Case Study: The Effects of Severe Winter Weather on Net Energy for Maintenance Required by Yearling Steers

INTRODUCTION

Maintenance requirement for energy has been defined as the amount of feed energy intake that will result in no net loss or gain of energy from the tissues of the animal body (NRC, 2000). Body temperature regulation, essential metabolic processes, and physical activity all contribute to maintenance energy requirements. Lofgreen and Garrett (1968) and Garrett (1980) described the NE^sub m^ requirement using the expression NE^sub m^ = 0.077 Mcal/EBW^sup 0.75^, where EBW is average empty body weight in kilograms. NRC (2000) calculates NE^sub m^ using 0.077 Mcal/SBW^sup 0.75^, where SBW is average shrunk body weight in kilograms.

Maintenance energy requirements vary with many factors including BW, breed, sex, age, season, temperature, physiological state, and previous plane of nutrition (NRC, 2000). Adjustments to NE^sub m^ for metabolic acclimatization of 0.0007 Mcal/°C that average monthly temperature is above or below 20°C are recommended by NRC (1981). Additional adjustments to NE^sub m^ for cold stress are recommended by NRC (1981) if cattle are exposed to temperatures below their lower critical temperature (LCT). Increases in maintenance energy are a function of surface area, LCT, and total insulation. Total insulation is a function of tissue insulation (hide thickness and subcutaneous fat) and external insulation (hair coat plus layer of air surrounding the body). Mud, wind, and precipitation influence the effectiveness of external insulation.

Severe winter storms in southeast Colorado in December of 2006 and January 2007 resulted in severe economic losses for the cattle feeding industry. Several research trials were lost at the Southeast Colorado Research Center (SECRC). Data from one of the studies were utilized to provide an assessment of the effect of severe winter weather on NE^sub m^ requirements. Therefore, the retrospective objective of this study was to examine NE^sub m^ required for maintenance during and in the aftermath of severe winter weather.

MATERIALS AND METHODS

Prior to the initiation of the original experiment, care, handling, and sampling of the animals defined in this research project were approved by the Colorado State University Animal Care and Use Committee.

Because of severe winter weather in December 2006, several research studies at secRC were terminated before their conclusion. Data from 214 yearling steers (557.8 kg ± 21.7) that were originally part of one of these terminated studies were utilized for this evaluation of NE^sub m^. Steers were of various Continental × British breed crosses, had been on feed for 95 d at the start of this evaluation, and based on visual appraisal, needed approximately 6 additional weeks on feed to achieve an acceptable slaughter endpoint. Steers were individually weighed on December 26, 2006, and on February 22, 2007. A 4% pencil shrink was applied to all weights before data analysis. Pen feed delivery records were not maintained for all days during the study period. However, for accounting purposes, total feed delivery records for this set of steers were maintained. Total DM delivered to the steers and total head days for the entire group was calculated. Dry matter intake was estimated by dividing DM delivered by total head days.

Empty body weight, empty body gain (EBG), retained energy (RE), and NE^sub m^ and NE^sub g^ required were calculated using equations described by NRC (2000). Diets fed to the steers during the trial period were typical winter finishing diets with steam flaked corn, corn silage, ground alfalfa hay, yellow grease, condensed corn distillers solubles, soybean meal, and supplement. Crude protein concentration was approximately 13.5%, and NDF supplied by the roughage in the diet was 6%. Average DM concentration was 70%, The ME, NE^sub m^, and NE^sub g^ concentrations in the diet (3.279, 2.263, and 1.573 Mcal/kg DM, respectively) were calculated from tabular values (NRC, 2000). The efficiency of ME use for maintenance (k^sub m^) was assumed to equal 0.69.

Daily high and low temperature and daily precipitation records for December 2006 through February 2007 for the Lamar Colorado recording station were obtained from the National Weather Service (NOAA, 2008).

RESULTS AND DISCUSSION

Figure 1 displays the daily high and low temperature obtained from the Lamar Colorado Reporting Station of the National Weather Service for December 2006 through February 2007. The Lamar weather station is located approximately 1.6 km southeast of SECRC. Average high and low temperature observed from December 26, 2006, through February 22, 2007 was -2.16°C ± 0.20 and -14.69°C ± 0.24, respectively. Average temperature that the steers were exposed to during the feeding period was calculated as the average of the daily high and low temperatures and averaged -8.43°C.

