Ken Odde, D.V.M., Ph.D.
Department of Animal Sciences
College of Agricultural Sciences
Colorado State University
Fort Collins, Colorado 80523
In recent years, scientists, clinicians, and producers have begun to realize the extent to which diet affects the health of cow-calf herds. Presented here are research data supporting the importance of sound nutritional management, as well as practical information on gauging cattle's nutritional status and adjusting their diets to optimize production.
One inescapable conclusion emerging from 30 years of research in ruminant nutrition is that cattle–like humans–are what they eat. Advising cow-calf clients about what this statement means in terms of adjusting herd feeding programs is critical to improving the profitability of their operations. (It is also one of the most important value-added services we can provide as veterinary consultants.)
This article provides a review of important investigations into nutrition's impact on each of the following factors that directly affect the bottom line of cow-calf operations:
• reproduction, particularly a cow's return to estrus
• calf vigor and survivability
• calf weaning weights
• resistance to disease, including immune response to vaccination.
The information is published here as a service to veterinarians whose clients have questions about the economic benefits that might be derived from monitoring and modifying a herd's nutritional program. Practitioners seeking more detailed discussions of specific topics are advised to consult the appropriate works listed under "References."
Nutrition and reproduction: some statistical realities
An article in the December 1991 issue of Drovers Journal summarizes the harsh economic realities facing cow-calf operations where pregnancy failures and delayed breeding are common:
Cow costs of $300 or more must be absorbed by the rest of the herd when there is a pregnancy failure. And, there is a loss of 21 growth days against a common weaning date when there's a pregnancy delay of one period.
Calves weighing 85 pounds at birth must gain 2.27 pounds per day to wean at 550 pounds in 205 days. Thus, a one-period pregnancy delay translates into 47.6 pounds less weaning weight--or $42.84 on a $90 market.
In a herd of 100 cows, if 10 cows miss just one cycle, there's a loss of 476 pounds, or $428 on a strong calf market. If a second set of 10 cows misses two cycles, and a third set misses three cycles, the loss increases to over $2,500.1
Unfortunately, economic statistics like this are all too familiar to veterinarians practicing in cow-calf country. National surveys show that first-service conception rates average only 65% for either natural service or artificial insemination.2
In specific herds and specific areas of the country, the numbers are even more dismal: first-cycle (21-day) pregnancy rates as low as 20% are relatively common, while a 35% rate is closer to average, and an 85% rate is considered exceptional.1 Typically in such herds, the highest percentage of females failing to conceive during the first cycle are first-calf heifers.
Animal health scientists acknowledge that environment, type of service (natural, artificial, or a combination of the two), and herd health all contribute to pregnancy rates. However, they consistently identify inadequate nutrition--both precalving and during the interval between calving and rebreeding--as the primary cause of delayed cycling.
In the 1960s, studies established that rebreeding was impaired in cows that were thin going into calving, even if they were put on a high-energy feeding program after calving.3 Since these initial investigations, various studies have been undertaken to gain a deeper understanding of the relationship between nutrition and reproduction.
In the mid-1970s, researchers at Colorado State University observed substantial differences in the re-breeding performance of a large number of cows that had been classified in one of three different body conditions (Table 1).4 Cows in thin condition at calving were able to achieve a cycling rate of only 66% at 90 days after calving. When cows were fed to moderate condition, the cycling rate 90 days after calving rose 26 percentage points to 92%; fed to good condition, to 100%.
During the 1980s, additional work linking nutrition to conception was done at Purdue University and the Eastern Colorado Research Center at CSU:
• The Purdue trial showed that as cow body condition scores at calving decreased, days to rebreeding and conception increased (Table 2).5
• In our Colorado trial, postpartum beef cows were estrus synchronized using melengestrol acetate and prostaglandin F2. A strong relationship between body condition score and pregnancy rate was observed. Cows in condition score 3, 4, 5, and 6 had synchronized pregnancy rates (percentage pregnant in the first five days) of 13.3%, 20.4%, 55.2%, and 68.8%, respectively.6
Statistics like the 90-day re-cycling percentages (Table 1) spell economic trouble for cow-calf operations where cows and first-calf heifers are in thin body condition at calving. Here's why. Only 66% of the poorly conditioned cows are cycling at 90 days after calving. They are the only cows with the potential of settling in time to keep the operation on a 12-month calving schedule. The remaining 34% cannot possibly conceive until sometime after the final 21-day cycle of an 80-day breeding season.
