A Little Science in Making Hay

Everyone knows you make hay when the sun shines. The reason depends on two scientific concepts you would learn in a good high school Physics course.

In making hay we dry the grass to a point where decomposition cannot occur, where the fungi and bacteria that cause decay can not multiply. In contrast, when making silage we prevent access to air, and need some moisture. This allows microorganisms to grow, producing alcohols and some organic acids which prevent further decay. It is well known that silage is very palatable to cattle, apparently the oxidation products first formed taste good to them. Humans also like some “partially spoiled” (really partially oxidized) foods: pickles, cheeses, alcoholic products, vinegar, yogurt, to name a few.

Likely there is some microbial action in most hay, you shouldn’t think of it as being 0% moisture. The objective is to keep the moisture content low enough so that it doesn’t progress to a stage where it is unpalatable to the animals. Frequently you can smell the action in hay after it is put up, for a week or two, but the hay is still quite acceptable to the animals.

Ideally, hay is put up as quickly as possible. You seek to avoid bleaching, which causes loss of the green color, an indicator of vitamin quality, so you want it to dry quickly. You also try to avoid getting it wet, because this will remove soluble compounds, among the most important being pentosans, five carbon sugars, which give it the characteristic odor of new mown (curing) hay. The sugars in a mammal’s body are almost entirely six carbon sugars, the most important of which is glucose, sometimes known as “blood sugar,” The cow’s metabolism changes five carbon sugars to six carbon sugars.

Ruminants (cattle, sheep, etc.) have a rich microbial flora in their rumen, a special stomach, which only ruminant animals have. The microbes are capable of digesting plant materials and converting them to compounds which the ruminant can readily absorb, and either use directly in its body or convert to compounds it can use. One kind of these is certain fatty acids which are absorbed directly from the rumen into the blood stream. Cattle can live on the fiber of grass alone, or on pure cellulose, if needed minerals and urea are also available. Cellulose is converted to glucose by the microorganisms.

There was a famous experiment several years ago in Sweden where a cow was fed shredded newspaper (almost pure cellulose), urea, the necessary minerals and water. She was able to survive and reproduce on that diet. But that was a well financed experiment, not something that is economically feasible. Well put up hay will contain protein, minerals and vitamins in addition to cellulose and the sugars which go a long way to make the hay palatable.

If hay is to be dried quickly it helps to have a warm day. Water evaporates more rapidly at higher temperatures. The vapor builds up near the source from which it evaporated, so a little wind helps by replacing the more nearly saturated air around the drying grass with lower moisture air. Turbulence carries it up away from the ground level.

The “dryer” the air, the more rapidly the water in the grass is removed. This brings us up to the first physics principle, relative humidity. A given volume of air is capable of containing only a certain mass (or weight) of water. If it contains all it can hold it is said to be saturated. If it has only half of what it can hold it is at 50% relative humidity. At 65% relative humidity air contains 65% as much water as it can hold. The lower the relative humidity, the more rapidly grass will dry. Hay makers in the West have an advantage over us in West Virginia, because of the dry air there!


Relative humidity varies widely from 100% to very low values. Water evaporates into the air over water and over vegetated land areas. The capacity of air to hold water is higher the greater the temperature. When air is cooled enough by contact with another, cooler, air mass it rains, because it is not able to hold all the moisture. If it cools just below its maximum capacity in any place, dew forms, if more cooling occurs, fog or clouds are formed.

Today I started to make hay when the relative humidity was 95%, but two things helped me, as the day wore on: the relative humidity dropped because it warmed up. The second thing was that it was a bright, clear day. The sun’s energy is about 446 watts per square meter (think square yard) at the top of the atmosphere. This is a little less than a 500 watt lamp. A lot of this gets through in summer, so the sold layer it hits first (the hay) is warmed considerably. This helps remove the water from the grass, into the air. The wind blows the water moisture away, and mixes it in the atmosphere. Presto! Dry hay! Tons of water gone from the hay field.

Now there is another principle we need to discuss here. It is the Latent Heat of Vaporization of water. You may recall that heat is measured in calories, and that one calorie is the amount of heat required to raise one gram of water one degree centigrade. In more modern courses this amount of heat is given as 4.184 joules, the measure of energy. (Heat is a form of energy.) In the physics class, it is explained that the heat energy is used (in large part) to increase the vibrational motion of the water molecules.

Now what is Latent Heat of Vaporization? It is the amount of heat required to break the molecules of water apart. It is 5.4 times the heat required to elevate the temperature of liquid water from freezing to boiling, some 2260 joules/gram. Latent Heat of Vaporization changes water from liquid to water vapor without increasing the temperature. This energy breaks the attractive forces between the molecules, and lets them evaporate to become a gas or vapor at the same temperature as the liquid water in the hay. A vast amount of energy from sunlight is used to dry hay. If it was not absorbed by vegetation through evaporating water the temperature would rise rapidly. If little sunlight is present hay will dry, but much slower, drawing the necessary energy from the surroundings, mostly by cooling the air.

The sun warms the hay at the surface of the ground, but the energy that goes to evaporating water cools the hay. You might say the two processes compete to change the temperature of the hay. When the hay begins to get dry, its temperature will rise, because water is not evaporating as rapidly, so the sun gets an edge.

As long as you keep water away from the dry hay, it will not decay further. Your cow will have a great assortment of compounds that were in the grass to flavor her dinner, and many compounds formed by the partial decay of the grass by the water you could not take out. She is a great gourmet and each day will savor the slight differences in hay from different parts of the field, including the effects of fertilizer on each part, different dryness conditions when it was put up, how broken the stems were, how many leaves were knocked off, weeds present, and a host of other factors. Put out two bales, one from a limed and fertilized field and one from a field without, both properly cured. See which disappears first.