Figure 2 shows the snowfall and snowpack records for December 2006 through February 2007. Snowfalls of 25.4 and 5.1 cm were recorded on December 20 and 21, respectively. An additional 25.4, 30.5, and 30.5 cm of snow fell on December 29, 30, and 31, respectively. Additional snow events occurred on January 13 and 14, January 21, and February 14 and 15, 2007. The average snow depth peaked at 91.4 cm on December 31 and averaged 32.3 cm ± 0.26 from December 26, 2006, through February 22, 2007. This snowpack contributed to extremely muddy pen conditions through the feeding period. By the end of the study, mud depth in the pens ranged from dew claw deep in some areas to nearly belly deep through much of the pen. There were no dry areas for the steers to lie down.

Death loss was 7% (15 of 214 steers) from December 26, 2006, through February 22, 2007. Steers weighed 557.8 kg ± 21.7 on December 26, 2006, and 550.3 kg ± 18.7 on February 22, 2007. Average shrunk weight was 554.1 kg. Average daily gain was -0.13 kg ± 0.21 over the 58-d feeding period. Steers were slaughtered at a commercial facility in Dumas, Texas on February 23, 2007. Average hot carcass weight, dressing percentage, 12th rib fat depth, percentage USDA low Choice and above, and percentage USDA YG 1 and 2 was 335.22 kg ± 4.45, 60.70% ± 0.45, 0.86 cm ± 0.05, 27.52%, and 82.18%, respectively.

Average EBW (497.74 kg) was calculated using Equation 3-4 in NRC (2000): EBW = 0.88 × SBW + 14.6 × NE^sub m^ - 22.9. Average EBG (-0.00686 kg/d) was calculated using Equation 3-5 in the NRC (2000): EBG = 0.93 × SBG + 0.174 × NE^sub m^ -0.28.

The energy content of empty body loss and gain are similar (NRC, 2000). Therefore, RE (-0.0283 Mcal/d) was calculated using Equation 3-1 in NRC (2000): RE = 0.0635 × EBW^sub 0.75^ × EBG^sup 1.097^, where EBG = absolute value of empty body gain or loss.

Total DM delivered to the group of steers was 53,436.7 kg. Total head days for the 58-d feeding period were 12,176. Therefore, average daily DMI was 9.67 kg/steer. Average NE^sub m^ concentration was 2.263 Meal/kg DM. Thus, 21.8893 Meal of NE^sub m^ were derived from the diet for the average steer.

Maintenance energy requirements under thermal neutral conditions may be described as 0.077 Meal × SBW^sup 0.75^ (NRC, 2000). NRC (1981) recommends adjusting the coefficient of this equation (0.077) by 0.0007 for each degree centigrade difference from 20°C that the cattle are acclimatized to the environmental conditions. Acclimatization to the environmental conditions is defined as one month exposure (NRC, 1981). Average temperature for December 2006 was approximately -1.9°C. The maintenance coefficient that describes NE^sub m^ required by these steers needs to be increased by 0.01533 Meal (21.9 × 0.0007). Therefore, the equation recommended by NRC (1981) describing NE^sub m^ required by steers acclimated to -1.9°C, assuming no acute thermal stress, is 0.09233 × SBW^sup 0.75^. Birkelo et al. (1991) demonstrated no increase in fasting heat production was associated with season. These authors concluded that acute cold stress was responsible for poorer performance typically observed during cold months of the year.

The observed coefficient (0.1919) is 2.5 times greater than the coefficient (0.077) suggested by NRC (2000) under thermal neutral conditions and 2.1 times greater than the coefficient adjusted for thermal acclimatization (0.09233) suggested by NRC (1981). These data suggest that steers fed during the winter of 2006-2007 were subjected to severe environmental stress. Additional adjustments to NE^sub m^ for cold stress are recommended by NRC (1981) if cattle are exposed to temperatures below their LCT. Increases in maintenance energy are a function of surface area, LCT, and total insulation. Total insulation is a function of tissue insulation (hide thickness and subcutaneous fat) and external insulation (hair coat plus layer of air surrounding the body). Mud, wind, and precipitation impact the effectiveness of external insulation.

