Feed & Nutrition

Since the onset of the modern era of biotechnology in 1973, scientists have made impressive strides in developing new agricultural biotechnologies. Biotechnologies that enhance productivity and productive efficiency (feed consumed per unit of output) have been developed and approved for commercial use. Technologies that improve productive efficiency will benefit both producers and consumers because feed provision constitutes a major component (about 70 percent) of farm expenditures.

[Today’s] distillers grains (DGS) tend to contain more protein, energy and available phosphorus than DGS from older ethanol plants, which likely reflects increased fermentation efficiency. Ethanol coproducts contain relatively high amounts of phosphorus, which can be a plus if additional phosphorus is needed in diets or a minus if excess phosphorus in manure needs to be disposed.

Dietary crude protein (CP) is an important determinant of milk production. Underfeeding CP is associated with reduced peak milk production. The partitioning of dietary CP into rumen degradable protein (RDP) and rumen undegradable protein (RUP) fractions has enabled a better understanding of protein utilization in the dairy cow.

It has been recognized that feeding balances of RDP and RUP consistent with requirements for rumen microbial synthesis and milk production can improve nitrogen (N) utilization efficiency. In addition to improving milk production, providing sufficient balances of RDP and RUP may enhance fertility and reduce environmental losses of N.

Protein and fertility

In general, increasing CP in dairy rations has been associated with reduced fertility, measured by increases in services per conception (reduction in conception rate (CR)) or days open (a measure of reproductive efficiency). However, when studies were combined in a meta-analysis, CP had no association with CR, but excess of RDP above that needed for rumen microbial synthesis was associated with reduction in CR.

Rumen requirement for RDP is largely determined by fermentable carbohydrate. NRC estimates the requirement of RDP as 1.18 times the yield of microbial crude protein, which is assumed to be 130 grams per kilogram (g/kg) total digestible nutrients (TDN) intake. This would equate to 24.5 g of degradable N per kg of TDN. Supplies of RDP providing more N per kg of TDN would increase rumen ammonia, plasma urea nitrogen (PUN) and milk urea nitrogen (MUN) concentrations.

MUN and PUN are highly correlated; relative differences in MUN and PUN concentrations will depend upon time of sampling PUN relative to feeding and sampling of MUN from composite or a.m.-p.m. milk samples. MUN sampled from composite a.m.-p.m. milk samples tends to be a more stable estimate of MUN concentrations. Plasma and MUN will be used interchangeably in this [article].

Ferguson et al. observed that fertility in a dairy herd was sensitive to elevated PUN. During periods when diets were offered with elevations in RDP, which increased PUN, CR declined in the herd. Cows with PUN greater than 20 milligrams per deciliter (mg/dl) had CR under 25 percent. The data suggested that PUN concentrations above 20.8 mg/dl were detrimental to fertility. Canfield et al. associated elevated PUN with reduced CR in an experiment with higher dietary RDP.

Several studies have examined increasing PUN or MUN and CR in dairy cows. A likelihood ratio test (LRT) for pregnancy was calculated for several of these studies. The LRT is calculated as the proportion of pregnant cows divided by the proportion of open cows within each urea category, as a proportion of the total cows. Pregnancy is more likely when the LRT is greater than one and less likely when below one. In general, as urea concentration increases in plasma or in milk, fertility declines.

However, the decline in fertility is not uniform across the studies, and the highest fertility group in Godden et al. was in the highest MUN category. This suggests that there is a general trend in reduction in fertility with increasing MUN, but MUN alone does not predict fertility. Multiple factors influence fertility.

Other risk factors for fertility, not identified in these studies, may modify the association of increasing urea on fertility in dairy cows. Westwood et al. found that cows consuming increased RDP had lower fertility when associated with greater weight loss in the early postpartum period, suggesting energy balance may play a modifying role on nitrogen effects on fertility. Other factors may include body condition loss, metritis and earlier days of first insemination.

Melendez et al. found negative associations of increasing MUN with fertility in summer versus winter months. Cows may adapt to high urea levels and maintain fertility. Godden et al. found the relationship of fertility with urea was quadratic; fertility was higher in cows with low and high MUN concentrations. Gustafsson et al. observed a similar relationship in Swedish herds.

