Pasture Utilization - The Critical Factor

Author: Sam Angima

Publish Date: Spring 2009

In the past decade, there has been rising interest in reducing livestock supplementation/hay and increasing grazing and use of stock-piled winter forages. While this can be very successful depending on your location, there is a very important component that applies to all grazing types and that is pasture or forage utilization. Forage utilization looks at how efficient your animals utilize available forage over a period of time and how this may translate to average daily gain. Forage utilization and intensively managed rotational grazing go hand in hand. Remember that animals will intensively graze by nature, and only you can intensively manage them. So management is what you have to intensify, not just grazing.

Let us take an example of cows. When they take a bite of growing forage, the bite size and biting rate is governed by forage quality and quantity available. Each time they are turned into a new pasture, they rapidly graze the best forage leaving poor quality forage behind. Through defecation and urination, the remaining forage quickly loses quality and utilization rapidly declines. In fresh pastures, intake rate is high, and the cows get ‘full’ quickly, therefore minimizing time spent grazing. Remember that grazing uses energy, therefore the conserved energy is used for growth or other metabolic functions that make livestock production profitable. That is why we need to rotate the animals to allow for re-growth of pasture.

To have a good rotation, you need to choose a proper stocking rate. If three cows are waiting for every blade of grass as it comes up, the “solar panel” (leaves) will never develop. On the other hand, if grazing pressure is too light and the plants grow up and are left ungrazed, dead material accumulates and the solar panel loses efficiency. As with most things in life, balance is the key. Several things govern how this sequence should happen.

We know that forage quality decreases as plants grow older because they become stemmy with nutrients tied up in non-digestible forms (such as lignin). In the rumen, the fibrous fractions of forage are less digestible and require more time for digestion than the non-fibrous fractions; therefore forage intake and total energy intake generally decline as forages mature. Because the rumen is filled to capacity with most forage diets, any factor that speeds the passage of the diet through the rumen will allow the animal to consume more feed. The intake of highly digestible materials can be as much as 3 times higher for very high-quality forage than for poor-quality forage.

Now, a good manager needs to balance pasture quality with pasture growth. Usually, the best time to graze is immediately following the most rapid growth but before flowering and seeding. If enough vegetative matter is left after grazing (4-5 inch stubble), pasture growth is the least affected. At this stage, sufficient carbohydrate reserves have been built up to allow for rapid re-growth. On the other hand, if grazing occurs before this stage when the forage has not had time to rebuild its carbohydrate reserves, the yield will be low, the next re-growth may be slow and reduced, and winter survival may be decreased. The frequency and pace of re-growth is higher in spring and early summer seasons.

One of the cornerstones of a successful grazing system is having rest periods that are long enough to allow for rapid forage re-growth. Rest periods allow plants to grow more leaves and more leaves provide more energy for root growth and carbohydrate storage and plant vigor.

Rest periods should be of appropriate length to create an efficient “solar panel” but shorter enough to prevent the plants from becoming decadent. We always have to compromise between yield and quality that takes place with every pasture growth cycle because as pastures grow during rest periods, yield increases, but quality declines. The optimum balance depends on the type of livestock and physiological stage of production. This is why it is impossible to say that a rest period should always be so many days or that a pasture should always be so many inches tall before grazing begins. We can control how often stock comes back to that pasture and how frequently it is grazed. In some environments that may be once or twice a year while in other environments it may be five to ten times a year. Knowing how to balance yield and quality for livestock requirements is a critical skill in management intensive grazing.

How do you allocate pastures to the number of animals you have? An average mature cow weighs 1000 lbs and consumes on average 3% (30 lbs dry matter) of its body weight per day. If there are 1000 lbs/acre of dry matter available for grazing in one year, then you will have 33 ‘cow days” grazing this acre-pasture in that year. These 33 cow days can only be achieved profitably if pasture utilization is high like in rotational grazing systems. A study carried out in the mid-west found that if cows are rotated every 3 days, 7 days, or 14 days that corresponded to 70%, 50%, and 40% utilization rates respectively. If you use these percentages for the 33 cow days above, you can see that you lose quite a lot of days where your animals will be underperforming and you are not profitable.

On fertilization, it is estimated that you can reduce recommended spring nitrogen rates by 20% for the same yield goal on intensively managed pastures. When forages are intensively managed, it is important to fertilize only when increased forage production is needed depending on your stocking rate. Growers should think carefully about whether the excess spring production is necessary unless the excess production is going to be harvested as hay.

How many paddocks are needed to provide adequate rest for a pasture? It has been shown that four paddocks can provide 75% rest opportunity while eight paddocks provide 87% rest. Therefore we can conclude that to keep pastures more vigorous, most of the rest benefit of rotational grazing is achieved with just four to eight paddocks. In summary then – take half and leave half. This is a safe guideline that can help you determine the length of the grazing period to ensure greater forage utilization.