When you work in the livestock industry it's fairly common to hear at least a few stories each summer about someone who suffered fairly extensive animal or production losses due to excessive heat or heat stress. Last year in Northeast Texas one cattleman lost about 12 head of mature, purebred brood cows in the course of a couple of days late last July to heat stress. Even more extreme, in July 1995, approximately 3,750 head of feedlot cattle in a yard in Western Iowa died over a 24 hour period due to the heat and humidity. We normally think of heat effects as isolated to the southern United States. Unfortunately, that's not true. We also tend to think that the heat affects only dairy or feedlot cattle. That is not true either. The animal and production loses represent millions of dollars not to mention the time and effort of dealing with the situation. This is true in all phases of the beef industry.
Heat Effects on Cattle
The thermocomfort zone for cattle varies depending on a large group of factors including body condition, hair coat length, plane of nutrition, health, breed, age and acclimation. In general, cattle do not handle heat as well as humans. In the midst of a typical summer, cattle are generally less comfortable than humans at the same environmental temperature. What that means is that producers need to consider the fact that their cattle are probably hot even when they themselves are not.
In the initial or early stages, when cattle start to suffer from heat, the early signs are not always apparent. Feed and roughage intake may drop a little but the animal may be fairly uncomfortable way before that. As cattle heat up and feed intake drops, cattle begin using additional energy in order to help keep themselves cool, therefore, heat stress reduces production and efficiency. Once this performance level drops it becomes very difficult to get it back. This is especially true in growing and feedlot cattle. Some of this loss is carried all the way through to the packinghouse. In many cases with growing and feedyard cattle the losses can equal 10 percent or more.
In breeding cattle, we see a similar response in terms of nutrient or feed intake and energy metabolism in an effort to stay cool. Often, this results in reduced breeding activity, reduced cycling and lower conception rates. A confounding factor in this scenario is that at a time when cows are hot and not grazing as heavily the forage quality has also deteriorated so that the roughage or pasture that is consumed is lower in nutrients as well as less digestible. This makes a pretty strong case for summer supplementation programs.
Another area of concern is immune response. This is especially critical in newly weaned cattle that are to be preconditioned and
backgrounded. It becomes even more of a concern if these cattle are transported, especially if they have been run through a sale facility of some type and co-mingled with other cattle. Although the exact relationship between heat stress and immune function is unclear, we do know that since much of the animal's maintenance energy is being used to cool itself, the requirements of the immune system may go unmet. Additionally we also know that under different types of stress, the body tends to deplete itself of critical nutrients such as zinc and copper that are vital to immune response. Also, it has been determined that under stress, the adrenal gland will secrete steroid mimicking hormones that will essentially turn off the immune system. Yet another factor is that respiration rate (panting) is accelerated which increases the susceptibility to respiratory disease, especially if conditions are somewhat dusty. Regardless, in many cases heat stress is only one stress component in the overall physiological challenges these cattle encounter.
It has been determined that three elements are critical in hot weather situations: intensity of heat, duration and the opportunity to cool down at night. The heat intensity means that the combination of heat and humidity create a seriously debilitating or killing situation. If you add an inch or more of rain just prior to an intense heat the humidity level is increased significantly. This elevates heat losses substantially. To help monitor this problem it helps to keep handy or even post a Temperature-Humidity Index
(THI) Chart and monitor it regularly in the summer months. Such as chart is found in Figure 1. When the THI reaches 84 or more for two to three days in a row, the producer needs to start taking action to help alleviate the heat.
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Figure 1. Temperature-Humidity Index (THI)
Chart.
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Air
°F
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Relative Humidity (%)
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Air
°F
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0
|
5
|
10
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15
|
20
|
25
|
30
|
35
|
40
|
45
|
50
|
55
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60
|
65
|
70
|
75
|
80
|
85
|
90
|
95
|
100
|
|
125
|
107
|
114
|
122
|
130
|
140
|
151
|
|
125
|
|
120
|
105
|
110
|
116
|
122
|
130
|
139
|
148
|
|
120
|
|
115
|
103
|
106
|
110
|
115
|
121
|
127
|
135
|
143
|
|
115
|
|
114
|
102
|
105
|
109
|
113
|
119
|
125
|
132
|
140
|
|
114
|
|
113
|
102
|
104
|
108
|
112
|
117
|
123
|
129
|
137
|
145
|
|
113
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|
112
|
101
|
104
|
107
|
111
|
115
|
121
|
127
|
134
|
142
|
|
112
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|
111
|
101
|
103
|
106
|
109
|
114
|
119
|
125
|
131
|
139
|
147
|
|
111
|
|
110
|
100
|
102
|
105
|
108
|
112
|
117
|
122
|
129
|
136
|
143
|
|
110
|
|
109
|
100
|
101
|
104
|
107
|
110
|
115
|
120
|
126
|
133
|
140
|
|
109
|
|
108
|
99
|
101
|
103
|
105
|
109
|
113
|
118
|
124
|
130
|
137
|
144
|
|
108
|
|
107
|
99
|
100
|
102
|
104
|
107
|
111
|
116
|
121
|
127
|
134
|
141
|
|
107
|
|
106
|
98
|
99
|
101
|
103
|
106
|
109
|
114
|
119
|
124
|
130
|
137
|
145
|
|
106
|
|
105
|
97
|
98
|
100
|
102
|
104
|
108
|
112
|
116
|
122
|
127
|
134
|
141
|
|
105
|
|
104
|
97
|
97
|
99
|
100
|
103
|
106
|
110
|
114
|
119
|
124
|
131
|
137
|
145
|
|
104
|
|
103
|
96
|
97
|
98
|
99
|
102
|
104
|
108
|
112
|
116
|
122
|
127
|
134
|
141
|
|
103
|
|
102
|
96
|
96
|
97
|
98
|
100
|
103
|
106
|
110
|
114
|
119
|
124
|
130
|
137
|
144
|
|
102
|
|
101
|
95
|
95
|
96
|
97
|
99
|
101
|
104
|
108
|
112
|
116
|
121
|
127
|
133
|
140
|
|
101
|
|
100
|
94
|
94
|
95
|
96
|
98
|
100
|
102
|
106
|
109
|
114
|
118
|
124
|
130
|
136
|
143
|
|
100
|
|
99
|
93
|
93
|
94
|
95
|
96
|
98
|
101
|
104
|
107
|
111
|
116
|
121
|
126
|
132
|
139
|
146
|
|
99
|
|
98
|
92
|
92
|
93
|
94
|
95
|
97
|
99
|
102
|
105
|
109
|
113
|
117
|
123
|
128
|
134
|
141
|
|
98
|
|
97
|
92
|
92
|
92
|
93
|
94
|
95
|
97
|
100
|
103
|
106
|
110
|
115
|
119
|
125
|
130
|
136
|
143
|
|
97
|
|
96
|
91
|
91
|
91
|
92
|
93
|
94
|
96
|
98
|
101
|
104
|
108
|
112
|
116
|
121
|
126
|
132
|
138
|
145
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|
96
|
|
95
|
89
|
90
|
90
|
91
|
92
|
93
|
94
|
97
|
99
|
102
|
105
|
109
|
113
|
118
|
123
|
128
|
134
|
140
|
|
95
|
|
94
|
88
|
89
|
89
|
90
|
90
|
92
|
93
|
95
|
97
|
100
|
103
|
106
|
110
|
114
|
119
|
124
|
129
|
135
|
141
|
|
94
|
|
93
|
88
|
88
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