Be aware of the challenges when switching cattle diets – a smooth transition from summer to winter diets is vital to maintain the delicate balance of the rumen and keep animals performing at their best. SAC nutrition specialist Colin Morgan highlights the aspects to keep an eye on.

A sudden change of diet can be upsetting enough for monogastrics but, for cattle relying on a delicately balanced fermenter such as the rumen to digest food, it can cause serious problems.

Livestock farmers need to be especially careful when changing cattle diets, particularly when bringing beef or dairy youngstock indoors for the winter.

Switching from grass to silage and concentrates, or to intensive grain diets, can cause problems in cattle of all ages, though youngstock being weaned at the same time as being housed perhaps face the biggest challenge.

Rush the job and growth rates will suffer unduly and serious health problems could soon follow.

Why are cattle so vulnerable to diet changes?

The rumen is designed to break down fibre, and contains billions of microbes - mainly bacteria - that work together to form a continuous fermentation system to digest this material.

It is a true ecosystem, and takes time to adapt when change is forced upon it.

It is happiest operating in slightly acid conditions, around pH 6-7.

Switching from grass to silage, which has been partially fermented and contains a lot of protein breakdown products and fewer sugars, can throw the system out of balance.

The problem increases when grain is added. This is high in starch, which is rapidly fermented causing excess acid production. An acidic rumen slows down bacterial fermentation of fibre, and, in severe cases, can stop it. This slows the passage of feed through the gut, which reduces intake as well.

As a rule of thumb, you should allow a minimum two-week transition period between diets to avoid rumen upset.

How do you help suckled calves?

Suckled calves are often weaned at housing. Creep feeding is an excellent way of getting calves used to concentrate in the diet, and to maintain growth rate as their mother's milk output reduces and grass growth slows.

As well as reducing the risk of a post-weaning check, creep feeding also helps guard against pneumonia as it reduces stress. However, because creep feed is a highly concentrated, starchy material and is fed to appetite to ensure all calves get their share, it needs to be managed carefully.

Gorging can be a problem, so it is best to dilute the feed with a digestible fibre such as sugar beet pulp, citrus pulp or maize gluten. Start with a 50:50 mix, then gradually reduce the fibre content over the next two weeks. Calves will probably be eating about 2-3kg by weaning or more if the feeders have been in for a longer period for moving on to an intensive diet.

What's the best way to introduce a silage-based diet?

This depends on the age of the cattle. A suckled calf's rumen will be less well developed than its early-weaned counterpart, so the microbes will not be working at maximum efficiency. Its rumen needs time to get going properly and to use silage efficiently.

Adding digestible undegradable protein (DUP) - soya bean meal is the classic choice - certainly seems to help in these early transition stages. Adding 0.25-0.5kg to the diet for six to eight weeks after weaning will also ensure the animal gets all the protein it needs, as a large proportion of it bypasses the fermentation process allowing the animal to use it directly.

After a couple of months calves can be left to their silage-based diet. Some cereal will need to be given, but as they will probably be going out again the following summer you don't want to overfeed when you can use cheap grass to put weight on later. Aim for a daily weight gain of 0.7-0.8kg by adding 2kg of concentrate a day to average quality ad-lib silage.

For finishing cattle, weight gain is all-important. They need to eat about 4kg of concentrate a day plus silage to put on 1kg a day, but in any one feed you only want 0.5kg for every 100kg of bodyweight. You need to build up to this - start with straight silage for a day or two, then introduce 0.5kg of grain and gradually increase over the next two weeks in two feeds.

In both cases, to give the appropriate amount and type of supplement, it is essential that the composition of the silage is known. A sample should be taken before the start of the winter feeding period so that it can be analysed and rations formulated accordingly.

What if you're using straw?

Straw is a much poorer feed than grass or silage, being high in fibre with low metabolisable energy and crude protein levels. It is not well digested, so stock will need more concentrate and protein supplement. The same principles should be used for the transition as before.

Remember to keep the forage element of the final diet at about 40% of the overall dry matter, and certainly no lower than 30%, otherwise the increased starchiness interferes with fibre digestion and you won't get the benefits of the fibre.

Intensive diets: How quickly can you introduce grain at housing?

Managed carefully, grain-fed animals will put on weight fast, so suckled calves should be ready to be sold at 13-15 months of age.

But this is a transition that cannot be rushed - the key thing is to build slowly starting with two feeds a day, then introducing a third in the middle of the day and a fourth in the evening. Animals should be leaving food after two weeks, at which point they are feeding ad lib, so you can fill the hopper and keep it topped up.

As with creep feed, you can dilute with digestible fibre to make the process safer. Correct grain processing is also important. It wants to be cracked open - if it is too finely ground then the microbes in the rumen will digest it too quickly, increasing the risk of acidosis and bloat. Coarse ground or rolled is fine.

Supplement it at the start with any good protein source like soya, distillers' grains, beans, peas or rapeseed meal to get crude protein levels up to 16-18%, then drop back to 13% after a couple of weeks.

Long clean straw should also be supplied to appetite. Don't rely on bedding straw - this soon gets fouled and stock won't touch it. Cattle won't eat much - possibly only 10-15% of the dry matter in the diet - but it will get them chewing which produces saliva and helps neutralise acid in the rumen.

And finally, don't forget that while all diets should be supplemented with vitamins and minerals, it is absolutely essential to use a calcium-rich supplement in an intensive grain diet to keep bones strong and healthy.

Golden rules

• Avoid any sudden change in diet when housing stock - ensure a two-week transition period to minimise problems
• Get silage analysed so you can supplement it appropriately
• If feeding an intensive grain diet, use coarsely ground or rolled grain and provide long, clean straw ad-lib