As I write this in early January, the northeastern U.S. is locked in a deep freeze. The polar vortex has descended upon us, with bitterly cold temperatures – worse this year than your average bitterly cold temperatures. Then again, for me, anything below 38 degrees Fahrenheit is considered bitterly cold.
During the winter months, it seems my main focus from day to day is to stay warm, and I’m certain that I’m in a “negative energy balance.” I need to consume more concentrated energy in my diet. So why is it that when I drink that extra cup of hot chocolate or consume that piece of fudge that’s still sitting around from the holidays, I’m still cold and my “body condition score” increases? If I’m cold and I need extra energy to compensate for that cold, then all those extra calories should not be adding to my weight. However, I learned a long time ago that even if it’s only 2 degrees outside, those calories and the energy they provide won’t necessarily get burned up unless you’re a marathon runner who pounds out 10 miles every day.
Unlike we puny humans, who might burn up 1 megacalorie (Mcal) of energy during the course of a day as we’re sitting at our desks shivering because it’s 66 degrees Fahrenheit in the house, a 1,500-pound dairy cow consumes 10 times that amount just to maintain body condition while in moderate temperatures. Then they have to consume that much energy again to produce several gallons of milk every day, and it all has to come from a properly balanced diet.
Carbohydrates are the most important source of energy for our animals. Unlike humans, who prey upon pasta dishes drenched in sauce, tasty breads and coffee shop muffins, cows get the majority of their calories from grasses and legumes. The primary sources of carbs for our dairy cows and heifers are cellulose from forages and starches and sugars that come from a variety of feedstuffs. All of that energy is produced in a big fermentation vat called the rumen.
During the fermentation process, the cellulose, sugars and starches are used by the rumen bacteria to support their own existence, and the byproducts from that fermentation are chemicals called volatile fatty acids (VFAs) that are then used for energy and glucose synthesis. There must be a proper balance between fibrous and nonfibrous carbohydrates in order for the rumen to function correctly and efficiently, for the rumen bacteria to remain healthy and abundant, and for cows to produce milk.
The three primary VFAs produced by rumen fermentation are acetate, butyrate and propionate, each a short-chain fatty acid. They are absorbed through the rumen wall and into the bloodstream. Most of a cow’s metabolizable energy comes from the conversion of propionate to glucose, a process that occurs in the liver. The primary driver of milk production in cows is blood glucose, along with a proper balance of amino acids. A cow requires copious amounts of blood glucose for metabolic needs as well as milk production.
Butyrate contributes to energy metabolism at the cellular level and has no part in the formation of glucose. Butyrate and acetate, however, are the precursors to milk fat. Some glucose is converted to glycerol, which is the foundational molecule of milk fat synthesis. Acetate and butyrate are used for the formation of the larger, more complex fatty acids that are attached to glycerol to form milk fat.
It’s important to understand that feeding a dairy cow involves more than just tossing a few bales of hay and some grain in front of her. Carbohydrates, while absolutely essential for growth, health and milk production, come in a variety of shapes and sizes. Too much forage, while good for rumen function, may limit feed intake as well as hinder adequate propionate and glucose production and limit milk production. Excessive starch and sugars coming from grains or silages may hinder forage fermentation and acetate production, depressing milk fat.
The nonfibrous carbohydrates that are found in soybean hulls and beet pulp promote the production of propionate, while also adding some bulk to the diet. They provide higher levels of VFAs than forages because they are more rapidly fermented in the rumen. Cellulytic carbohydrates, coming primarily from forages, can range from simple molecules that are easily broken down by rumen microbes and quickly fermented to highly complex and lignified matrixes that are nearly impossible for microbes to do anything with. The potential energy available from forages is highly dependent upon the stage of maturity of the plants at the time of harvest.
As dairy farmers attempt to improve the revenue produced by their cows, it has become necessary to design and feed diets that maximize both milk production and milk components. Changes in the proportion of forage and concentrate – and thereby the makeup and combination of carbohydrates – in dairy cow diets can have a significant effect on the amount and the percentage of each VFA produced in the rumen.
In recent years, it’s become more common to find increased grain levels in newer varieties of corn silage. While supplying a higher energy level in the diet through homegrown forage appears to be a good thing, dairy farmers must be careful not to overfeed starch, which can overacidify the rumen and depress the synthesis of acetate, thereby depressing milk fat, which will reduce the value of the milk produced in the long run.
Early lactation is the time when cows are the most profitable, based on the amount of milk they produce. Cows that spend a month or more in the brutally cold weather either as dry cows or fresh cows need significantly more energy to support total metabolic needs. Energy status during the dry period will influence total milk production during the next lactation. Your dairy cows essentially give exactly as much milk as their nutritional plan allows for, and carbohydrates play a significant role in producing milk.