Impact of hair coat color
What role does it play on body temperature?
By Adam McGee
Recently, the Midwest experienced record high daytime and nighttime temperatures. These record high temperatures can negatively affect animals not adept at dissipating excess body heat, such as cattle (Blaxter, 1962).
Several factors contribute to heat stress in cattle: amount of subcutaneous fat, production level, adaptation to the environment, coat thickness, and coat color (Wijffels et al., 2013).
In 2000, Hungerford et al. found cattle with a black hair coat had a 5.7% greater chance of death related to heat stress than cattle of other coat colors. Since the majority of the U.S. cow herd has a black hair coat and given the fact that dark colors absorb radiant energy, we designed a study to determine any differences in body temperature between cows with either a black or red coat color.
Materials and methodsThis study was conducted at the Journagan Ranch in in Southcentral MO on June 30 – July 14, 2022. Forty-Five bred heifers were sorted into two treatments based upon their coat color, red (26 hd) and black (13 hd). To minimize differences between treatments, all heifers were from the same spring calving herd and have therefore been under similar management and trait selection.
On day 1 of the study a blank CIDR with a temperature logging device (Burdick, 2012) was placed in each heifer and vaginal temperatures were recorded every 5 minutes for 24 hours and then averaged into one-hour increments. The data loggers remained in the animal recording temperatures for 14 days.
Initial body condition scores (9 = obese and 1 = emaciated; Richardson, 1986) were recorded at day 1. At the same time, hair coat scores were determined using the 5-point hair coat shedding score (5 = full winter coat and 1 = fully shed summer coat; Gray, 2000) by two trained scorers.
Heifers were then placed into a 23-ha fescue-dominated pasture where they remained for the duration of the 14-day study. The pasture had shade trees scattered throughout with about a 10% canopy coverage of the pasture.
To account for environmental impacts a weather station located on the ranch recorded temperature, relative humidity, wind speed and solar radiation. Cattle were minimally supplemented during the study to allow for management and care of the animals.
ResultsAverage wind speed was 2.5 mph, average relative humidity was 85%, average solar radiation was 24 MJ/m, and daytime temperatures (Figure 1) were between 38.3°C and 9.4°C during the 14-day study.
Body condition scores were similar regardless of coat color (P < 0.01) with an average of 6.1 and 5.8 for the heifers with black and red coat colors, respectively. Hair shedding scores were also not significantly different (P = 0.04) with an average score of 1.2 for heifers with a black coat and 1.5 for the heifers with red coat color.
Vaginal temperatures were statistically different for treatment (P < 0.01), hour (P < 0.01), and the interaction of treatment x hour (P < 0.01). When averaged across all days of the study, vaginal body temperatures were statistically warmer for cattle with a black hair coat for all time points except 2:00, 6:00, 8:00, 10:00, and 17:00 - 21:00 when the treatments were not statistically different from each other (P > 0.05; Figure 2).
DiscussionWhile vaginal temperatures were statistically different at most of the time points, it is important to note that the differences in temperature between the two treatments averaged only 0.08°C. Part of the reason for this small difference in vaginal temperature is likely due to the ability of the heifer to find ways physiologically (panting, increased respiration, etc.,) and behaviorally (more time in shade, pond, etc.,) to lessen the heat stress in an attempt to maintain a thermoneutral constant core body temperature.
During the study, the high temperature each day exceeded 25°C, above which respiration and panting rates increase (Gaughan et al. 2008), and each night the temperature dropped below 25°C giving the cattle a period of nighttime recovery. Nighttime cooling provides an opportunity for cattle to return to a normal respiration rate and reduces the chance that cattle will enter the following day with a carryover heat load further compounding their heat stress (Hahn and Mader, 1997).
Peak body temperature during this study lagged the peak ambient temperature by 2-3 hours. This is consistent with previous researchers who found body temperatures lagged the ambient temperature by 1—5 hours (Hahn et al., 1999; Mader, 2003; Brown-Brandl et al., 2005).
While hair coat may have a role in body temperature, not all data supports this theory. Parish et al (2017) also looked at the impact of coat color on body heat in cattle and found heifers with a red hair coat were warmer than the heifers with a black hair coat. This study is supported by similar results from Finch and Western (1977) and Peters et al. (1982).
However numerous studies align with our results indicating that cattle with black hair coats have a heat load during warm temperatures (Gaughan et al., 1998) and have lower performance and seek shade faster (Gaughan et al., 1998; Sullivan et al., 2011).
Based on the results of this study and differences found in other studies, coat color likely plays a role in heat stress but there are likely many other factors that have a greater impact than coat color alone.
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McGee is an assistant professor in the William H. Darr College of Agriculture at Missouri State University.
