 ± 22 days postpartum (Ambrose et al., 2006). Diets were isonitrogenous, isoenergetic, and isolipidic. Diets contained either rolled flaxseed (high in linolenic, omega-3) or rolled sunflower seed (high in linoleic, omega-6). Cows fed flaxseed were twice as likely to become pregnant. Embryo mortality from day 32 post AI to calving was lower for cows consuming flaxseed compared to those fed sunflower seeds (9.8 vs. 27.3%). In summary, supplementation with omega-3 fatty acids may aid in suppressing prostaglandin F_2α to prevent regression of the corpus luteum in order to maintain progesterone synthesis and sustain pregnancy (e.g., prevent early embryonic death).

Summary

It has been known for many years that early postpartum dairy cows usually produce more milk when fed a moderate amount of supplemental fat. There is growing evidence, as summarized in Table 2, that lactating dairy cows can benefit reproductively as well. Fat sources enriched in omega-6 or omega-3 fatty acids that deliver these fats to tissues beyond the rumen may be the most effective ones to feed, but this cannot be firmly concluded because other fats having very low amounts of these omega fatty acids have improved conception rates in single studies. The fats were fed at a minimum of 1.5% of the diet in studies in which conception rates were improved. Feeding less fat than this may be beneficial, but there is no supporting research behind it. Improved conception rates by fat-supplemented cows may be due to an improved progesterone status of the cow by 1) increasing the size of the dominant follicle and corpus luteum on the ovaries and 2) by helping the corpus luteum survive and continue to produce progesterone during the early days of pregnancy. In addition, the growth and quality of the young embryo appears to be superior when polyunsaturated fats are fed. If fed in moderate amounts, initiation of fat supplementation when the cows enter the close-up group may be beneficial to the reproductive tissues as they transition from pregnancy to pregnancy. Nevertheless, it is suggested that fat supplementation be initiated at least 40 days prior to artificial insemination in order to label reproductive tissues with key fatty acids.

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Author Information

Charles R. Staples, Bruno Amaral, Albert DeVries, and William W. Thatcher
University of Florida, Gainesville 32611
staples@animal.ufl.edu