en Español: La Economía del Estrés por Calor: Implicaciones para el Manejo
Introduction
Virtually the entire southern United States and areas closer to the equator are subject to extended periods of hot weather which stresses the lactating dairy cow. Cows have optimal temperature zones within which no additional energy above maintenance is expended to heat or cool the body (West, 2003). This thermo-neutral zone for dairy cattle is estimated to be from 32° to 68°F with an upper critical air temperature of approximately 77°F (West, 2003). Dairy animals become heat stressed when the effective temperature conditions exceed the animals’ zone of thermal comfort. Temperature-humidity indices (THI) are frequently used measures of heat stress in dairy cattle and are calculated using the ambient temperature and relative humidity. Heat stress in dairy cattle starts at a THI of 72, which corresponds to 72°F at 100% humidity, 77°F at 50% humidity, or 82°F at 20% humidity (Ravagnolo et al., 2000; Jordan, 2003). Lately, even lower thresholds for THI have been identified in high-producing dairy cows, such as 68°F (Collier et al., 2011). Heat stress reduces milk production, feed intake and reproduction and increases the risks of lameness and culling. Seasonal milk production is a problem when seasonal demand does not follow the seasonal supply.
This paper first reviews some estimates of the economic losses from heat stress effects on dairy cattle production. Secondly, some implications of heat stress on reproductive management are discussed.
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Economic Losses from Heat Stress
Much work has been done to identify the physiological effects of heat stress and the mechanisms by which dairy production is reduced (St-Pierre et al., 2003). The extent of production loss is often difficult to estimate; however, some estimates exist.
Based on a milk price of $13/cwt, St-Pierre et al. (2003) calculated annual losses of $897 million for the dairy industry in the United States due to heat stress even when current, economically optimal heat abatement systems were used. This loss is almost $100 per dairy cow per year. Without heat abatement systems, the total annual loss would be $1.5 billion, or about $167 per dairy cow per year, with approximately 9.2 million dairy cows in the U.S. Their analysis included effects of heat stress on dry matter intake, milk production, reproduction, culling, and death of both young stock and adult cows. In addition, they considered three heat abatement systems and chose the economically optimal system for each state: fans or forced ventilation, fans and sprinklers, or high-pressure evaporative cooling. Annual losses assuming minimal heat abatement systems in Arizona, California, Florida, Kentucky, New Mexico, Texas, and Wisconsin are shown in Table 1. Hours of heat stress during the day were calculated when the THI was greater than 70. The U.S. average for annual hours of heat stress is 14%. With only minimal heat abatement intensity (natural ventilation), economic losses would be large in areas with high temperatures and humidity that lasts for several months each year.
|
State |
Dry matter intake reduction (lb/cow/yr) |
Milk production loss (lb/cow/yr) |
Increase in average days open |
Increase in reproductive culling (%) |
Increase in deaths (%) |
Annual hours of heat stress (%) |
Economic loss ($/cow/yr) |
|
WI |
201 |
403 |
9 |
0.6 |
0.1 |
9 |
72 |
|
CA |
320 |
646 |
12 |
0.9 |
0.2 |
12 |
110 |
|
NM |
370 |
745 |
23 |
2.2 |
0.5 |
20 |
168 |
|
AZ |
798 |
1609 |
26 |
2.5 |
0.5 |
22 |
256 |
|
KY |
1100 |
1779 |
27 |
2.7 |
0.6 |
21 |
285 |
|
FL |
1971 |
3975 |
59 |
8.0 |
1.7 |
49 |
676 |
|
TX |
2196 |
4425 |
54 |
7.4 |
1.6 |
36 |
698 |
The amount of heat stress within a location depends on the season and the type of heat abatement system (St-Pierre et al., 2003). Table 2 shows milk production and economic losses per month for naturally ventilated dairy barns (minimal heat abatement) and barns that use fans and sprinklers (high heat abatement) for temperatures and humidity recorded in Orlando, Florida. Economic losses were calculated from the costs of reduced DMI, reduced milk sales, and increased days open. Clearly, heat stress is most severe in June through August. Under natural ventilation, dairy cows in Orlando would be under heat stress 51% of the time and lose $687 per cow per year due to heat stress. Using fans and sprinklers, heat stress would be reduced to 19% of the time, and losses would be reduced to $125 per cow per year. The cost of the heat abatement system is not included in these calculations. However, Flamenbaum (2010) has assumed a cost of $36 per cow per year for an intensive heat abatement system. In such a scenario, the return on $1 investment per year would be $16. Obviously, the effectiveness and cost of heat abatement systems vary from farm to farm and over time.
