Because heifers are still growing, even when calving at 24 months, they need a diet rich in protein but moderate in energy to avoid getting too fat.

It’s no secret that good reproductive performance on dairies involves a variety of factors, but often components of reproductive programs get blamed when cows don’t get pregnant – which may be the wrong thing to focus on.

“People often focus on just the reproductive part of a program at one point in time, such as synchronizing cows,” says Pedro Melendez, DVM, PhD, College of Veterinary Medicine, University of Florida. “They focus on GnRH, prostaglandin, timed AI, etc., but aren’t looking at what were the situations surrounding parturition and before that, such as did she have difficulty calving, how was the transition and pre-partum period management, what was the incidence of retained fetal membranes, hypocalcemia, ketosis, etc. We blame reproductive management, but we often don’t look at the other factors that occurred before this time.”

Jose Santos, DVM, PhD, Veterinary Medicine Teaching and Research Center, University of California, believes it is very important that producers, veterinarians and nutritionists evaluate reproductive problems not only by paying attention to the reproductive management but also to other aspects, such as nutrition. “When cows experience excessive metabolic problems after calving, that will influence subsequent fertility and response to the reproductive program implemented in the herd,” he says.

Inadequate feeding of transition and early lactation cows that leads to more ketosis, hypocalcemia, displaced abomasum, etc., normally reduces subsequent fertility. In addition, it has been demonstrated that mastitis, which is prevalent early postpartum and can be influenced by nutrition, is negatively associated with conception and pregnancy maintenance in dairy cattle. “Unless these problems are corrected,” says Santos, “it is unlikely that the reproductive program will be successful.”

Melendez agrees and says studies relating calving-related disorders to fertility found that cows with hypocalcemia or milk fever were six times more likely to develop a retained fetal membrane and probably three times more likely to develop metritis. And indirectly, those cows are more likely to develop ketosis. “Finally, we end up with an animal that is cycling very late, so it’s very important to pay attention to the basic concept and look back to see what’s going on in the transition period,” says Melendez.

Nutrition can influence fertility at all stages of the reproductive cycle, but the period between late gestation and first postpartum insemination is probably the most critical. Santos says this is when a cow experiences most of the diseases during the entire lactation cycle and what happens to her in this period will determine how successful the subsequent breeding will be.

Melendez echoes that point of view. “Starting in the dry-off and transition period, the last 21 days before and after calving are the most important time periods for nutrition. The more deficiencies we have at that time, the more infertility we’re going to have in the herd. If we have a dry-off and dry period in bad shape, the transition period won’t improve anything.”


Pedro Melendez, DVM, PhD, says nutrition in the transition period has a significant effect on later fertility.

Fertility problems and nutrition
Nutrition can have remarkable influences on fertility. Inadequate energy intake and/or inadequate body fatness early postpartum can retard resumption of ovulatory cycles, which extends the interval from calving to first estrus, reduces conception rates at first postpartum AI and increases late embryonic losses after first postpartum AI.

In addition to energy status, there are some well-characterized effects of nutrition on fertility. Nutritional deficiencies, such as antioxidant nutrients (vitamin E and Se), influence immunity status of the uterus and the ability of white blood cells to kill bacteria that invades the postpartum uterus (see sidebar at right). “This makes the cow more likely to develop retained fetal membranes and metritis, both of which reduce fertility,” explains Santos.

Another important example is nutritional management during the transition period to prevent subclinical and clinical hypocalcemia. Hypocalcemia can be minimized by manipulating the mineral composition of the prepartum diet (feeding low Na and K diets and including acidogenic salts). Hypocalcemia results in increased blood cortisol concentrations after calving, impairs uterine involution, is associated with increased uterine infections and reduces fertility.

On the other hand, excess of nutrients or dietary components containing toxins can have negative effects on fertility. One example is a diet too high in protein. “In order to get milk production of 25,000 pounds per lactation, some farms formulate diets too high in crude protein, such as 19 to 20 percent,” says Melendez. But he adds that high protein levels lead to high levels of urea and ammonia in the blood, and studies have demonstrated that high levels of urea will affect the uterine environment and can introduce toxic effects to gametes and the embryo.

High-producing lactating dairy cows do not require diets with 19-20 percent crude protein to achieve productions above 26,000 pounds, says Santos. “What these cows really need is a diet that is balanced for the energy and protein components and provides adequate amounts of metabolizable protein with an adequate amino acid profile.”


Jose Santos, DVM, PhD, says ketosis and associated diseases have been negatively associated with conception rates.

