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Nutrition strategies

Dietary fibre: Should I feed more or less during hot conditions?

With daily feed intake reduced, and more grain / concentrates being fed to maintain energy intake, the quality and amount of fibre sources fed is critical.

High quality fibre is the best tool you have to maintain rumen stability and to increase nutrient density without producing excessive metabolic heat. Low quality forage (high NDF) simply takes up too much space in the diet and makes it difficult to achieve the required daily nutrient intakes needed for milk production given the cow’s reduced appetite.

Consider the quality of the fibre first, then decide how much fibre to feed. Higher fibre intakes add safety and help cows bounce back better after an excessive heat load event has passed.

If you have a mixer wagon, and can feed fibre with other feeds in a partial mixed ration (PMR), this gives you much more flexibility. It is best to feed the PMR under shade between the morning and afternoon milking, and allow cows to graze best-quality pasture overnight.

When feeding out quantities of forage fibre to your herd, ensure all cows get equal access. Heifers and less-dominant cows may be more at risk of acidosis than others. Good feeding management is critical.

Slowly fermenting starch sources

As mentioned in Summer nutrition program, heat stressed cows have a greater need for glucose. Providing starch in a more slowly fermented form assists this in two ways:

It takes some of the starch fermentation away from the rumen, assisting the rumen to maintain feed digestibility, and energy yield from the rumen that can be converted to glucose in the liver. It also reduces the risk of ruminal acidosis.
The starch that is not digested in the rumen will normally be digested in the small intestine. At this site of digestion it produces glucose directly for use by the gut tissue (a huge energy user in the cow), and can be transported and converted to energy for use around the body more widely. This form of digestion also releases less heat then ruminal starch digestion.

Corn (maize) is the most readily available slow fermenting starch source of all the grains.

Dietary fat: How much should I aim for?

Feeding fat has an added advantage in hot conditions - it is digested and utilised by the cow more efficiently than starches and fibre, producing less metabolic heat and thereby adding less to heat load. However, too much fat interferes with microbial digestion in the rumen and depresses feed intake. Aim for a maximum of 6-7% total fat in the diet (DM basis).Dietary supplementation with extra fat is a good way to help increase the energy density of the cow’s diet and maintain the daily energy intake.

The best results are likely in higher producing cows and those under greater metabolic stress. A number of supplementary fat sources may be used, including straight vegetable oil and commercial ‘by-pass fat’ supplements. It is important to manage the ratio of saturated versus unsaturated fats being fed in the diet. In the US, diets used in hot conditions typically contain 3% fat from the main feeds in the diet, 2-3% fat from added vegetable oil, and 1-2% fat as supplementary by-pass fat. Consult your nutrition adviser.

Protein: How much in the hot season?

In hot conditions, cows still need sufficient amounts of protein in their diet to maintain rumen microbial function and supply good flows of amino acids to the intestine. However, they are faced with three challenges:

their daily feed intake is reduced,
their rumen microbial function is compromised, and
summer pastures are lower in protein.

So, feed higher-quality protein sources in the diet during the hot season. Higher ‘by-pass’ or ‘escape’ protein sources that are readily digested in the cows’ small intestine can help offset lower yields of microbial protein from the rumen during hot weather. Consult your nutrition adviser.

Essential minerals: potassium, sodium & magnesium

Cows lose enormous amounts of potassium (K+) in hot conditions through sweating. Potassium bicarbonate is the preferred potassium source. Between 1.3-1.6% potassium in the diet (DM basis) is recommended during the hot season. See your nutrition adviser.

In hot conditions, cows excrete more sodium (Na+) through the urine. Sodium bicarbonate is the preferred sodium however, sodium chloride (salt) is satisfactory. Approximately 0.45 – 0.6% sodium in the diet (DM basis) is recommended during the hot season. See your nutrition adviser.

Magnesium inputs should also be increased during the hot season, especially if you are supplementing with extra fat. Approx. 0.35% magnesium in the diet (DM basis) is recommended. Consult your nutrition adviser.

Buffers: Are they necessary?

Cows normally produce large amounts of bicarbonate in their saliva (more than 2.5 kilograms per day!), which helps keep the rumen pH in the optimal range for the growth of the microbes that digest food. Hot conditions lead to a drop in the flow and the bicarbonate concentration of the cows’ saliva, thereby reducing the natural buffering activity in the rumen. On top of this, the cow may be consuming less effective fibre and more grain / concentrate, so the risk of a fall in rumen pH and ruminal acidosis problems is increased.

Dietary supplementation with a buffer is therefore good insurance during the hot season. Recommended daily feed rates vary, depending on what is fed and how it is fed. Consult your nutrition adviser.

Feed additives

In addition to minerals and buffers, a number of other feed additives are available that have potential to assist cows in hot weather. These include:

Rumen modifiers
Yeast and yeast metabolites
Betaine
Niacin

Rumen modifiers such as monensin, tylosin, virginiamycin, lasalocid and bambermycin may assist by beneficially altering the balance between the different populations of microbes in the rumen and the proportions of VFAs (volatile fatty acids) they produce. For more information, see feed.FIBRE.future fact sheet H or visit the Dairy Australia website.

Yeast and yeast metabolites may assist by increasing fibre digestibility and the utilisation of lactic acid, and helping the cow produce more glucose from propionate.

Betaine may assist in maintaining feed intake and also help by reducing the amount of energy (and glucose in particular) the cow has to burn to stay cool and carry on her normal metabolic processes.

The vitamin Niacin has been shown to play a role in energy metabolism, so additional niacin supplementation may be helpful in the hot season.

Consult your nutrition adviser for more information on what place these or other feed additives may have in your summer nutrition program. Remember that there are no silver bullets! The most important thing with your summer nutrition program is to provide a diet which includes high quality forage fibre, slowly-fermenting starch sources, fats, buffers and minerals.