Increased MUN is correlated with increased urinary urea. Urinary urea breaks down rapidly to ammonia when mixed with feces. Ammonia volatilizes rapidly from barn floors and contributes to air particulate matter and acid rain. Therefore, reducing MUN has other benefits[besides] reproduction.

Together, the results suggest that fertility and environmental impact (and milk production) may be minimized when MUN concentrations are maintained between 9 to 16 mg/dl on a herd basis. Individual cow concentrations may range from 4 to 22 mg/dl, but the majority of animals will cluster between 9 to 16 mg/dl. Thus, high production can be supported with adequate protein and minimal urea concentrations.

Mechanisms reducing fertility

Specific actions by which increasing urea concentrations associated with excess RDP reduce fertility have not been identified. Effects may be associated with alterations in the uterine environment which are detrimental to the early embryo or effects may be detrimental to the oocyte, retarding development of the early blastocyst. Blanchard et al. observed embryo quality was reduced in cows consuming a 16.5 percent CP diet that contained 70 percent RDP compared with 62 percent RDP. The effect was not apparent in all cows, but particularly was seen in a higher proportion of cows in their 4th or greater parity. Approximately one-third of cows consuming the higher RDP diet failed to yield any fertilized embryos.

Larson et al. found that cows with higher MUN had more failed pregnancies, which were associated with regular inter-estrous intervals, based on sequential milk progesterone testing. These data suggest higher RDP and urea concentrations are associated with fertilization failure as a cause of repeat breeding and should result in regular inter-estrous intervals.

However, Elrod et al. observed that reduced fertility with increasing serum urea nitrogen in heifers was associated with increased inter-estrous interval and reduction in uterine pH early in the luteal phase. Infertility was associated with increased embryonic loss. Elrod’s work suggested that loss of embryos occurred after maternal recognition of pregnancy, which extended the inter-estrous interval, resulting in reduced fertility.

These results are in contrast to Blanchard et al. and Larson et al. Blanchard and Larson’s studies were in lactating dairy cows, whereas Elrod’s studies were in primiparous, nonlactating cows. Therefore, mechanisms may be different. In addition, Blanchard’s study involved embryo’s collected from super-ovulated cows, seven days post-insemination, whereas Larson’s data was based on progesterone profiles post-insemination. Embryo loss prior to day 15 may have resulted in normal inter-estrous intervals in Larson’s study.

Sinclair et al. found higher dietary RDP increased serum ammonia and effected oocyte maturation and early blastocyst development. McEvoy et al. observed that plasma ammonia concentrations measured at or near insemination in sheep were negatively correlated with pregnancy. These studies suggest that increases in serum ammonia may play a role in reducing reproductive performance in cows fed high RDP diets by influencing oocyte quality and blastocyst maturation.

DeWit et al. and Ocon and Hansen found that oocytes incubated in increasing concentrations of urea had reduced proportions of fertilized oocytes that developed to blastocysts. DeWit et al. found that increasing urea was associated with reduced fertilization and cleavage rate, but had no effect on embryos after fertilization. Ocon and Hansen reported that fewer oocytes developed to blastocysts due to decreased developmental competence.

Urea reduced fertilization and cleavage rate of developing embryos. Armstrong et al. found increased urea associated with increased nutrient supply decreased oocyte quality. However, Lavan et al. observed that Holstein cows fed diets high in rapidly rumen degradable nitrogen experienced no negative effects on follicular development or embryo growth despite increases in serum urea and ammonia, suggesting cows can adapt to short-term increases in RDP.

Few studies have examined the relationship between RUP and fertility. Westwood et al. concluded that increasing RUP in isonitrogenous diets, improved feed intake, reduced serum nonesterified fatty acids postpartum and improved reproductive performance particularly in cows of high genetic merit. Triplett et al. fed a basal diet to postpartum beef cows with three supplements of increasing RUP (low RUP, 38.1 percent; moderate RUP, 56.3 percent and high RUP, 75.6 percent). Cows receiving the low RUP supplement had lower first-service CR than cows receiving the moderate and high RUP supplement (29.2 percent versus 57.6 percent and 54.6 percent, respectively).