Photo credit: Jose Santos, DVM, PhD

However, producers and nutritionists often conclude that cows need very high crude protein diets. “In fact, well-balanced diets that contain 17 to 18 percent crude protein are enough to provide all the amino acid needs of high-producing cows even in early lactation,” explains Santos. “The problem is that high-producing cows that consume a lot of feed will also consume a lot of protein, although the diet is presumptively balanced. Even in those cases, urea nitrogen in the blood will be high as a consequence of the high protein intake.”

Feeding excessive amounts of gossypol is another example, adds Santos. “We have shown that feeding cottonseed that results in plasma gossypol concentrations above 5 micograms/mL reduces conception rates and increases fetal losses, and this is, in part, mediated by reduced embryonic development.”

When pregnancy gets beyond 25 days, the cow is fairly adept at protecting the conceptus from nutrition-related problems, but even still factors, such as excess copper or iodine, toxic plants, mycotoxins or fungal-infected silage, can produce abortions.

Feeding heifers versus cows
There’s a vast difference in how heifers utilize and respond to nutritional factors than cows because heifers are still growing, even when calving at 24 months. Because of that, heifers need a diet rich in protein but with moderate energy to avoid getting too fat and risking dystocia when calving.

Additionally, the pre-calving cow differs from the heifer in that because she’s not still growing, in the transition period she’s more likely to develop hypocalcemia. “Those animals can be fed acidogenic salts to prevent hypocalcemia,” says Melendez. “Heifers are less likely to develop hypocalcemia and don’t need acidogenic salts. The take-home message is that there should be two different rations – one for heifers and one for cows.” Melendez adds that first-lactation heifers should continue to be fed in a separate group until the second calving when they become mature adults.

Santos agrees that heifers prior to calving should be fed differently than cows. In the last three weeks prepartum, heifers have lower dry matter intake relative to their body weight than cows do. Because they are still growing, their requirements for metabolizable protein is greater and, as opposed to cows, increasing the concentration of crude protein in close-up rations above 13 percent benefits postpartum performance.

“The key here is to feed diets that provide adequate metabolizable protein according to the expected intake of the animal,” says Santos. Furthermore, prepartum heifers should not be fed acidogenic salts to prevent hypocalcemia as cows are because heifers do not experience this problem and those salts reduce feed intake and might impair the secretion/activity of some regulatory hormones, such as insulin, that are important during transition. “Whenever possible, cows and heifers should be fed separately pre- and postpartum, and prepartum, they should be fed diets that are tailored to their nutrient needs.”


Cows that are obese at calving with body scores of 3.5 to 4 or more can have more problems with retained fetal membranes and ketosis, which impact cyclicity.

Melendez suggests that the older the animal becomes, the more chronic these nutrition problems can become. “If they are not well-fed, they will probably start with some lameness problems, and sometimes it’s so acute they are culled in the first lactation. They are more likely to develop hypocalcemia, chronic metritis, etc.”

Using body-condition scores
Body-condition scores (BCS) indicate the energy reserves of the body. Typically, every cow loses BCS after calving. Cows calving in obese conditions or too thin can have decreased fertility later on for different reasons. For example, an obese cow calving at 3.5 to 4 will eat less, develop more retained fetal membranes and ketosis because they are mobilizing fat, which can then impact cyclicity.

Alternatively, a too-thin cow at calving will not have enough reserves to support milk production and reproduction.

Melendez recommends that at dry-off a cow in general should score at a 3 and gain a .25 to .5 during the dry period so she will calve at 3.25 to 3.5. “Dry cows should never lose body-condition score in the dry period,” he says. In addition, low energy during the postpartum period can affect the first ovulation and expression of heat.

For heifers, preferred scores at calving are a bit lower, such as 3.25, otherwise dystocia problems can occur. Too much fat can affect the birth canal causing more dystocia.

Body condition’s affect on fertility has a lot to do with the change in score from one time point to another. Studies have demonstrated that between calving and the first 60-90 days, the cow should not lose more than 1 point of condition. Beyond that, fertility can be affected. “A cow may be at 2.5 and gaining weight or a cow may be at 2.75 and losing weight,” says Melendez. “That cow will have lower fertility even though it has a higher BCS. The change in BCS is more important than just the score itself.”

When to body-condition score
Melendez likes cows to be scored at calving, between 60-90 days post-calving to determine how much condition is lost in the first 100 days, at mid-lactation and dry-off.

At dry-off, she should score at 2.75 to 3 and be able to recover a quarter or half of a point before calving. Below 2.75 makes it difficult for a cow to gain up to 3.25 by calving time.

At calving, heifers and cows should be at 3.25 to 3.75 and not lose more than .75 units on the 1-5 scale, says Santos. Cows with low BCS at calving have reduced potential for milk production in the first four to six weeks post-partum. “Ketosis and the associated diseases have been negatively associated with conception rates at first postpartum AI.”