If these cows are retained and are bred late (beyond the limits of the set breeding season), they will extend the next calving season. And their offspring will decrease the total calf crop weight by 21 growth days for each of the four cycles that their dams failed to conceive during the previous breeding season. The reduction in uniformity among the calves due to variations in weight ultimately translates into reduced revenues at market time.
Additional financial damage is sure to occur the following year because the late-calving cows stand very little chance of cycling along with the rest of the cow herd, and thus cannot settle before bulls are removed from pasture. In this case, the cost of maintaining the open cows must be absorbed by the rest of the herd.
Even on operations where estrus synchronization procedures are followed, poor pregnancy results will continue until managerial changes are made in the cow herd's nutritional program.
Because cows cannot settle until they cycle and because nutrition directly affects onset of cycling, the most practical advice we can give cow-calf operators is to have all cows in proper body condition by calving. By doing so, they can stop cows from slipping back from one cycle to the next, and can begin to move others forward so that they too are on a 12-month calving schedule.
The nutritional background of bulls can also be an important factor in realizing the full reproduction capacity of cow-calf herds. When bulls fail to reach their maximum growth potential because they are underfed, they are less active at mating and may be poor sperm producers. In a trial where bulls were fed only 60% of their calculated needs for total digestible nutrients from 8 weeks of age to 44 months of age, the undernourished bulls showed reduced sperm-cell production, even though they showed the same sexual interest as adequately nourished bulls.7
Poor nutrition also affects the time bulls require to recover maximum sperm-producing capacity after service. Research shows that when bulls are adequately fed, their sperm count recovers within 7 days; in contrast, the sperm count of underfed bulls is not fully restored until 26 days.8
Nutritional management of dam affects calf vigor
Cow nutrition also directly affects calf development and survivability. Some of the earliest work on neonatal growth of beef cattle was done during the 1970s when researchers showed that nutritional deprivation of the dam reduced birth weights but did not simultaneously lower the percentage of dystocias or increase calf survival rates.9,10 Related studies conducted during the 1970s and 1980s showed that calf mortality increased as prepartum nutritional stress increased.11,12 Findings like these are especially important to share with our clients.
Another factor affecting calf survival that we at CSU have linked to cow nutrition is body heat production in neonatal calves. We knew that low-birth-weight calves were less tolerant of cold stress,13 but we did not yet know the relationship between birth weight and subsequent body heat production.
The study we conducted involved 24 two-year-old beef heifers pregnant with either an identical twin or a full sibling calf, while the paired twin and another full sibling were carried in a separate heifer, resulting in 12 pairs of females.14 (The embryos had been obtained from super-ovulated donor females and micro-surgically divided into demi-embryos. ) During the final trimester, one study group of pregnant heifers was given a protein-restricted diet (55% of the level recommended by the National Research Council) while pregnant control heifers received 91% of the recommended protein requirement. The protein-restricted heifers also consumed 8.3% less dry matter, resulting in a slight reduction in metabolizable energy intake compared with control heifers.
At calving, colostrum was completely milked out and quantified. All calves were given 1 liter of their dam's colostrum by esophageal feeder, and were then placed in metabolic chambers from hours 5 to 13 after birth to have their heat production measured.
Heifers fed the protein-restricted diet gained 84 kg/head less in body weight during the last trimester (P < .001) and had lower body condition scores at parturition (P < .05). The protein-restricted heifers also produced a reduced volume of total colostrum, but their concentrations of IgG1 and IgM were higher than those of heifers receiving an adequate diet.
This observation showed that an inverse relationship exists between colostral volume and colostral immunoglobulin concentration: higher volumes of colostrum tend to have lower concentrations of immunoglobulin. Because the trial artificially restricted intake to one liter of their dam's colostrum, and colostrum from protein-restricted heifers had higher concentrations of immunoglobulin, serum lgG1 and IgM concentrations were higher in calves from protein-restricted heifers.