The increase in NE in an environment (NE^sub mi^) that is colder than the animal's LCT is described by the formula NE^sub mi^ = 0.69 × [A × (LCT - EAT/I], where 0.69 = k^sub m^ or the efficiency of converting diet ME into NE^sub m^, A = surface area (0.09 × SBW^sup 0.67^), EAT = effective ambient temperature, and I = total insulation. In this analysis, NE^sub m^ increase was approximately 11.373 Mcal. By solving the above equation for LCT and making some assumptions concerning total insulation, a value for LCT can be calculated that would result in a NE increase of 11.373 Meal: LCT = [(I × (NE^sub ml^/0.69)/A] + EAT.

Total insulation is the sum of tissue insulation and external insulation (EI). For yearling steers in a feedlot, tissue insulation ranges from 5.5 to 8.0°C/Mcal per m^sup 2^ per day (NRC, 1981) and depends upon subcutaneous fat and skin thickness. The steers in this evaluation averaged 554.1 kg and averaged 0.86 cm subcutaneous fat at slaughter on February 23. In addition, all steers were of Bos taurus breeding. They likely did not have thin skin thickness characteristic of Bos indicus breed types. Thus, the steers in this evaluation likely possessed some degree of tissue insulation. External insulation is a function of wind speed and hair coat depth. With wind speed listed at 12.8 km/h, NRC (1981) suggests that for cattle with 20 mm of hair coat, EI is 8°C/Mcal per m^sup 2^ per day. The presence of moisture, wet snow, or mud in the hair coat can reduce EI by 50 to 80% (NRC, 1981). NRC (2000) reduces the value of EI by 80% if the cattle are covered with wet snow and mud.

The EAT that cattle are exposed to depends upon several factors including thermal radiation, humidity, wing speed, contact surfaces, and precipitation (NRC, 1981). The impact of thermal radiation is the difference between combined solar and long wave radiation received and the long wave radiation emitted by the animal. Exposure to sunlight causes a gain in heat and effectively increases EAT 3 to 5°C (NRC, 1981). Conversely, the rate of heat loss on cold clear nights reduces EAT 5°C within the temperature range of -10 to 10°C (NRC, 2000). Lying on the cold and frozen or wet and muddy pen surface increases the rate of conductive heat flow from cattle.

Birkelo and Johnson (1993) used a simulation model to evaluate energy exchange through conduction, convection, long and short wave radiation, and losses associated with respiratory and cutaneous evaporation. Results indicated a 20.9% increase in apparent NE^sub m^ associated with long wave radiation into the cold, clear, night sky in winter, and short wave heat absorption during the day was of little value in offsetting this phenomenon. This effect of long wave radiation at night may have had a major impact on the steers in the current study. Only 4 snowfall events occurred during the 58-d study period (Figure 2). The vast majority of the nights were very clear, dry, and cold.

Table 1 shows the effect of EAT and I on the LCT needed to result in a NE^sub mi^ of 11.373 Mcal. If one assumes an I value of 9°C/Mcal per m^sup 2^ per day and an EAT of -8°C, LCT would need to be 15.9°C to result in a NE^sub m^ increase of 11.373 Mcal. If I was 1°C/Mcal per m^sup 2^ per day and EAT was -12°C, LCT would need to be -9.3°C to increase NE^sub m^ by 11.373 Mcal. Cattle are generally very tolerant of cold environmental conditions. This is due to the large amount of heat produced during digestion of the feed they consume. Lower critical temperatures as low as -36°C have been reported for feedlot cattle (Webster, 1974).

The ability of cattle to tolerate cold requires that they remain well insulated from the environment. Maintaining effective insulation requires protection from the wind, maintenance of a dry hair coat, and protection from cold and frozen or wet and muddy pen conditions. Snow covered the ground during the entire 58-d feeding period. This snowpack resulted in extremely muddy pen conditions. The high LCT values presented in Table 1 demonstrate that the destruction of the insulation value of the hair coat and the amount of heat loss from cattle in the aftermath of a catastrophic blizzard is tremendous.