At 60 days postpartum, pre-breeding, they should be at 2.75 to 3. “This is important because overconditioned animals have less appetite during transition, which makes them more susceptible to ketosis and hepatic lipidosis,” says Santos. Cows with BCS less than 2.75 at 60 days postpartum are much more likely to have not experienced any ovulation prior to first postpartum AI (anovular or anestrous cows), which normally reduces conception rates and increases the odds for late embyronic loss, adds Santos.

Melendez does not like to see animals below 2.5 at 90 days post-calving. At mid-lactation, she should score at 2.75 to 3. “Losing just one body-condition score can affect her fertility,” says Melendez.

Most BCS loss after calving occurs in the first 40 to 60 days; scoring immediately prior to calving or at 40 to 60 days postpartum will allow the producer to evaluate how his/her fresh cow program is. “At mid-lactation, the producer can decide if cows should be fed differently or not if he/she wants to manage cows based on BCS,” suggests Santos. “At this point, there still is time to avoid overconditioning or to improve low BCS by moving cows to different groups if different rations are fed to the lactating herd. Lastly, the measurements taken at dry-off and immediately prior to calving will allow the producer to evaluate the dry cow program.”

For lactating dairy cows, a key factor beyond nutrition that will influence BCS is the interval from calving to conception. “Cows that become pregnant early in the postpartum period rarely will be overconditioned when they go dry,” explains Santos. “However, they can be on the thin side. On the other hand, cows that have prolonged lactations because of poor reproduction are more likely to be overconditioned at next calving.”

Heat stress, nutrition and fertility
Heat stress is the most important environmental factor influencing fertility of lactating dairy cattle. Once the body temperature of the cow rises above 40°C (~104°F), fertilization and early embryonic development are impaired. “At this point, there is not much that you can do in terms of nutritional management to improve fertility,” says Santos. “What you really have to do is to provide adequate cooling to avoid hyperthermia.”

University of Florida research demonstrated that cells under thermal stress are more prone to oxidative stress, which harms cell membranes, and indicated that one possible mechanism by which heat stress influences fertility is by causing more cellular damage to the embryo because of oxidative stress. However, it’s not been demonstrated that there is any benefit to increasing the supply of antioxidant nutrients (Vitamin E, B-carotene) on conception rates of heat-stressed cows.

Because heat stress negatively influences feed intake, it might be prudent to increase the energy content of the diet to offset reduced feed intake and minimize energy deficit that impairs reproduction, says Santos. “However, one might be cautious because cows under heat stress are more likely to develop lameness, which can be exacerbated by hotter diets. In addition, Melendez’s research demonstrated that high urea nitrogen in the milk of cows reduced fertility when associated with heat stress. This indicates that diets that are unbalanced in protein can be more detrimental to fertility during heat stress.

Nutrition and expression of estrus
Textbooks used to say estrus lasts 18 hours, but today’s cows don’t seem to be reading those old books. “Today you can see a cow in estrus for just two hours,” says Melendez. “They are completely different today, as is nutrition and the high levels of production we now have.” One of the first things affected by poor nutrition is going to be the expression of estrus.

Poor nutrition, especially inadequate energy intake, can lead to extended postpartum anestrus, so diets that cause excessive body weight loss can reduce expression of estrus. “In
addition to that, I am not aware of any research that has evaluated the effect of specific nutrients on intensity, expression and rates of estrus detection in lactating dairy cows,” says Santos. “We and others have demonstrated that 10 to 20 percent of the postpartum cows and 20 to 50 percent of the postpartum heifers have not cycled until 60 days postpartum, and these numbers are highly influenced by body-condition score. So, if cows and heifers are underfed early postpartum or experience more postpartum problems that lead to reduced BCS, they will have extended period of lack of estrus expression.”

Furthermore, recent studies by Milo Wiltbank’s group at the University of Wisconsin have demonstrated that intensity and duration of estrus are negatively correlated with level of milk production. This is one reason today’s cows are less likely to be seen in estrus. The increase in milk production has changed, to some degree, the biology of the cow and this has made estrus detection more difficult.

Investigating problems
A dairy has to have good records to begin with in order to know when reproductive problems are abnormal. A major indicator of fertility in the herd is the pregnancy rate, which is conception rate x heat detection. “Conception rate is affected by nutrition and other factors,” says Melendez. “When conception rate drops 5 to 6 percent, that’s a red flag. Pregnancy rate changes over time because of seasonality, but when you compare summer-to-summer, winter-to-winter, that number is very important and can indicate problems.”