Although energy intakes were similar for calves from both test groups, calves born to heifers fed the protein-restricted diet produced 11.4% less body heat and required more time to stand after calving than did calves born to dams receiving adequate levels of protein (Table 3). In addition, heat production per square meter of surface area in large calves was nearly double that in small calves. Thus, maintenance of a constant body temperature during cold stress would be more difficult in the smaller calves.
The difference in body heat production was detectable even though other vital signs--respiratory rate and rectal temperatures at birth--were unaffected by prepartum protein nutrition. While the birth weight of calves from protein-restricted dams was also less than that of calves from non-protein-restricted dams, the weight difference was not statistically significant.
The impaired ability of the neo-natal calf to produce heat, as a result of prepartum nutritional stress and/or small birth weight, may be one of the factors contributing to neonatal mortality, which other studies indicate is as high as 8% to 13.5%.15,16
Heifer body condition affects calf Ig concentration, weaning weight
Calf heat production, however, is only one of the developmental and survivability factors affected by nutritional management of dams. Work done in 1975 by Corah and others showed that pregnant cows fed 70% of their calculated energy requirements during the last 90 days of gestation produced calves with increased morbidity and mortality rates.12 Subsequent studies that we conducted at CSU established a correlation between body condition of the dam at calving and calf serum immunoglobulin (Ig) concentrations.
In the first of several trials, we used 73 two-year-old crossbred pregnant beef heifers.17 At calving, the heifers' body condition was scored on a scale of 1 through 9, where 1 indicated very thin, and 9 indicated very fat. Sex and birth weight of the calves were recorded soon after calving. Calves born unassisted were allowed to suckle their dams naturally, while calves requiring assistance at birth were fed either with a nipple bottle or esophageal feeder. Calves and dams were then placed in pens together, and the dams were scored for mothering abilities on a scale of 1 to 3, with 1 indicating immediate acceptance of the calf, and 3 indicating physical abuse of the calf.
About 24 hours after calving, we took jugular blood samples from the calves, and separated and froze the serum. Thereafter, IgG1 and IgM concentrations in the calf serum were quantified by single radial immunodiffusion. To determine the relationship between immunoglobulin concentrations and calf growth, calves were weighed at the ages of 60,180, and 240 days (weaning).
Table 4 summarizes various physiological measurements of the calves that were affected by the body condition of their dams. Calves born to heifers in thin body condition (scores of 3 and 4) required more time to stand following birth than did calves born to heifers in moderate condition (scores of 5 and 6), but the time differences were not statistically significant.
Calves from heifers in better body condition also had higher levels of IgG1 and significantly higher levels of IgM in their serum, a finding likely due to one or more of the following factors:
• increased volume of colostrum produced
• increased calf vigor and sucking intensity, which allowed calves to take advantage of the immunoglobulin that was present
• increased ability of the calf to absorb immunoglobulins.
Since immunoglobulin concentration in calf serum is a measure of protection against disease, calves from cows in good body condition stand a much better chance of resisting infectious agents than calves from cows in poor body condition.
The relationship between serum immunoglobulin concentration and calf growth is shown in Table 5. At 60, 180, and 240 days, the correlation between IgG1 and IgM concentrations and calf weights was positive. There are at least two possible explanations for this observation:
• calves with higher immunoglobulin concentrations were more resistant to disease and therefore had fewer restraints put upon their growth
• calves with higher immunoglobulin concentrations received more milk from their dams because of the positive relationship between colostral immunoglobulin production and subsequent milk production.
Regardless of the reason, the study points up an important consideration for cow-calf veterinarians and their clients: body condition of 2-year-old beef heifers clearly affects the serum immunoglobulin concentrations and weight gains of their calves. Heifers in proper body condition at calving gave birth to calves that were consistently more vigorous and that showed higher concentrations of serum immunoglobulin after ingestion of colostrum.