|
|
Natural ventilation |
Fans and sprinklers |
||||
|
Month
|
Milk production loss (lb/cow) |
Monthly hours of heat stress (%) |
Economic loss ($/cow) |
Milk production loss (lb/cow) |
Monthly hours of heat stress (%) |
Economic loss ($/cow) |
|
Jan |
0 |
0 |
0 |
0 |
0 |
0 |
|
Feb |
0 |
0 |
0 |
0 |
0 |
0 |
|
Mar |
7 |
25 |
4 |
0 |
0 |
0 |
|
Apr |
75 |
40 |
22 |
0 |
0 |
0 |
|
May |
209 |
58 |
49 |
5 |
23 |
4 |
|
Jun |
717 |
100 |
126 |
84 |
44 |
25 |
|
Jul |
851 |
100 |
159 |
152 |
54 |
39 |
|
Aug |
875 |
100 |
162 |
162 |
55 |
41 |
|
Sep |
602 |
100 |
116 |
49 |
42 |
17 |
|
Oct |
161 |
56 |
41 |
0 |
13 |
0 |
|
Nov |
13 |
29 |
7 |
0 |
0 |
0 |
|
Dec |
0 |
0 |
0 |
0 |
0 |
0 |
|
Total/year |
3510 |
51 |
687 |
454 |
19 |
125 |
1 Calculated based on formulas in St-Pierre et al. (2003). Economic losses calculated from the costs of reduced DMI, reduced milk sales, and increased days open. Cost of heat abatement system not included. Temperature and humidity data from http://countrystudies.us/united-states/weather and http://www.cityrating.com/relativehumidity.asp.
Impact of Heat Stress on Milk Production, Fertility, and Survival
In addition to cooling cows, dairy producers might alter their reproductive program to cope with the negative effects of heat stress. Reproduction suffers more from heat stress than milk production. Using typical data for Florida, De Vries (2004) assumed a loss in milk production of at most 15% and a reduction in conception rate of at most 53% in the summer compared to the data for February. Data from Israel showed a decrease in the summer of 7% for milk production and 51% in conception rate in herds producing at an average rate (Flamenbaum and Galon, 2010). Estrous detection efficiency may be reduced as a result of heat stress (Jordan, 2003).
To evaluate the economics of reproductive policies to cope with heat stress, it is necessary to have quantitative associations between animal performance and heat stress. St-Pierre et al. (2003) developed equations that quantify the association between THI measures and milk production, dry matter intake, reproduction, and survival based on an extensive review of the literature. Some of these equations are in a format not suitable for the computer programs we have to analyze heat stress effects. Therefore, other published relationships between heat stress and loss in milk production, reductions in conception rates, and increases in culling were used. However, there is a dearth of good quantitative relationships between heat stress (as measured by THI) and cow performance, which makes the results from economic evaluations less accurate.
Improving Fertility in the Summer
The large decrease in reproduction due to heat stress has motivated much research into ways to increase pregnancy rates in the summer (Hansen and Aréchiga, 1999; Jordan, 2003). Embryo transfer can significantly improve pregnancy rates during the summer months by bypassing the period in which the embryo is more susceptible to heat stress. In a recent study, lactating dairy cows were bred using in vitro-produced embryos with sexed semen that were either frozen and thawed or remained fresh and were transferred after a timed embryo transfer program into lactating dairy cows during summer or were bred with conventional AI semen (Stewart et al., 2011). Conception rates were doubled with fresh embryos compared to AI.
Using mathematical programming and assuming a doubling of the conception rate and a $60 cost per embryo transfer, calculations showed that annual profit/cow increased by $5 when the maximum total number of cows (dry + milking) was constant in each month. When the total number of milking cows was constant in every month, profitability increased by $14 per cow per year from embryo transfer over conventional AI. These benefits arise from increased fertility in the summer and more heifer calves, but the benefit is reduced by more cows calving in the late spring, which is not advantageous for cows.