Overall pregnancy proportion tended to be lower for the cows receiving the low RUP supplement than the moderate and high supplements (43.2 percent, 61.5 percent and 56.4 percent, respectively). It is difficult to separate the effects of increasing RUP on fertility from the simultaneous reduction in RDP which occurred in these studies.

Conclusion

Risk factors which modify N effects on fertility have not been clearly identified. Although it seems fertility may be maintained at higher MUN concentrations, the general trend across the literature is a reduction in fertility. In addition, elevations in MUN are associated with increased urinary losses of N, a form of N which will be rapidly lost as ammonia to the environment.

Nutritionists and veterinarians can monitor milk urea nitrogen (MUN) as a tool to assess efficiency of protein feeding. Mean MUN between 9.0 to 14 mg/dl is sufficient for adequate milk production and will ensure there are no negative effects on reproduction. Concentrations of MUN between 14 to 16 mg/dl should not significantly impair fertility but indicate some wastage of dietary N is occurring. MUN concentrations above 16 mg/dl not only may decrease fertility but also increase the risk of environmental pollution from ammonia volatilization. PD

References omitted but are available upon request at

—From 2007 Mid-Atlantic Nutrition Conference Proceedings

The saying goes, “You can’t have your cake and eat it, too.” Sometimes we’re faced with the tough decision of choosing one action or the other rather than getting everything we want. Fortunately, when it comes to making money and keeping cows healthy on your dairy, things aren’t as complicated. You can have your tasty treat, by reaping more profits in your milk check, and savor it too, by ensuring rumen health and good protein nutrition for your herd.

Research and practical on-farm experience over the years has shown us that grouping cows according to age, nutritional needs and milk production at specific stages of the lactation can provide an economic benefit to many dairy farms. The dairy farmer who’s able and willing to group cows can do a more efficient and effective job of managing his herd. It opens the door for fine-tuning of feed rations, which has the potential to increase overall lactational performance and maximize income-over-feed-costs (IOFC) for individual groups. Properly formulated feed rations targeted for specific stages of lactation will result in a more productive and healthy cow.

“Hey, Doc, my cows are eating dirt. Waddya got for that?”

A few years ago, I posed this question at several dairy seminars in the Midwest: “Do your animals chew on wood or eat dirt if they have the chance?” A few said their cows would chew on wood. Almost all indicated their cows would eat dirt, if available. One fellow said he had to haul in dirt around the foundations of his buildings to replace the soil his cows had eaten over a period of years. Strangely enough, a few even told of their cows licking or drinking from urine puddles, if they could get to them. As bad as that sounds, it is even more alarming when conventional opinion regards this eating behavior as being almost normal because it is so common. It’s the “everybody’s doing it, so it must be OK” syndrome. And it may be “normal” in the sense that it is appropriate, compensatory behavior for animals forced to subsist on a mineral-deficient ration.

Eating dirt and other abnormal appetites are attempts to secure some vital element or attain some nutritive balance that is not otherwise present in their diet. It should be considered a warning signal that something is amiss in the ration.

To examine the problem from a holistic viewpoint, let’s go back in time and look at the effect of domestication on today’s dairy cattle. Most authorities agree that primitive cattle or Aurochs (Bos taurus primigenius) were first domesticated about 8,000 years ago. Before domestication, cattle lived a lifestyle similar to that of bison in the American west. They were free to roam over wide, naturally fertile areas. Specific imbalances of soil in one area would be offset by excesses or adequacy of the same element in other areas. A multitude of different plants were available. Many plants had the ability to absorb and concentrate different minerals and trace minerals, giving the grazers even greater nutrient options. Thus, over a period of time they could seek out and obtain balanced mineral and nutritional needs. Predators strengthened the genetic pool by culling the weak and unfit.

It’s a lot different today. Dairy cattle have been genetically modified to produce at levels never intended by nature, increasing their need for minerals.

