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.

Conception rates measured for cows managed under controlled experimental conditions as reported in scientific journals also have decreased. Rates dropped from 55 to 45 percent (breeding at spontaneous estrus) to 35 percent (timed A.I.) over a 50-year period.

The dollar value assigned to pregnancy for dairy cows varies because it is dependent upon 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 days of 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.

Even when a cow conceives, the pregnancy does not go to term about 50 percent 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. Efforts to reduce this loss are certainly justified.

Many reasons for these declines 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 that cycle later, an increase in genetic inbreeding and an increase in milk production. Average milk production per lactation has increased by 57 percent from 12,000 to 19,000 pounds per cow in the last 25 years. However, amount of milk production has not been an accurate predictor of the chance for pregnancy.


For example, higher producing cows that ate very well cycled sooner after calving than lower producing cows that ate poorly. Those poor eaters lost more bodyweight in the first two weeks postpartum and had not cycled by nine weeks postpartum, and fewer became pregnant. Extent of negative energy balance (energy output in milk plus body maintenance minus energy intake from the diet) might be a more important factor influencing pregnancy.

It is not known if the modern-day dairy cow is in a more negative energy state than dairy cows from 30 years ago. Nevertheless, the lactating cow is more difficult to get pregnant than the nonlactating cow. In conclusion, several suspects have been identified that may be contributing to this lowered fertility. What about some management steps to help promote reproductive efficiency?

Fat supplementation and reproduction introduction
The influence of nutrient intake on reproductive performance is a growing field of study, including the effect of feeding supplemental fat. Supplementing fat (the most energy-dense nutrient) in small amounts seems reasonable because milk production is such an energy-demanding process. Fat supplementation in moderate amounts often stimulates milk production and has improved reproductive efficiency. However, its impact on reproduction has not been consistent. Questions such as which fat sources can be most effective and how these fats work have not yet been adequately answered. The purpose of this [article] is to review some of the effects of fat supplementation on reproductive tissues and pregnancy.

Fats defined
Many different types of supplemental fat have been fed to lactating cows. 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 percent). Granular fats are dry fats prepared commercially and are composed mainly of palmitic acid (36 to 50 percent).

A variety of vegetable oils can be fed as free oil or in the seed form. The oil seeds contain from 18 percent oil (such as soybeans) to 40 percent oil (such as flaxseed). The selection of a vegetable oil will bring with it particular fatty acids. Canola oil is high in oleic acid. Cottonseed, safflower, sunflower and soybean oils are high in linoleic acid. Flaxseed is high in linolenic acid. Linoleic acid and linolenic acid are essential fatty acids for the cow because neither her body nor her ruminal microorganisms can synthesize them.

Fresh, temperate grasses contain 1 to 3 percent fatty acids of which 55 to 65 percent is linolenic acid. Corn silage lipid contains much more linoleic acid (49 percent) than linolenic acid (4 percent) due to the presence of corn grain. 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. The whole oil seed is frequently fed rather than the oil alone. Fish oil is unique in that it contains eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are fatty acids found in fish tissue due to their consumption of marine plants.

Dietary fats are modified in the rumen by bacteria
The ruminal microbes will convert unsaturated fats to saturated fats. Some scientists have speculated that this act by bacteria is an attempt to protect themselves as unsaturated fats can be toxic to bacteria, primarily the bacteria that digest fiber. The majority of the consumed unsaturated essential fatty acids, linoleic and linolenic acids are converted by the bacteria to stearic acid. During the process of biohydrogenation of unsaturated fats in the rumen, several intermediate forms of fatty acids, called trans fatty acids, also are formed.

Some of the trans fatty acids can influence the cow’s metabolism, including 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. The conception rates are sometimes reported for first insemination or for cumulated inseminations. The average improvement in conception rate was 21 percentage units. This is not to imply that feeding one of these feedstuffs to cows on your farm will increase herd conception rate by 21 percentage units. The margin of increase in conception rate due to fat feeding needs to be very great in trials involving a few number of cows in order for the fat supplement to be detected as having a significant effect.

If a fat supplement is to be beneficial on a dairy farm, it is more likely that the benefit will be less than 10 percentage units. In three of the four studies in which at least 250 cows participated in the study, the margin of increase due to fat supplementation ranged from 5 to 9 percentage units.

It is somewhat surprising that fat supplementation improved conception in experiments using between 30 and 132 cows. This may be partially due to the tighter management of cows used in experiments compared to that used on commercial farms. Normally about 300 cows per treatment are required to have a high chance of detecting a 10 percent increase in conception rate due to a treatment. Certainly other studies have been published in which fat supplementation did not improve pregnancy rate.