Earlier, De Vries et al. (2011) evaluated the economic potential of embryo transfer in the summer with a different program (v.Dairyplan) and slightly different effects of heat stress on milk production, culling, and fertility. The amount of heat stress was different as well. In this analysis, using in vitro-produced embryos with sexed sorted semen with a cost of $60 for an embryo with transfer and associated hormones for timed embryo transfer compared to $20 for a conventional AI improved profit/cow per year $22 to $42, depending on herd constraints. When the number of milking slots was fixed throughout the year, improvement of fertility in the summer was again more valuable than when the number of total cows was fixed throughout the year. These results show the importance of the limiting factor (total cows versus milking cows) on the value of reducing heat stress in the summer.
Bell et al. (2009) studied the effects of bST to improve first insemination conception rates and found that improvements in the cooler times of the year were more valuable than improvements during heat stress. The constraint was a fixed total number of cows throughout the year. Other constraints, such as fixed number of milking cows throughout the year or a fixed average number of cows per year, will alter these results.
The varying results show the difficulty in providing general estimates of the value of improvements of fertility in heat-stressed dairy cows. A major limitation is the lack of quantitative relationships between heat stress and cow performance.
Delayed Insemination
Another reproductive management implication of heat stress is not to inseminate certain cows during certain times of the year. DeJarnette et al. (2007) surveyed dairy producers who participated in a progeny test program on their voluntary waiting periods (VWP) for first insemination. These producers were primarily located in the western part of the United States. Eighteen percent of the responding dairy producers varied the VWP based on season, with the major motivation to avoid calving in cold weather. In such herds, heifers and cows were not inseminated in the spring.
In Florida and other locations where summer heat stress is severe, many dairy producers without adequate cooling facilities will not inseminate cows in the summer and the early part of the fall. The two main reasons are decreased conception rates, and therefore wasted expenses and effort, and the desire not to have calvings in the late spring and summer. Cows calving in the late spring will have most of their peak milk production in the summer, which will be depressed from heat stress. Cows calving in the summer, and their calves, are more at risk for death and involuntary culling. Therefore, the value of improving conception rates in the late summer is compromised by the negative effects of calving in the late spring. A consequence of this breeding philosophy is that such herds are more seasonal, with more calving and milk production during the cooler season and consequently larger swings in cash flows.
The DairyVIP program includes the option to evaluate delayed insemination. The program can automatically calculate whether and how long not inseminating certain cows is profitable. Using the inputs with the climatic data for Dallas, Texas, annual profit/cow without the option to delay insemination was $357. When delayed insemination was allowed, annual profit/cow increased by $16 to $373. Most of the inseminations in July, August, and September would be delayed until the cooler time of the year.
Optimal Insemination Mix
These embryo transfer results assumed that all cows were treated equally in the summer and the other seasons. But it is hypothesized that the optimal insemination decisions vary between cows. Some scenarios are shown here.
The v.Dairyplan program was used to evaluate when and for which animals increased fertility would be advantageous. Three types of inseminations were considered which varied by fertility and price:
- 100% of the conventional conception rates (which decreased with parity and insemination number) at $20 per insemination,
- 125% of the conventional conception rates at $40 per insemination, and
- 150% of the conventional conception rates at $60 per insemination.
These costs are proxies for any technology that increases the conception rates – for example, estrous synchronization – and are not necessarily the cost of semen. The last scenario is a proxy for embryo transfer. Thus, increased fertility was assumed to come at a higher cost.
Profitability remained very similar ($350 per cow slot per year) when $40 inseminations were allowed only in one season of the year in both heifers and cows and only $20 inseminations in the remainder of the year. Improving fertility in the winter was not found to be the most advantageous season as described by Bell et al. (2007), but neither was there a relative advantage of improving fertility in the summer. In these four scenarios, delayed inseminations were not allowed, so eligible heifers and cows were always inseminated.
The use of inseminations with higher cost and fertility year-round, combined with the option to delay inseminations, increased milk yield and profitability (Table 3). Milk yield and profitability were less when the number of milking cows was constrained. This constraint prevents lack of parlor capacity to milk all lactating cows in the winter and spring in a seasonal herd. The $60 inseminations (150% relative conception rate) increased profitability by about $25 per cow slot per year in both cases.
|
|
Herd constraint: |
Added herd constraint: |
||
|
Insemination cost and relative conception rate2 |
Milk yield (lb/cow slot/year) |
Profit ($/cow slot/year) |
Milk yield (lb/cow slot/year) |
Profit ($/cow slot/year) |
|
$20, 100% |
20,831 |
354 |
20,820 |
341 |
|
$40, 125% |
21,008 |
371 |
20,940 |
360 |
|
$60, 150% |
21,138 |
378 |
21,052 |
368 |
|
Optimal mix3 |
21,116 |
390 |
20,981 |
378 |
1 At most, 90% of the cow slots contain a milking cow, in addition to a constraint on the available number of cow slots per week. This herd constraint prevents too many milking cows in the winter or spring, which could be the result of summer heat stress effects on milk yield and fertility.