A follow-up study, which included older cows as well as heifers, was conducted to determine whether cow body condition produced similar effects on calf immunoglobulin concentrations when the dams were mature cows.18 As in the previous study, calves from heifers with condition scores of 3 and 4 at calving had lower serum immunoglobulin concentrations than calves from heifers with condition scores of 5, 6, and 7.
In comparison with 3-year-old and older cows, thin first-calf heifers produced calves that had lower levels of serum immunoglobulins at 24 hours of age, even though colostral immunoglobulin concentrations in the heifers and older cows were similar. The lower volumes of colostrum produced by the first-calf heifers is likely a principal factor (dystocia being another key cause) responsible for increased disease susceptibility in calves from first-calf heifers.
Body condition and immune response
Work on the relationship between cow nutrition and calf serum immunoglobulin concentrations led to various research projects on nutrition's effect on the dam's response to vaccination. Early in the 1980s, researchers demonstrated that the nutritional condition of animals greatly influenced their immune response to vaccines.19 Previous studies had already established that young stock deprived of protein were able to mount only a weak immune response to vaccination.20
Under conditions of poor nutrition, protein used in the production of antibodies is rechanneled to meet heifer energy requirements. Nutritionally deprived, these young cows simply are incapable of mounting an adequate immune response following vaccination and thus are susceptible to infectious disease.
For much the same reason, when poorly nourished first- and second-calf heifers are vaccinated against viral and bacterial scours agents, they may be incapable of producing the quantity of colostrum necessary to protect their calves against neonatal scours.21 While various factors--including imprecise timing of vaccinations--can be involved in vaccine breaks, energy depletion of the dam is certainly one of the key causes. When first- and second-calf heifers are protein starved as a result of energy deprivation, their production of antibodies against scours agents is compromised, and the immunoglobulin concentration of the colostrum in these young cows is inadequate to protect their newborn calves against disease challenge.
Energy supplies available for the production of antibodies are drained even further during periods of cold weather. Studies have shown that a cow's energy needs increase by at least 1% for each 1_F drop below the animal's critical temperature (temperature at which the cow has to burn energy to stay warm).22 In cold, wet, and windy conditions, cow energy requirements may increase as much as 2 % for each 1_F decrease in temperature.23
Widespread effects of poor nutrition
The various research findings cited in this article underscore the consequences of failing to satisfy the nutritional needs of pregnant beef cows and heifers:
• Thin cows cycle late after calving and have low pregnancy rates.
• Thin cows that do become pregnant probably will conceive late in the breeding season, calve later, and produce lighter weight calves at weaning.
• At market time, light-weight calves from poorly conditioned cows depress the total calf crop weight by 21 growth days (40 to 50 lb) for each cycle their dams failed to conceive.
• At 24 hours of age, calves from thin first-calf heifers have lower levels of serum immunoglobulins than calves from mature cows.
• Cows thin at calving are more likely to produce less vigorous calves that are more susceptible to disease.
• Calves born to heifers on restricted protein diets have a reduced ability to produce heat soon after birth. As a result, the calves are more susceptible to cold stress.
Body condition scoring: key to adjusting feeding programs
The most direct way that cow-calf producers can help cows and heifers deliver live, healthy, disease-resistant calves is to maintain the breeding females in good body condition through adequate nutrition. An added benefit is that cows and heifers in good condition are more likely to conceive early in subsequent breeding seasons, thereby improving production results and extending cow longevity within the herd.
To determine the plane of nutrition a cow herd requires, producers need a practical method for assessing the effectiveness of existing feeding programs. An excellent tool that veterinarians can teach is the body condition scoring (BCS) system.24
BCS is nothing more than assigning each cow a numerical value based on the amount of the animal's body fat (an indicator of energy reserves). The system we recommend uses a 9-point scale, where
1 = emaciated, 5 = average, and 9 = obese. Body condition should be determined by observation and touch (wherever possible) and recorded twice a year--once at calving and once at pregnancy checking.
Using the BCS system at pregnancy testing, producers can sort their cow herd for separate nutritional management so that all breeding females can reach proper body condition at calving (see box for background on meeting the nutritional needs of replacement heifers). Cows with a BCS of 3 or below may be starting to show muscle degradation and need to be put on a higher quality feed to reach a BCS of 5 or 6 by calving. Overconditioned cows (BCS of 8 and 9) are expensive to maintain and may encounter calving difficulties. Appropriate cutbacks in feed may be required.