Although the main nutrient in fish meal is protein and not fat, it is included here because there is growing evidence that the oils unique to fish may play a very important role in establishing pregnancy. The inclusion of fish meal in the diet (2.7 to 7.3 percent of dietary dry matter [DM]) also 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, suggesting that the positive response was due to something other than a reduction in intake of ruminally degradable protein. The unique omega-3 polyunsaturated fatty acids in fish may have been responsible for the improvement in fertility, hence their inclusion in the current discussion.

Oil seeds have not been well evaluated for their ability to improve conception. Those seeds that can deliver the key fatty acids past the rumen may be good candidates for the diet. Although the oil in many oil seeds contains more than 50 percent linoleic acid, the delivery of linoleic acid past the rumen to the small intestine is not the same for all oil seeds.

If we use an increase in the linoleic acid concentration of milk as an indicator that an oil seed can deliver linoleic acid to the tissues, then soybeans appear to be most effective and cottonseeds seem to be ineffective. Sunflower seeds and safflower seeds also can increase the linoleic acid of milk fat but not quite as effectively as that of soybeans.

The processing of whole seeds can also influence their ability to deliver unsaturated fat past the rumen. Roasting of soybeans and rolling of sunflowers seemed to increase delivery of linoleic acid. Regarding linolenic acid, whole flaxseeds fed at about 10 percent of the diet can deliver some of its omega-3 fatty acid to the tissues.

Grinding the flaxseed may deliver even more linolenic acid to the tissues. In the United Kingdom, a process has been developed in which cracked linseeds or soybeans are processed with steam in order to create Maillard products, which help to protect the seed’s unsaturated fatty acids from ruminal microbes. Obviously, more research needs to be done to better identify the most effective fat sources, whether from seeds, oils or calcium salts.

Amount of fat to feed and economic assessment
A frequently asked question is: “How much fat or of a specific fatty acid should be fed in order to improve reproduction?”

In previous studies, the fat sources were fed at a minimum of 1.5 percent 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. People are interested in feeding a smaller amount of fat for various reasons including keeping feed costs down and minimizing the potential negative effects of supplemental fats on the cow’s bacteria in her rumen. These negative effects can include reduced fiber digestion and reduced fat and protein concentrations in the milk.

Generally speaking, fat supplementation at 1.5 percent of the diet is usually safe in terms of cow performance with the exception of fish oil. Feeding fish oil at more than 1 percent of dietary DM will usually reduce feed intake or milk fat and protein concentration.

If the fat concentration of the base diet without a fat supplement is 3 to 4 percent, then increasing it to 4.5 to 5.5 percent by fat supplementation should not be a problem if the dietary fiber is sufficient and effective. Certainly diets containing higher fat ingredients (like distillers grains, hominy or whole cottonseeds) need to be watched closely so that the total fat content stays below 6 percent.

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 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. A small but steady supply of these key fatty acids streaming to the tissues will allow the tissues to accumulate the fatty acids and have them ready at the proper time for reproductive purposes. Therefore, even a smaller fat feeding rate than the 1.5 percent as used by one of the published experiments could prove beneficial.

The economic assessment of fat supplementation on herd performance is not a straightforward calculation. It is based upon many factors including the base conception rate of a herd, improvement in conception rate or milk yield, milk price, replacement costs, etc. Based upon some preliminary calculations using the breeding and replacement model described by De Vries, the economic benefit of fat supplementation to lactating dairy cows was evaluated. If conception rate was improved from 17 to 20 percent and milk production ($14 per hundredweight [cwt]) was improved by 2 pounds per day through supplementation of fat, an additional $100 per cow would be realized.

If the fat supplement was fed at 1.5 percent of dietary DM, intake of fat would be about 0.7 to 0.8 pounds per day. The break-even price in the diet to realize this benefit would be $0.38 per cow per day over 300 days. To have a 2-to-1 benefit-to-cost ratio, one could not afford to pay more than about $0.25 per pound of fat supplement. Other possible benefits from fat supplementation such as improved health or body condition were not considered in these calculations.

Certainly fat costs would change if the same results could be achieved by feeding less fat than 0.7 to 0.8 pounds per day. If conception rate or milk production could be increased further than outlined here, then benefits would increase.

When to initiate fat supplementation
Fat feeding must be initiated long enough 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 four 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 begun supplementing cows in the close-up nonlactating period (three to five 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 (2 percent of dietary DM) should begin at five weeks prepartum, at calving or at 28 days post-calving. Cows fed fat starting in the prepartum period produced more milk and had fewer health problems in the first 10 days after calving than cows in the other groups.