2 Relative conception rate multiplies the default conception rates which decrease with parity and insemination number. The optimal mix consists of the optimal insemination type per week of the year, parity, and insemination number.
3 Optimal mix of $20, $40, and $60 inseminations.
Figure 1. Seasonality of milk production, animals present, and the optimal mix of three types of inseminations for the average seasonality scenario with a constraint on available cow slots per week.
Figure 1 gives some insight into the optimal insemination mix throughout the year where the number of cows milking is allowed to float for the most economical combination. Observe that the cow slots (milking + dry) are always constant, but the fraction of milking cows is seasonal with a high of 95% in the spring and a low of 75% in the late summer. The optimal insemination mix includes all three types of inseminations for heifers and only $20 and $60 inseminations for cows, although the $40 inseminations are used scarcely. In heifers, the $60 inseminations (with highest fertility) are mostly chosen in the summer and the $20 inseminations in the winter. But among cows, the optimal insemination mix is different. In cows, the $60 inseminations are primarily chosen in the winter and to a lesser extent in the summer. As a result, there is heavy calving in the late fall and very little in the summer. Heifers and first-lactation cows use relatively more $60 inseminations than older cows. Within lactations, more expensive inseminations are used more around the fifth and sixth insemination opportunity than earlier or later. This pattern agrees with the value of a new pregnancy which peaks at that time for lactating cows (De Vries, 2006). Early in the lactation, cows have more remaining opportunities to get pregnant before voluntary culling, and hence not getting pregnant is not as expensive. Later in the lactation, the value of getting pregnant is reduced by the long period with low milk production before calving. However, heifers are more profitable inseminated with $60 inseminations after the third insemination opportunity.
The optimal insemination mix changes again when an additional constraint is added that at most 90% of cow slots contain a milking cow (Figure 2). This constraint relates to the maximum parlor capacity in the spring. With this constraint, there is still slack in the parlor in the late summer, which means that the parlor is not used to capacity. In heifers, the $60 inseminations are also used in the spring compared to the situation in Figure 1 where no constraints were used. For cows, there are now two peak times during the year (winter and summer) when $60 inseminations are mostly used.
Therefore, if a more even milking pattern is desired throughout the year, improved fertility in the summer is greatly advantageous. Use within and across lactation numbers is similar to the situation without the parlor constraint. These optimal insemination mixes add approximately $36 per cow slot per year to the herd’s profitability. They are also approximately $11 per cow slot per year more valuable than the best situation where only one type of insemination ($60) is used.
Figure 2. Seasonality of milk production, animals present, and the optimal mix of three types of inseminations for the average seasonality scenario with constraints on available cow slots per week and at most 90% of the cow slots contain a milking cow.
When lack of milk production in the summer is a concern, price incentives could be applied that encourage producers to produce more milk in that season. The most important permanent strategy would be to use facilities that cool cows. In addition, an optimal insemination and replacement policy could be used that results in more calvings in the spring and hence more milk production during the summer. A price incentive plan was employed in Florida from 1993 through 1995, but it was withdrawn because it lacked full participation by the cooperatives’ membership (Washington et al. 2002). Dairy producers who did participate changed the distribution of heifers calving, voluntary cow culling, and average days to first insemination to reduce seasonality in their milk production. Price incentives were not evaluated with the spreadsheet for this paper.
Seasonality and Sexed Semen
Commercialization of sexed semen took off in 2006 and has led to increases in the number of heifer calves born (De Vries, 2010a; Norman et al., 2010). Sexed semen is typically more expensive than conventional semen, and conception rates are approximately reduced to 80% of the conventional conception rates. Standard sexed semen results in 90% heifer calves. Female sexed embryos for embryo transfer can be produced fairly cheaply, and conception rates were found to be double those of conventional AI when used in the summer in Texas (Stewart et al., 2010). Hence, transfer of sexed female embryos is an excellent way of getting cows pregnant in the summer. A scenario was analyzed with the option of using $60 female sexed embryos with 150% relative conception rate in cows during the summer and $20 inseminations (conventional) year-round in the average seasonal situation. Initial results suggest that profitability was $365 per cow slot per year, an increase of $11 over the scenario where only $20 inseminations were available. When the mathematical model was used, the optimal use of sexed embryos varied from 81% to 96% of all impregnations in the summer, with exceptions in older cows (Figure 3). Seasonality of milk production was reduced. Further study must reveal if embryo transfer in heifers and other seasons might improve profitability even more.