For all practical purposes, the target to shoot for in mature cows is a BCS of 5 to 6 at calving; for heifers, the target is slightly higher at 5.5 to 6.0. Data from many different trials show that when cows and heifers reach these scores, they are the most efficient at cycling earlier after calving, getting pregnant earlier in the breeding season, and producing more vigorous calves earlier in the calving season. In addition, calves from BCS 5 and 6 females have higher immunoglobulin concentrations in their serum at 24 hours of age, and thus are likely to be more resistant to infectious diseases.
Conditioning replacement heifers
Replacement heifers must reach puberty between 13 and 14 months of age if they are to become profitable cows capable of achieving the following production goals:
• becoming pregnant in the first 25 days of the breeding season
• giving birth to live calves with little calving difficulty
• raising the calves to weaning at or about average weaning weight
• breeding back as 2-year-olds within the first 45 days of the breeding season
• continuing to reproduce and wean calves every year for 6 to 9 years.
Research has shown that the two most important factors influencing onset of puberty are age and weight. Feeding replacement heifers so that they weigh about 65% of the mature weight for their respective breed prior to the breeding season enhances the odds of achieving long-term production goals.
Since conception rates are higher on the third estrus of replacement heifers than the first,25 producers should be encouraged to get their heifers to the 65% target weight as early as one month before the set breeding season. Such a practice increases the odds of heifers conceiving early in the season and keeping them on the same yearly schedule as the cow herd.
NRC requirements: a good starting point
The National Research Council (NRC) has established guidelines and feeding schedules for keeping cows of different weights in optimum body condition at various stages of production. Our advice is to start with these requirements, keep a close watch on the cow herd, and adjust feeding schedules accordingly for all of the following factors:
• Age. First- and second-calf heifers have higher nutritional requirements because they are still growing while maintaining a fetus and lactating. To achieve optimal performance levels, these heifers must be kept on a high plane of nutrition, especially during the critical pre- and postcalving periods. Since first-calf heifers tend to get less feed when group fed, sorting the 2-year-old females from the older cows helps assure delivery of superior quality hay and forage to those animals most in need. The same quality ration would overfeed mature cows and would be wasted.
• Breed. High-milking beef breeds may have higher maintenance requirements and do have higher requirements for lactation. A BCS of 6 at time of calving is desirable for young higher-milk-producing cows.
• Grazing activity. A study conducted in 1987 suggests that maintenance requirements for grazing cows may be 30% to 40% higher than those of cows in drylots.26
• Weather conditions. At least 1% more energy must be supplied to cows for each 1_F drop below critical temperature. Maintenance requirements for thin cows are much higher than they are for fleshy cows in cold temperatures.22,23
• Stage of production. As Table 6 shows, the beef-cow-year is divided into 4 periods.27 Nutrient requirements are highest following calving when the cow is lactating heavily and trying to recover from calving to cycle and rebreed (Period 1). An increase in milk production from 10 pounds to 30 pounds per day increases nutrient requirements for lactation by 200%.28 In contrast, requirements are lowest after weaning when the cow is only maintaining herself.
Help in determining dietary levels of protein and energy is now available in the form of an inexpensive, simple-to-operate spreadsheet template from Iowa State University.* Veterinarians or producers can enter variable factors--hay quality (obtained from laboratory analysis), current weather conditions, stage of reproduction, cow size, and body condition and the spreadsheet analyzes the ration for nutrient adequacy while simultaneously projecting feed costs and daily gain.
With this information, producers can quickly and accurately adjust feeding programs to meet the changing needs of the cow herd. Some producers, for example, may be surprised to discover that a cow herd's energy requirements in relatively mild (about 40ºF) but wet and muddy conditions are practically the same as they are in windy 0ºF conditions. The key practical advantage of the Iowa State spreadsheet over NRC guidelines and most computer programs is the ability to factor environmental effects into the rations recommendations.29
Meeting the energy and protein requirements set by the NRC helps assure the maintenance of adequate body condition in cows and heifers. Extra energy must be added to the maintenance requirements if gain is to be produced in heifers, young cows that are still growing, and thin mature cows.