If some fat sources provide a benefit to the cow’s immune system, then the fat feeding should begin during the transition period. In summary, research studies that have documented the benefits of fat supplements for reproduction fed the fats at a rate of at least 1.5 percent of the diet DM. Feeding fat supplements at a lower inclusion rate may prove beneficial in the field, but the research studies to support a lower feeding rate have not been done, as far as we know.

How might fat supplementation help improve conception rates?
Improvements in reproductive performance through dietary fat supplementation may result for a variety of feasible reasons, either acting alone or in combination. The research support behind each of these is not equal, although each has a biological basis to be true. The main hypotheses are listed below as a group and then are discussed individually in the following paragraphs.

1. The feeding of additional energy in the form of fat reduces the cow’s negative energy status so that she returns to estrus earlier after calving and therefore is more fertile at insemination.

2. Feeding additional essential fatty acids in the diet cures a fatty acid deficiency that has developed in the modern-day lactating dairy cow as she is managed today.

3. Cows fed fat develop larger ovarian follicles that develop into larger corpus lutea (CL) which produce more progesterone, a hormone necessary for coordinating nutrients for the developing embryo and for maintaining pregnancy until calving.

4. Specific individual fatty acids taken up by the uterus help inhibit the production or release of prostaglandin F2α (PGF2α) by the uterus when the embryo is 2.5 weeks old. This helps prevent the regression of the corpus luteum on the ovary so that progesterone continues to be produced and the newly formed embryo survives.

5. The fertilization rate and embryo development is improved when fat is fed.

6. The immune status of the cow may be improved, reducing her susceptibility to disease and improving her chances of becoming pregnant.

Improving energy status
Those lactating dairy cows which 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. Adding a very energy-dense nutrient such as fat to the diet will usually increase the cow’s energy intake. However, the energy status of the cow is usually not improved because of a slight to moderate depression in feed intake or an increase in milk production.

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
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 sub model of the Cornell-Penn-Miner (CPM) Institute Dairy Ration Analyzer 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.

Feeding fat sources rich in linoleic acid that can reach the small intestine may reduce the negative balance of linoleic acid and improve performance. Nonruminant animals such as pigs and poultry had their reproductive performance 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 severe.

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 disappear. 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 millimeters (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 by maintaining the uterus until parturition.

Cows that have a greater concentration of progesterone in their blood after insemination (during days four to 15) also have a better chance of becoming pregnant. What leads to greater progesterone in the blood? 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 millimeters larger (a 23 percent increase) in fat-supplemented cows compared to control cows. A variety of dietary fat sources have had this effect on cow ovaries. Yet are certain fats more effective? In various studies, polyunsaturated fats were most effective in increasing follicle size.

In summary, cows fed fats enriched with the essential fatty acids are likely to have more progesterone being synthesized due to larger ovarian corpus lutea, thus making them better candidates for a successful pregnancy.

Less embryonic loss
Here too, progesterone plays an important role. The embryo must signal to the uterus that it is present so the uterus does not release PGF2α. If PGF2α 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 percent of embryos die. Embryonic loss is a significant problem in the dairy industry.

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 PGF2α. Can omega-6 fatty acids have a similar beneficial effect? Not likely, because omega-6 fatty acids are used to synthesis PGF2α.

Supplementation with omega-3 fatty acids may aid in suppressing PGF2α to prevent regression of the corpus luteum in order to maintain progesterone synthesis and sustain pregnancy (e.g., prevent early embryonic death).

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 published studies, 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.

Improved immune status
During the first four weeks postpartum the cow’s immune system is severely challenged. The incidence of diseases and disorders can be high during this time, and these, in turn, can have a negative impact on reproductive performance. We are learning that the fatty acid profile of the lymphocytes (the primary cells that recognize and respond to pathogens) can be changed by the type of fat the cow consumes.

This, in turn, can change the way animals handle an infection, although cows have not received much attention by scientists in this area. If positive health benefits should result from fat supplementation, it seems logical to begin moderate fat supplementation in the close-up period.

It has been know for many years that early postpartum dairy cows usually produce more milk when fed a moderate amount of supplemental fat. There is growing evidence 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 percent 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 have been associated with an improved progesterone status of the cow by:

1. increasing the size of the dominant follicle and corpus lutea on the ovaries

2. helping the corpus luteum survive and continue to produce progesterone during the early days of pregnancy

Early research results appear promising that some fat supplements may prove helpful to the health of the cow as well, but much more work is needed. If fed in moderate amounts, start feeding the fat when the cows enter the close-up group, especially if benefits to cow health and the ovaries are desired. PD

References omitted but are available upon request at
—Excerpts from 2007 Western Dairy Management Conference Proceedings

Charles R. Staples
Bruno Amaral
University of Florida – Gainesville

Albert De Vries and William W. Thatcher
Department of Animal Sciences
University of Florida – Gainesville