Dairy--Animal Management

HEAT STRESS AND MILK PRODUCTION

General

A cow's response to weather is affected by breed, animal size, level of production, and stage of lactation in addition to factors such as nutrition and caloric content of feed. Milk producers know that hot, humid weather lowers production. They also know that cattle have the ability to acclimate to changes in weather. This acclimatization process, however, requires time. Thus effective summer season herd management requires advance warnings of impending heat stress periods.

Heat stress is defined as any combination of temperature, humidity, radiation and wind producing conditions that are higher than the temperature range of the animal's thermal neutral zone. An animal's performance during heat stress periods depends upon several weather factors, especially temperature and humidity. Thus we can use these factors in our mamagement planning.

The Temperature Humidity Index (THI), a measure of heat stress, combines the effects of temperature and humidity into one value. The chart shows THI for a range of air temperatures and relative humidities.

Chart 1. Temperature Humidity Index at specific temperatures and relative humidities.

Air	Relative Humidity, %
Temp	40	50	60	70	80	90

76	70	71	72	73	74	75
78	71	73	74	75	76	77
80	73	74	75	76	78	79
82	74	75	77	78	79	81
84	75	77	78	80	81	83
86	77	78	80	81	83	84
88	78	80	81	83	85	86
90	79	81	83	85	86	88
92	80	82	84	86	87	90
94	82	84	86	88	90	92
96	83	86	88	90	92
98	85	87	89	91
100	86	88	91
102	87	90	92
104	88	91

We developed a heat stress milk production model based upon two factors, total hours when THI values are above 74 and total hours with THI exceeding 80. During a typical South Carolina summer day, THI is greater than 74 from late morning through early evening. THI exceeds 80 when air temperatures are greater than 86 degrees.

The model revealed that cows acclimate to heat stress in about 4 days. During a period of stress, when THI hours above 74 no longer increase, production stabilizes. It also showed that high stress, hours with THI above 80, immediately effect production. This models says that heat stress relief by sprinkling animals with water will have maximum impact when started at THI values near 80. This will not only keep the animal more comfortable, it may also reduce water usage.

A good rule of thumb for starting your sprinklers will depend upon the minimum temperature recorded in the morning. If the minimum temperature was 74 or higher, start sprinkling when air temperature reaches 85. If the minimum temperature was 66, wait until air temperature reaches 88. At minimum temperatures between these lows, start sprinkling before air temperature reaches 88 degrees.

For additional information on animal heat stress, consult your county Extension office.

76	70	71	72	73	74	75
78	71	73	74	75	76	77
80	73	74	75	76	78	79
82	74	75	77	78	79	81
84	75	77	78	80	81	83
86	77	78	80	81	83	84
88	78	80	81	83	85	86
90	79	81	83	85	86	88
92	80	82	84	86	87	90
94	82	84	86	88	90	92
96	83	86	88	90	92
98	85	87	89	91
100	86	88	91
102	87	90	92
104	88	91

76	70	71	72	73	74	75
78	71	73	74	75	76	77
80	73	74	75	76	78	79
82	74	75	77	78	79	81
84	75	77	78	80	81	83
86	77	78	80	81	83	84
88	78	80	81	83	85	86
90	79	81	83	85	86	88
92	80	82	84	86	87	90
94	82	84	86	88	90	92
96	83	86	88	90	92
98	85	87	89	91
100	86	88	91
102	87	90	92
104	88	91

76	70	71	72	73	74	75
78	71	73	74	75	76	77
80	73	74	75	76	78	79
82	74	75	77	78	79	81
84	75	77	78	80	81	83
86	77	78	80	81	83	84
88	78	80	81	83	85	86
90	79	81	83	85	86	88
92	80	82	84	86	87	90
94	82	84	86	88	90	92
96	83	86	88	90	92
98	85	87	89	91
100	86	88	91
102	87	90	92
104	88	91