Have you ever heard someone state categorically that "animals know what they need to eat"? I hear claims like this quite often. In general I hear statements like this when we are discussing supplementary feeding programs, in particular the need for adding minerals and trace elements.
There's no doubt animals need minerals and some trace elements for their well being. In general animals obtain the minerals and elements they require as part of their daily feed intake.
In most circumstances the feed cattle eat each day has enough of minerals and trace elements to satisfy the animals needs.
There is another way cattle obtain them in their diet. Thats through the soil. I remember being told in a lecture from a vet that cattle eat roughly about 1kg of soil a day!
It makes sense when you think about it. If plants are pulled out from the soil during grazing, some soil will make its way inside!! Fortunately it comes out in the dung!
In that process cattle can access some of the minerals that may not be available form the feed itself.
Having said all that, there are times when the plants or the soil can't supply the minerals and elements cattle need.
So, do cattle know they need to have these minerals and elements, and will they actively search for them? Many people say that cattle will do this. They use plenty of examples to demonstrate this position.
In my experience and from many years working with researchers, I'm not so sure. I reckon my position is best summed up in this picture.
More recent research suggests that animals will look for particular minerals, but only when they have been deliberately depleted of that mineral and when the deficiency is major. It seems that when the deficiency is minor, the animals don't know they are missing it and don't go looking for it!
So what does this mean for producers designing feeding programs? I reckon it means that in the short term, deficiencies are not recognised by animals, and you can't expect them to go and eat a supplement to correct the problem. If you do want them to eat something, its more likely they will go to something they are used to eating, suck as a block. This is why sulphur blocks work quite well on some forage crops.
In the long term, deficiencies like calcium and phosphorus seem to be felt more by the animal. In these longer term deficiencies, animals do seem to look for options to address the absence of these minerals. This is when cattle will lick the soil, and in many cases chew bones.
While its easy to think they know that chewing bones is a result of nutritional wisdom, most researchers think its more likely that they do it because it makes them feel better.
There is one big exception to this! And its the need for sodium, in other words, the need for some salt.
We know there are some specific metabolic needs that will trigger the animals brain to crave salt. When this happens you will see cattle licking the ground or looking for salt deposits.
It was actually this behaviour that first made people think cattle possessed nutritional wisdom.
So what does this mean at a practical level? I reckon the first thing is to realise your animals are not smart enough to choose their specific mineral needs in the short term. Its up to you to monitor your animals and ensure they are not deficient.
You also need to think of how you supply your animals needs. Don't rely on the animals to pick and choose. If there is a deficiency in your animals intake, then design a program that will actually correct it quickly and effectively. In the long run it will be much more cost effective.
I reckon the final thing is don't have a guess! If you're not sure about your cattle and their nutritional needs, get some advice. If you don't have nutritional wisdom, just remember your cows can't tell you what they need either!