In southern New England, we experienced funny weather this past spring. March came and went with temperatures reaching above freezing on just a few days. At the end of April, when many dairy farmers are usually turning cows out on pasture, there was still snow on the ground. Then in the middle of May it got hot and suddenly the orchard grass was two feet high and already heading out. So much for the first cutting.

If you’re a dairy farmer or a forage producer you know how challenging it is to produce consistent, high quality hay crops in the northeastern United States. The challenge is getting the grass mowed and baled before it gets too mature and trying to get it all done between rain storms. Grasses and legumes become more fibrous as they mature. That fiber, made of complex cellulose and lignin, becomes more difficult for rumen microbes to break apart and utilize. Digestibility, the nutritional value and the “net energy” of forages are significantly affected by their maturity.

Nuts and bolts of net energy

The net energy system is a convenient way to measure the net calories retained or available in an animal feed product. These values tell us that the feed supplies different levels of calories for different functions or life stages of the cow or heifer. It gives us the values of the energy available for both maintenance and growth in heifers, dry cows and beef animals and energy available for lactation in milk cows. Net energy for maintenance is abbreviated as NEm, net energy for gain is abbreviated as NEg, and net energy for lactation is abbreviated as NEl.

Net energy values are calculated based upon the protein, ash, fat, lignin and neutral detergent fiber found in feedstuffs. The lower the protein and the higher the fiber content of forages, the lower the net energy values will be. Excessive ash content (including dirt from a field) will also lower the net energy value of a forage. All forage testing labs should compute the NEm, NEg and NEl for forages. The values are listed in mega-calories per pound of feed (MCal/lb).

Following are two examples of hay crops:

Maintenance nutrition

Mature cows or dry cows that are not growing or producing milk need only consume enough dietary calories to maintain a constant body weight or level of activity. For our forage Hay A, 0.48 MCals are available for maintenance in a pound of that hay. If, for example, a cow requiring a total of 10 MCals of energy per day for maintenance eats 25 pounds of this hay, her energy needs will be more than met, consuming 12 total MCals (25 × 0.48). She could actually put on some weight.

Animals that are growing or lactating must first meet the NEm requirement before the diet can meet NEg or NEl requirements. You will notice that the NEm value and the NEl value on a forage report are similar in value. This means that the cow needs about as much energy for her maintenance needs as she does for producing milk – but again, nutrition for NEm must be met first and whatever is left can be partitioned for milk production.

It takes more energy for an animal to grow. That’s why the NEg value is always the lowest value in the forage analysis. It’s not because the feed is somehow deficient in calories. Depending on how fast (rate of gain) a producer wants a calf or heifer to grow, a diet of all hay or pasture grass may fall well short of supplying the necessary energy to support that growth rate after the NEm requirement has been met.

A growing dairy heifer weighing 1,000 pounds requires about 8 MCals per day for maintenance. We are also still expecting her to gain 1.5 pounds per day as she approaches two years of age and freshening. At this age she may consume about 20 pounds of feed dry matter per day. Since the initial net energy calculation must be for NEm, the heifer must consume nearly 14 pounds of hay to meet her NEm requirement (14 × 0.59 = 8.26) with Hay B. In comparison, she would need to consume nearly 17 pounds of Hay A to meet her maintenance requirement (17 × 0.48 = 8.16 MCals). In this example, both Hay A and B easily supply maintenance nutrition.

The energy requirement for a daily gain of 1.5 pounds is about 4 MCals for this size animal. The question is: can either Hay A or B supply 4 MCals with either 3 pounds or 6 pounds of dry matter remaining in either of the diets, knowing that the heifer can only consume 20 pounds of dry matter total for the day? The answer is “no.” (6 × 0.33 = 1.98 MCals for Hay B and 3 × 0.23 = 0.69 MCals for Hay A). Both forages fall well short of the necessary 4 MCals needed to support 1.5 pounds of weight gain after maintenance has been met. The producer must decide to settle for a diminished rate of gain or supplement the diet with a higher caloric feedstuff such as corn silage or beet pulp, for instance.

Due to more advanced nutritional modeling systems, net energy calculations are seldom used in calculating diets for lactating cows these days. Producers may still consult the National Research Council recommendations for NEl requirements for milk cows if desired. Milk cow diets that strive to feed all forages (even on lush, high protein pastures) seldom provide enough net energy to produce more than 50 pounds of milk per day after maintenance needs have been met by the forage.

It’s important to recognize that though forages are essential in ruminant diets, even the best forages are limited in supplying adequate net energy for high levels of milk production lactation or aggressive growth rates after maintenance needs have been met. Knowing the various net energy values of forages allows dairy farmers to mix and match feeds to their best application.

Cover photo DenisTangneyJr/istock