Heat stress and nutrition

A cow feeling effects of heat stress
A Holstein Fresian dairy cow panting from heat stress

One of the major impacts of heat stress on dairy cow production appears to be through its effect on feed quality and dry matter (DM) intake. Potential pasture and cow management decisions that can be made to help alleviate this problem would be an important step towards improving milk production throughout the hotter months.

Impact of heat stress on cow nutrition

High temperature and humidity have a two-fold impact on dairy cow nutrition.

1. A direct effect on DM intake

Cows have been shown to voluntarily reduce intake by 9-13% in hot conditions. Factors reducing feed intake include panting, which reduces cud chewing, slows the breakdown of feed, and reduces the amount of water and buffers from saliva reaching the rumen.

Standing in the shade to keep cool also restricts grazing time and decreases intake. Any form of stress, including heat stress, can slow rumen contractions, which in turn slows digestion.

2. A direct effect on forage quality

Humidity and high temperatures increase plant growth rate, which increases the neutral detergent fibre (NDF) content of plants and reduces potential intake.

Feeding strategies to reduce the impact of hot conditions

Increase feed access

Take feed to cows, rather than cows to feed in hot weather. Walking to feed increases a cow´s heat load, so reduce their walking during the hottest time of the day.

Increase night grazing

Allow greater access to pastures for grazing at night. Cows will do up to 70% of their daily grazing at night in hot weather.

Provide high-quality forages

Compared with concentrate diets, high-fibre forages will generate more heat through digestion. Cows will voluntarily limit forage intake during hot weather, so provide good quality forages during these periods.

Supplement the diet

Increase the energy content of the diet with good quality forages and concentrates to make up for the shortfall in reduced intake and to reduce the metabolic heat load.

Increase minerals and vitamins

To compensate for the reduction in feed intake, increase the concentration of minerals and vitamins in the diet, particularly sodium, potassium, magnesium and niacin.

Supplementing cows with 1.5-1.6% DM of potassium and 0.5-0.6% DM of sodium will potentially improve milk yield in heat-stressed cows. Include magnesium at 0.35-0.4% DM to help avoid metabolic problems (grass tetany) when feeding higher amounts of potassium.

Including niacin (6 g/cow/day) may also be beneficial. It has been reported to reduce skin temperature and increase milk yield.

Buffer the rumen

Improvement in milk yield has also been reported by feeding cows sodium bicarbonate at 150-200 g/cow/day during hot weather to help buffer the rumen.

Cows in shaded feedpad to reduce heat stress
Holstein Fresian dairy cows on a shaded feedpad to minimise the effects of heat stress.

Management strategies to reduce the impact of heat

Water

Provide cool, clean water and ample trough space in close proximity to cows at all times. In hot weather, lactating cows have the capacity to consume more than 100 L a day.

Cows may drink 50% of their daily water intake as they exit the dairy. Therefore, a 200-cow herd may require a supply of up to 5000 L of cool, clean water during the 1-2 hours that the cows exit the dairy.

Shade

Allow access to shade throughout the day. Shade can reduce a cow's heat load from the environment by up to 50%. Provide shade in feed-out areas, grazing areas and over the milking yards.

Sprinklers

Cooling cows at the dairy with shade, sprinklers and/or fans before and after milking will improve their comfort and enhance their capacity to eat.

Let cows wander home and stand under sprinklers before the afternoon milking. Lowering body temperature will encourage higher feed intake during milking and higher pasture/forage intake after milking.

Queensland trials have shown that 30 minutes of wetting cows with sprinklers can produce an extra 1 L of milk/cow/day; 60 minutes has produced an extra 1.5 L of milk/cow/day in hot weather. Sprinklers should deliver large drops to thoroughly wet the cows.