When investigating fertility problems on a dairy, Melendez suggests to first look at the dairy’s management in general, heat detection, cow comfort and other factors. “When looking at nutritional influences, work with the nutritionist and see the interactions with rations, the levels of energy, protein, NDF, ADF, nutrients, etc., and see if there have been changes, to start to slowly expose the different pieces of the puzzle.”

Today, there are many useful diagnostic tools that can point you in the right direction when analyzing nutrition problems. One way is to measure urine pH in cows fed acidogenic salts prior to calving to prevent hypocalcemia. In addition, producers can evaluate ketones in milk by using strips to determine if changes in prevalence of subclinical ketosis early postpartum, says Santos. Other tests that can be used that are associated with the nutritional status of the herd are plasma concentrations of nonesterified fatty acids and ketones in late gestation and early lactation, and milk urea nitrogen (MUN) during lactation. Melendez says high MUN indicates too much protein, and studies have indicated too high MUN might impair fertility.

Obviously, keeping good records of occurrence of disease events is important to evaluate if the health of the herd has not deteriorated. “In addition to these tools, producers, veterinarians and nutritionists should do the common things that normally improve production, such as evaluating forage quality,” suggests Santos.

Melendez says monitoring transition cows after calving for 20 days in a separate group is one way to head off problems, then after 21 days, they can go to the milking ration. “But we need to monitor those animals for the first 20 days,” he says.

In Florida, the university’s Food Animal Group is monitoring temperature after calving within the first 10 days to see if cows have a fever, which is a red flag for metritis. Melendez says, “We’re monitoring ketone bodies by taking urine or milk samples to see if they are ketotic. If they are, the more likely they are to develop an LDA. It’s a fresh cow program, and it’s helped a lot to diagnose these problems at this time.”

Other nutrition and management factors come into play in the relationship with fertility. Cow comfort, heat abatement, bunk space, ration mixing, etc., need to also be considered, as is quantity and quality of water. Dry matter content of the TMR is very important as well, says Melendez. Veterinarians and nutritionists need to look at the dry matter content during times of wet weather, or when forages and feeds are wet so they can adjust rations to get the proper dry matter intake.

Santos believes it is critical that veterinarians and nutritionists work together. “By working together, we will be able to make more educated decisions and complement each other on our expertise in problem solving,” he says.

“The most important thing to remember is that nutrition and fertility is a highly complex relationship,” says Melendez. “And everything is related.”

Trace minerals and fertility

Adequate balances of major and minor trace minerals and vitamins play important roles in health as well as reproductive efficiency. For example, calcium is involved in any physiological and molecular process. Selenium is very important in preventing retained fetal membranes before calving. Zinc, especially in bulls, is important for semen quality. “In general,” says Pedro Melendez, DVM, PhD, “every nutrient in the end is related to fertility.”

Trace minerals, such as Cu, Se and Zn, influence the immune system of the cow by being part of enzymes that promote cell integrity at the level of membrane or cytoplasm. This is important for proper postpartum uterine health. Also, vitamins such as E and A have complementary effects to some of these trace minerals – an example is vitamin E and selenium working together as scavengers of free radicals at the membrane and cytoplasm, respectively, and they are both important for proper reproductive performance.

“However, feeding these trace minerals and vitamins in excess of the daily requirements is unlikely to improve reproduction,” notes Jose Santos, DVM, PhD. “In fact, excess of some of these trace minerals, such as copper, can be detrimental to fertility.”


Bull nutrition is important, too. Overconditioned bulls may not mount cows, but too-thin bulls may have poor semen quality.

Nutrition and bull fertility

Nutrition is important in dairy bull fertility. Overconditioned bulls can be lazy and not want to mount cows. Thin bulls may have poor semen quality.

The problem is that on dairies using bulls, the bulls consume the same diet as the cows. These diets contains more energy and protein than they need and also contain other ingredients that might be detrimental to bull fertility.

“Bulls fed lactating rations normally become overconditioned, and the fat accumulated in the neck of the scrotum can impair thermoregulation of the testes, which is critical to sperm viability,” says Jose Santos, DVM, PhD. In beef cattle, young bulls fed an excess of energy after puberty showed reduced sperm reserves in the epididymus and increased testicular degeneration.

Lactating rations can also contain considerable amounts of cottonseed and cotton byproducts, which are the source of gossypol. Most lactating cows will consume diets with 500 to 1,000 ppm of total gossypol, most of it in the free form if from whole cottonseed, leading to intakes of 15 to 30 g of gossypol/cow/day. “These amounts of gossypol are detrimental to bull fertility as they increase testicular degeneration and sperm abnormalities and might reduce libido,” says Santos.

One option is to rotate bulls continuously from the lactating group to a group in which the energy and protein content of the ration is more adequate and contains no gossypol. However, this requires more breeding bulls in the herd.