posted Dec 24, 2010, 12:51 PM by Bob Whitney
updated Feb 20, 2011, 1:11 PM
Daily energy intake is the primary limiting factor affecting beef cattle performance on forage diets like coastal hay or pasture or even native grasses. This energy intake can be further limited when forages supply an inadequate amount of crude protein. The reason for this is that the amount of crude protein in the cow’s diet needs to be in balance with the energy content. This is usually expressed as a ratio of 6 parts energy to 1 part protein. For instance, hay with 60% total digestible nutrients or TDN would need a protein content of 10% to be in balance and allow the microbes in the rumen of the cow to properly digest all the energy and also have enough energy to digest all the protein. The problem is that we allow pastures and hay fields to get too mature so that they have less protein and energy as a percent of the total. It is not unusual for coastal hay to test 45% TDN and have a 5% crude protein (CP). This is a ratio of 9 TDN:1 CP which is way out of line. This kind of ratio has been researched to limit dry matter intake to just 1.6% of body weight(BW) when it should be above 2% of BW. When the crude protein of a forage falls below about 8% the dry matter intake of cattle declines rapidly. This decline is attributed to a loss of rumen microbes as they die for lack of protein. This is why feeding a high quality protein supplement will improve both the protein and the energy status of cattle.This is simply because the rumen microbes can now work better so that they improve the forage digestibility for the cow and when this happens the cow can now take in more forage so that forage intake increases. I said earlier that at 5% CP, forage intake is limited to about 1.6% of the cow’s BW. When the forage is improved to 8% CP or if a high CP supplement (25% or greater) is added with the forage then forage intake increases to 2.3% of BW. This is a 44% increase in forage intake or nearly half again more. This means that she will eat more of the lesser quality pasture or hay and lose less weight in the winter because as she eats more forage she gets more energy. If you remember what I first said was that energy is the limiting factor for cow performance on forage diets. Most of the time we say it is protein and we will usually buy hay based on protein but what we really mean is we want the higher protein in order to balance the energy. Again that ratio needs to be about 6 TDN : 1 CP or you will find that the cattle will perform poorly even though you have plenty of feed.Now for some rules of thumb you may want to go by for supplementing cows. First if you have plenty of pasture (I doubt you do this year) but you know it is low quality then feed a protein supplement but no hay. This will help them digest the low quality forage. Second, remember cattle are designed to lose up to 15% of their BW in winter we just don’t like to see them do it. Force them to eat the low quality stuff as long as possible. Third when you start feeding hay you can’t quit. This is mainly because they don’t want to if you will feed them and secondly the higher energy in the better hay puts the rumen on a new level and so they really can’t go back to the pasture as easily. Fourth know what your hay tests. You may need to add a protein supplement to your hay if it is not balanced to be 6 TDN: 1 CP. Fifth stay away from energy supplements as long as you can since they are the most costly. This may not be possible if your hay and pasture are really poor quality and/or you don’t have much forage which is definitely the case this year.