Ever-more restrictive confinement limits their ability to seek out and consume adequate diets. In a natural grazing situation herbivores probably had hundreds of different plants from which to choose. Today they are limited to six or less: grass, alfalfa, corn, soybeans, cottonseed and maybe some oats or barley. Seeds and grains in the amount currently fed can be detrimental to dairy cow health. Cows are ruminants and need a high-forage diet!

Crop quality has declined. Every crop harvested or animal removed from a farm or ranch takes with it a finite amount of life-supporting nutrients. Major elements can be replaced, but it is difficult to restore a natural balance that includes high organic matter, adequate trace minerals and vibrant biological life. Intensive NPK fertilization results in higher yields at the expense of nutritive values and mineral content in the crops.

“Average” is a myth! A total mixed ration (TMR) is the industry standard feeding strategy that attempts to provide, in one total mix, all the nutrition required by the ‘average’ cow in the group. This concept fails to consider the individuality of each animal’s nutrient requirements. No two animals have the same needs. Variables such as breed, age, pregnancy, stage of lactation, weather, season of the year and others have a marked influence on the need for mineral supplementation. With a TMR, probably no one animal will get exactly what it needs. A few may get pretty close, but many will be lacking in some nutrients while others will have excesses. This limits their production, eventually depresses their immune response and ultimately may result in various herd health problems. Eating dirt, if available, is their way of responding to these imbalances.

Unfortunately, mainstream nutritionists tend to downplay the ability of animals to balance their nutritional needs. Anyone who doubts that cattle can make valid nutritional choices needs to watch cows graze in a mixed pasture. They do not just mow grass like a lawnmower, but they pick and choose each mouthful. They avoid eating the bright green grass surrounding ‘cow pies’ in the pasture but will search the fence-rows for weeds that concentrate various essential trace minerals. Given the chance, they will balance their nutritional needs during each feeding period.

The following incident illustrates another aspect of this ability. Weather had made it a bad year for crop quality. In late winter, a good client called me about two problems. His cattle were eating excessive amounts of mineral and his heifers would abort a live calf about 10 days before they were due to calve. The calf would live, but the heifer would usually die. Focusing first on his mineral problem, he decided to try a “cafeteria” mineral program in which each mineral was fed separately. He had to carry each bag of mineral through his cow lot to get to the mineral feeder. His first few trips were uneventful. Then suddenly several of the normally docile cows surrounded him, tore a bag of mineral from his arms, chewed open the bag and greedily consumed the contents … a zinc supplement.

Within a week after the mineral change, consumption returned to normal and his remaining heifers calved normally. Apparently, the previous year’s stressful growing season had resulted in crops that were deficient in zinc or perhaps high in zinc antagonists. His mineral mix was high in calcium with only small amounts of zinc. Their quest for zinc impelled them to overeat the mixed mineral. Excess calcium interferes with zinc absorption. Every mouthful they took increased the imbalance and escalated their need for zinc. Inevitably, metabolic problems began in the most vulnerable group – young, growing heifers in the last stages of pregnancy. Finally, they just gave up and checked out ... all for want of a few grams of zinc.

If your cows are eating dirt or if you just want to experiment, give your cows a chance to participate in their own diet formulation. Do not change your current ration, but do provide separate free-choice sources of these six items: salt, bentonite, bicarb, a basic mixed mineral with a 2-to-1 Ca/P ratio, one with a 1-to-2 Ca/P ratio, and kelp. Cows with rumen acidosis will prefer bicarb or bentonite. The separate sources of Ca and P allow them to adjust that critical ratio. If they lack trace minerals they may also eat a lot of kelp. If kelp consumption remains high you may want to provide separate sources of some of the trace minerals. There are commercial companies that provide a broad range of separate free-choice minerals and trace minerals.

We should use our nutritional knowledge to formulate dairy rations, but we should also rely on the nutritional wisdom of animals to fine-tune their individual needs. It doesn’t hurt to have two opinions; one from your nutritionist’s computer and one from the real experts – your cows. PD