Use of supplements in cattle rations during cold weather
Maintaining our cows through the various stages of pregnancy, especially through the winter months, has some unique challenges, especially when feeding low quality forages. A forage is considered low in quality when it has digestible energy of less than 1 Mcal/lb. and less than 7 percent crude protein. Examples would be late-cut weathered hay or straw.
Feeding straw when the weather is very cold
A 1,213-pound mature cow, six months pregnant, needs about 30 Megacalories of digestible energy per day. Even if the weather is not cold, the cow can eat a maximum of about 18 pounds of straw on a dry matter basis (about 21 pounds as-fed). The amount of digestible protein needed by the bugs in the rumen to digest the straw is about 0.65 lb., of which only 62 percent is supplied from the 18.7 pounds of straw. This cow will lose half a body condition score in about 33 days. At -13 F, the amount of energy required to maintain the cow goes up to approximately 41 Megacalories per day. The cow will attempt to increase straw intake by about 20 percent and may get impacted. Even at the increased intake of 22 pounds of dry matter, the amount of digestible protein supplied is only 39 percent of what is required. The cow will lose half a body condition score in about 20 days.
Energy and protein supplements
All nutritionists agree that the most important consideration to get a cow through the winter is the provision of adequate energy and protein. Cows can not survive harsh winters and be productive on straw alone. However, in recent times, there have been reports suggesting that feeding barley grain as an energy supplement is not a good idea. Those against feeding grain say that feeding cows grain could actually serve to reduce energy intake through a decrease in the digestibility of straw. On the other hand, the recommendation usually given is that feeding a protein supplement is the way to go. Let us illustrate the basis of this recommendation with numbers in Table 1.
On the other hand, the assumption is that feeding one pound of a protein supplement increases straw intake by about 3 pounds. Using those assumptions, the digestible energy intake from adding grain is about 20.28 versus 22.26 Megacalories per day from adding the protein supplement. The conclusion drawn is that feeding grain with straw may not be a good nutritional management in winter when the animal is cold stressed.
What are the facts? Fact No. 1: In a cold stress situation, the cow does not use the digestible energy system! She will use what is called the metabolizable energy system. Feedstuffs that cause less wastage of energy (less methane production and energy loss in urine) become extremely useful to the cow. For example, straw loses about 18 percent of its energy value as methane and urine. Barley grain loses about 13 percent or even less. A protein supplement such as the range pellet loses about 16 percent as methane and urine energy. When you take these wastage factors into consideration, you move from the digestible energy into the metabolizable energy system. The metabolizable energy system measures the total amount of energy actually available to the cow.
Fact No. 2: More recent research data indicates that the so-called substitution effect is not one-to-one. The substitution rate is actually only 43 percent instead of 100 percent when you supplement low quality forage with grain. What this means is that when you add 3 pounds of grain to straw, only 1.3 pounds of straw will not be eaten out of the 20 pounds offered. In other words 18.7 pounds of straw will be consumed.
When these two facts are added together, the total metabolizable energy intake from feeding straw only, straw and protein, and straw and grain becomes 14.76, 18.28 and 18.11 Megacalories per day, respectively.
Note that the difference between supplementing with protein and grain is about 0.17 Megacalories per day and that is nothing to write home about.
The take-home message is that when it is cold, the cow needs better quality feedstuffs, such as good-quality hay. If you do not have goodquality hay, your choice is supplementing your straw with either grain or a protein supplement or both.
My recommendation is that when it is very cold, feed some grain. It is a faster and often cheaper method of increasing the energy intake of your cows. — Erasmus Okine, Department of Animal Science, University of Alberta, Edmonton