Understanding plant physiology improves pasture management

beef cattle on pasture

As the old saying goes: to be a successful cattle farmer, you must be a successful grass farmer.

The meaning behind the saying is that fat or good milking cattle are the product of plentiful, high-quality forage.

The key to being good at something will boil down to the ability of your understanding of it. Apply this to anything and everything, but for this article let’s apply it to the pasture field.

In general, grasses are four-fifths water. Out of the one-fifths of dry matter (DM); 80 percent are carbohydrates (both digestible and indigestible), 10-15 percent are proteins, and 5-10 percent oils and minerals.

These ratios change with the type of plant and maturity of the plant as well. Dry matter increases with maturity while protein levels typically fall.


If we focus on protein, which could be deemed as the most important indicator of forage quality, the main location of this nutrient is in the leaves.

In fact, the most abundant protein in the planet is something called Rubisco. To simplify it, Rubisco is the main protein/enzyme that takes carbon dioxide and puts it into a pathway to make sugar.

Rubisco is found in all plant life (with very few exceptions) but in higher abundance in C3 type plants, which encompasses the cool season grasses as well as Alfalfa.

Therefore, when we apply nitrogen fertilizer to our C3 forages, most of it goes into the production of Rubisco, which leads to photosynthesis and growth.

Rubisco is mainly found in the leaves of plants; that is why the highest quality portion of most forages are the leaves.


Nitrogen is one of the fastest responding nutrients that we apply.

Plants have an exponential type growth curve when conditions are favorable; they grow slow in the beginning and end of their growth cycle but very rapidly in the middle — the vegetative three-inch range.

This is one reason for specific applications times for nitrogen, a nutrient that does not stick around but is vital for plant growth.

The recommended nitrogen application for cool season forages is in the late spring and late summer. This is designed to capture the nutrient and ramp up vegetative growth in forages.

Nutrients like phosphorous are found in plant DNA and used for replication of cells and growth. Phosphorous deficient plants do not grow well.

Phosphorus is also a very responsive nutrient in terms of soil chemistry (pH). Phosphorous availability drops greatly when the pH is below 6.0 and plants have a hard time growing in these conditions, especially in phosphorus-deficient soils.

Liming is a crucial factor for making phosphorous more available to forages. Unlike nitrogen, lime takes months to react and neutralize soil acidity.

A fall application of lime is typically recommended if necessary. I always say, fall is the best time to lime but there is never a bad time to do so.

Potassium and magnesium are co-factors that enzymes use to work. They help transfer energy and make things happen.

Potassium will drive the pump in cattle rumen for the exchange of magnesium into the blood.

Too much potassium in the forage can inhibit magnesium exchange and lead to grass tetany especially in early spring.

This is the reasoning behind not spreading potash early in the spring.

Magnesium and nitrogen both are required for photosynthesis to happen. If either magnesium or nitrogen is deficient, yellowing occurs in the leaves.

Warm or cool

Most of us north of the Carolinas and Tennessee will probably step out onto cool-season grasses. By definition, these grasses prefer or rather grow best at cool temperatures, 40-75 F.

Warm-season grasses do best in temperatures of 75-95 F. These two types of grasses accomplish this because they have different ways of growing and photosynthesizing.

These differences also affect the composition of the plant altering its management, nutrient quality, and animal palatability.

Warm-season grasses are C4 plants, they require less water than C3 and are more efficient in photosynthesis.

Here is the kicker, they are more efficient and have less rubisco making the quality of these plants lower to livestock.

Additionally, they are less palatable due to higher lignin and waxes, which is why they do so well in hotter climates.


I will end by saying this, whether you are dealing with cool or warm-season grasses, vegetative growth will always be of higher quality than mature forages.

This is the pivotal point in having warm season paddocks in your pasture fields.

The understanding of how plants respond to various interventions by you, the manager, will make for a more productive pasture and by default make you a better cattle farmer/rancher.

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