For a spring-calving herd, a good time to improve or maintain cow condition is the fall when a large quantity of good quality grass is still available and cold stress is not yet a problem. Decreasing the nutrient demand on cows can be accomplished by:
• weaning calves at an earlier age
• supplementing protein to allow maximum utilization of energy from the grass
• moving cows to higher quality pasture (meadow regrowth, small grain fields, alfalfa aftermath).30 Where circumstances dictate (first-and second-calf heifers, for example), it is advisable to wean calves early and provide supplemcntation.
The impact of early weaning and supplementation on cow weight gain and calf growth was shown in a trial conducted by Adams and Short (Table 7).31 When calves were left on cows that were not supplemented, cows lost 2 pounds for every 1 pound calves gained. When calves were left on cows but the cows were supplemented, cows lost only 0.3 pounds for each pound in calf gain.
Cows that were supplemented and whose calves were weaned in September gained 80 pounds from September to December. In contrast, cows whose calves were weaned but that were not supplemented lost 23 pounds. Overall, the difference in cow weight between cows that were supplemented and whose calves were weaned and those that were not supplemented and whose calves were not weaned was 210 pounds.
A final consideration: input costs
As we can see from the various studies cited in this article, reproduction, calf development to weaning, onset of puberty, and disease resistance all respond to adjustments in the nutritional management of cow-calf herds. In herds where cows, heifers, and bulls are maintained in optimum body condition and preventive health measures are implemented to control reproductive and enteric diseases, significant progress can be made toward the goal of having each breeding female provide a live healthy calf every year.
Specific feeding programs, however, must be customized for the resources of individual operations. Only when individual input costs are assessed against calculated potential production gains can a successful nutritional program be implemented.
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| Thin (1-4) |
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| Moderate (5-6) |
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| Good (7-9) |
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(Whitman, Colorado State University, 1975)
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| 3 (thin) |
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| 4 (thin) |
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| 5 (moderate) |
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| 6 (moderate) |
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| 7 (good) |
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a
Body condirtion scores have been converted from a 5-point system to a 9-point system.(Houghton, et al., J Anim Sci, 68:1438, 1990. Reprinted by permission of the publisher.)
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Protein intake (kg/day) and performance valuesb |
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Metabolizable energy (Mcal/day) |
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| Colostrum quantity (mL) |
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| Colostral IgG1 (mg/dl) |
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| Colostral IgM (mg/dl) |
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| Total colostral IgG1 (g) |
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| Total colostral IgM (g) |
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Calf serum IgG1 24 hours (mg/dl) |
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Calf serum IgM 24 hours (mg/dl) |
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| Heat production (Kcal/MBS) |
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Interval from calving to standing for calves (min) |
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a
Twelve pairs of heifers were involved in the study Mcal = megacaloriesb
Values given are means ± standard errorKcal/MBS = kilocalories per unit of metabolic size
(Carstens, et al., J Anim Sci 65:745-751, 1987. Reprinted by permission of the publisher.)
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| Parameter |
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Interval from calving to standing for the calf (min) |
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| Colostrum production (mL) |
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| Calf serum (IgG1 (mg/dl) |
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| Calf serum IgM (mg/dl) |
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a
Condition was scored on a scale of 1 through 9, where 1 indicated very thin, and 9 indicated very fat.b
Numbers in parentheses are the number of observations.|
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| Parameter |
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| Weight at birth |
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| Calf weight at 60 days |
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| Calf weight at 180 days |
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| Calf weight at 240 days |
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| Average daily gain from birth to weaning |
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a
Adapted from Corah, Kansas State University, Ext Rep C68, p.2, February 1987.Reprinted by permision of the publisher.
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| Protein Supplement |
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| Cow weight change (Sept-Dec) |
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| Cow condition change |
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| Milk Production |
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| Weaning Weight |
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a
Adapted from Short, et al., Factors Affecting Calf Crop, pp., 176-186, 1993.Reprinted by permission of the publisher.