Lower critical temperature is also described in NRC (2000) by the formula: LCT = 39 - I × (HE/A - He), where I = total insulation, HE = heat production, A = surface area, and He = heat of evaporation. Heat production is defined in NRC (2000) by the equation: HE = ME - RE. NRC (2000) recommended calculating He using the equation: He = HE/A × 0.15.

Metabolizable energy for maintenance concentration in the diet was calculated from tabular vales (NRC, 2000) and was 3.279 Mcal/kg DM. Total ME intake was 31.72 Mcal. Retained energy was -0.0283 Mcal/d. Therefore, daily HE was 31.75 Mcal and He was 0.77 Mcal/m^sup 2^.

Lower critical temperature calculated using the above equation is 17.24, -0.16, and -17.57°C assuming total insulation of 5, 9, or 13°C/Mcal per m^sup 2^ per day, respectively. These values indicate that the assumed I value for the cattle greatly influences the estimate of LCT. In addition, cattle with satisfactory amounts of I can withstand very cold temperatures. Conversely, as the value for I is reduced, LCT increases, subjecting cattle to cold stress.

Cattle feeders in the high plains are reluctant to provide bedding and windbreaks for cattle. Mader et al. (1999) concluded that although windbreaks effectively alleviated the negative impact of wind on winter performance, air flow in the summer was compromised and performance was reduced in pens with windbreaks during summer months. Therefore, windbreaks, unless they were portable, will likely not become common in the hot climates of the high plains.

Providing bedding has been shown to effectively combat cold stress in northern climates (Birkelo and Lounsbery, 1992). High plains feeders are reluctant to use bedding because of the cost of removing bedding plus manure from the pens. They also believe that bedding will hold moisture in the pen and delay drying of the pen surface. Providing bedding as a routine management strategy in the high plains will likely not become common. However, the economics of providing bedding in the aftermath of a catastrophic winter storm should be evaluated.

Total cost per head associated with the poor performance observed in this study includes feed, interest, yardage, and death loss costs over the 58-d study period. These costs are shown in Table 2. Several assumptions were made to calculate these costs. Yardage rate was assumed to equal $0.35/d per head. Ration costs were assumed to be $5.16, $6.33, $7.50, $8.68, or $9.85/45.45 kg as-fed ration based on corn costing $2.50, $3.50, $4.50, $5.50, or $6.50/25.41 kg corn, respectively, at 15% moisture. Ration DM concentration was 70%. Initial pay-weight of the steers was 403.8 kg, and initial feeder cattle price was assumed to be $80, $100, $120, or $140/45.45 kg. Shrink for the steers was 4% upon arrival. Interest rate of 8% was applied only to the initial cost of the steers. Steers were on feed 95 d before the study period and feedto-gain ratio before the study period was 5.34 kg of DM/kg of gain. Processing and medicine costs before the study started were $20.00 per head. Death loss costs were 7% of the steer value at the start of the study period. Steer value at the start of the study period included initial steer cost plus processing and medicine, yardage, interest, and feed costs before the study period. These calculations show that economic losses associated with a catastrophic winter weather event may range from $185 per head when feed and feeder cattle are inexpensive to over $323 per head when feed and feeder cattle are expensive.

IMPLICATIONS

These data show that performance by cattle in the aftermath of a severe winter storm event may be severely impaired leading to catastrophic economic losses. A significant part of the poor performance may likely be attributed to an increase in maintenance energy requirements. Calculations in this study suggest that a reduction in the insulation value of the hair coat may explain the increase in maintenance requirements. Bedding is commonly used in northern climates to protect cattle from the adverse effects of cold weather. Bedding is rarely used in the high plains cattle feeding region because of the cost of application and removal from pens and because of the perception that bedding will retain moisture and delay drying of the pen surface. The logistics and economics of providing bedding to cattle in the aftermath of a catastrophic winter storm in the high plains cattle feeding region needs to be evaluated.

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