Figure 3. Seasonality of milk production, animals present, and the optimal mix of conventional inseminations and female sexed embryos for the average seasonality scenario with a constraint on available cow slots per week.
Options that result in more heifer calves than are needed to replace culled cows has led to an interest in the use of beef semen for a percentage of the cows to create crossbred calves for beef production. The optimal insemination mix then might consist of a combination of female sexed dairy semen, conventional dairy semen, and (conventional) beef semen. Some initial results for a non-seasonal herd indicate use of sexed semen mostly in heifers and high-producing younger cows and beef semen in older cows with lower milk production (De Vries, 2010b). To evaluate the effect of seasonality on the optimal insemination mix, we assumed a cost of $20 for conventional dairy and beef semen, both with 100% relative conception rates. Female sexed semen cost $40 with 80% relative conception rates. Crossbred calves were sold for $150, dairy bull calves for $50, and excess dairy heifer calves for $200. This scenario resulted in a profit of $362 per cow slot per year. The seasonal pattern of this optimal insemination mix is shown in Figure 4. In heifers, sexed dairy semen was used in the fall, winter, and spring but not in the summer. The highest use was 77% sexed semen and 23% conventional dairy semen during the winter with sexed semen being used in first and second insemination opportunities. In cows, sexed semen was used only in the summer, but only up to 12% of all inseminations. Conventional dairy semen was used year-round but with an increase in the summer. Beef semen was used in the fall, winter, and spring but hardly in the summer. During the summer, conventional dairy semen was used the most in both heifers and cows.
Figure 4. Seasonality of milk production, animals present, and the optimal mix of conventional and female sexed dairy semen and conventional beef semen for the average seasonality scenario with a constraint on available cow slots per week.
Delayed Replacement
In addition to a modified insemination policy as a result of heat stress, the cow replacement policy should be considered as well. De Vries (2004) studied the economics of delayed replacement of cows in the summer. The idea was that cows that were culled in the summer might be replaced with heifers purchased in the fall. Heifers that calve in the fall reach higher peaks and are easier to get pregnant in the cooler part of the year. A consequence of this policy would be empty slots in the summer. A computer model that optimized replacement decisions based on dynamic programming was developed (DairyVIP). The results showed that in most realistic cases of heat stress and prices, immediate replacement was optimal. Only if seasonality was rather severe, and fixed cost low compared to variable cost, would delayed replacement be economically advantageous. Regardless of delayed replacement, springing heifers would be primarily purchased to calve in the fall. Twenty years ago, Delorenzo et al. (1992) already showed that due to summer heat stress, heifers should primarily be purchased in the fall. This is a common practice among Florida dairy producers. In closed herds, where calves are kept and raised to become calving heifers, the optimal replacement policy depends on the availability of heifers. The economics of these constraints are difficult to calculate and not published in the literature yet, but work is under way to provide better insights.
Software to Evaluate the Economic Consequences of Heat Stress
The DairyVIP program is available for download from the Florida Dairy Extension website at http://dairy.ifas.ufl.edu/tools. The program is continuously updated as part of NIFA-AFRI grant 2010-85122-20623. A user-friendly interface has been developed that allows for easy comparison of two scenarios side by side.
Conclusions
Dairy producers have several reproductive management options to deal with heat stress. Options include delayed inseminations, delayed replacement of culled cows, and improved fertility through hormonal manipulations and embryo transfer. The value of these options depends not only on the effects of heat stress on fertility but also on the effects on milk production and risks of death and involuntary culling, as well as herd constraints. The best policy is cow dependent, and tools are being developed to assist dairy producers with the most economical reproductive management options. Non-reproductive management options, such as cooling of cows, need to be considered as well.
Author Information
Albert De Vries, Ph.D.
Department of Animal Sciences, University of Florida
Gainesville, Florida, USA
Acknowledgments
The Dairy Heat Stress Roadshow and proceedings were supported by Agriculture and Food Research Initiative Competitive Grant No. 2010-85122-20623 “Improving Fertility During Heat Stress in Lactating Dairy Cows” from the USDA National Institute of Food and Agriculture.
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