* Cost of the spreadsheet template, "MCS 13 Beef Cow Ration Analysis," is $20 for Iowa residents, $25 for nonresidents. Orders can be placed with Extension Software Service, Iowa State University, 108 Atanasoff Hall, Amess, Iowa 50011, 515-294-8658.
REFERENCES
1. Allen, K.: Solving production's complex puzzle. Drovers Journal, pp. 10-11, Dec 1991.
2. Knop, F.: For a better ROA, improve reproduction. Drovers Journal, pp. 8-9, Dec 1991.
3. Wiltbank, J.N., Rowden, W.W., Ingalls, J.E., et al.: Effect of energy level on reproductive phenomena of mature Hereford cows. J Anim Sci 21:219-225, 1962.
4. Whitman, R.W.: Weight change, body condition and beef cow reproduction. Doctoral dissertation, Colorado State University, Ft. Collins, Colorado, 1975.
5. Houghton, P.L., Lemenager, R.P., Horstman, L.A., et al.: Effects of body composition, pre-and postpartum energy level and early weaning on reproductive performance of beef cows and preweaning calf gain. J Anim Sci 68:1438, 1990.
6. Yelich, J.V., Mauck, M.S., Holland, M.D., et al.: Synchronization or estrus in suckled beef cows with melengestrol acetate (MGA) and PGF2 Col State U Beef Program Rep, P. 71, 1988.
7. VanDemark, N.L., Mauger, R.L.: Effect of energy intake on reproductive performance of dairy bulls. I. Growth, reproductive organs, and puberty. J Dairy Sci 47:798, 1964.
8. Salisbury, G.W., VanDemark, N.L.: Management factors affecting reproductive efficiency of the bull. In Physiology of Reproduction and Artificial Insemination of Cattle. San Francisco, W.H. Freeman & Co., 1961, pp. 592-631.
9. Laster, D.B.: Factors affecting pelvic size and dystocia in beef cattle. J Anim Sci 38:496, 1974.
10. Bellows, R.A., Short, R.E.: Effects of precalving feed level on birth weight, calving difficulty and subsequent fertility. J Anim Sci 46:1522, 1978.
11. Warrington, B.F., Byers, F.M., Schelling, G.T., et al.: Heifer and calf growth as affected by prepartum energy levels. J Anim Sci 61 (Suppl 1):458, 1985.
12. Corah, L.R., Dunn, T.G., Kaltenbach, CC.: Influence of prepartum nutrition on the reproductive performance of beef females and the performance of their progeny. J Anim Sci 41:819, 1975.
13. Young, B.A., Okamoto, M., Robinson, J.B., et al.: Cold weather calving: metabolic heat production and thermostability. 65th Annu Feeder's Day Rep Agric and For Bull (Special issue). Univ of Alberta, Alberta, 1986.
14. Carstens, G.E., Johnson, D.E., Holland, M.D., et al.: Effects of prepartum protein nutrition and birth weight on basal metabolism in bovine neonates. J Anim Sci 65:745-751, 1987.
15. Wiltbank, J.N., Warwick, E.F., Vernon, E.H., et al.: Factors affecting net calf crop in beef cattle. J Anim Sci 20:409, 1961.
16. Bellows, R.A., Short, R.E., Staigmiller, R.B,: Research areas in beef cattle reproduction. In H. Hawk (ed): Animal Reproduction. New York, Hasted Press, 1979, pp. 3-18.
17. Odde, K.G., Abernathy, L.A., Greathouse, G.A.: Effect of body condition and calving on calf vigor and calf serum immunoglobulin concentrations in two-year-old beef heifers. Col State U Beef Program Rep, p. 16, 1986.
18. Odde, K.G.: Survivial of the neonatal calf Vet Clinics of N America: Food Animal Practice 4:501-508, 1988.
19. Sheffy, B.E., Williams, A.J.: Nutrition and the immune response. JAVMA 180:1073-1076, 1982.
20. DeLong, W.J., Waltham, D.G., Hall, R.F.: Restricted dietary protein in pregnant beef cows. II. Effect on the immune response. Theriogenology 12:69-77, 1979.
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