How Much Do Cows Eat on Rangeland?

Recent research conducted in a University of Idaho study aims to gather more information on this age old question

BY JIM SPRINKLE

UI Nancy M. Cummings Research Extension & Education Center, Carmen, ID and

Carmen Willmore

Livestock & 4-H; UI Extension Bingham County, Blackfoot, ID

Introduction

How much do cows eat on rangeland? This is a simple question that has proven difficult to answer. In a confined feeding situation, it is easy to measure how much is fed to animals daily. This is a much more difficult proposition with free-ranging livestock. Yet, animal scientists conducted research decades ago to attempt to answer this question and the University of Idaho conducted research led by Carmen Willmore in 2016 at Rinker Rock Creek Ranch to further address this question.

Some of the first attempts to estimate forage intake are illustrated by Figure 1. If we can obtain an estimate of forage digestibility through representative samples of the diet, we can calculate forage intake as fecal output ÷ (1 – digestibility). In the research shown in Figure 1, Chad Gibson was obtaining total fecal collection on this steer and others with a big diaper. (He was also obtaining grazing and resting time with the vibracorder shown attached to the steer’s halter.) This was a very laborious effort, and it can be expected that carrying around a big diaper would impede livestock movement. However, at the time this was the best that could be done and was a real game changer for getting estimates of forage intake. Later on, some ingenious scientists came up with a plan to orally administer a unique marker such as chromic oxide that was not commonly present in the diet. Thus, fecal output could be estimated by measuring the dilution of the known dose of the marker within the grab fecal samples obtained once or twice a day. This eliminated total fecal collection and allowed one to use females in the forage intake research. Administering a daily dose of the marker required a concerted daily effort and it also disturbed grazing activity of the cow. Later on, an agricultural company in New Zealand developed a spring-loaded plastic capsule with a pellet of the desired marker that lodged in the reticulo-rumen and constantly released marker over a period of 21 days. Unfortunately for range livestock nutritionists, this product has not been available for over two decades.

Ui Forage Intake Project

It was our desire to characterize the forage intake for 24 twoyear-old lactating cows at both mid- and late-lactation while grazing rangeland at Rinker Rock Creek Ranch in June and August of 2016. We further split these cows into cows that had previously been characterized as yearling heifers as either efficient (ate less than expected for the weight and growth) or inefficient (ate more than expected, greater appetite) with the GrowSafe units in the feedlot at the Nancy M. Cummings REEC. Milk production was estimated at peak lactation (March) and late lactation (August) using weigh-suckle-weigh procedures.

Since there were no constant release boluses available to dose cattle with and because we did not want to disturb grazing behavior with daily dosing of a marker, we opted for an alternative dosing of the marker. We can do a one-time dose of a larger amount of marker, called pulse dosing, and recover the marker in repeated fecal samples in the field. Carmen absorbed a C32 alkane marker that was commercially available onto filter paper after heating the alkane in heptane (80° C) and pouring it over the filter paper to dry. The filter paper was then shredded to 2-inch strips with a paper shredder and then loaded into 6 gelatin capsules for each cow. The actual dose of the alkane was only 3.91 grams and the entire weight of the gelation boluses for each cow was 39 grams. These boluses were given to the cows with a bolus gun (3 boluses fit into the gun, given twice) as shown in Figure 2. The pulse dose was administered to the cows 18 hours prior to obtaining repeated fecal samples starting at daylight on the following day. We cordoned the 24 cattle off within an 80-acre piece of rangeland with a temporary electric fence. We had 4 to 5 people available to follow the cows and obtain fecal samples (Figure 3). On the first day we collected all the samples we could during the daylight hours. On the second and third day, we attempted to collect three fecal samples early morning, mid-day, and midafternoon. On the fourth and final day of sampling, we attempted to collect fecal samples in the morning and early afternoon. From this, Carmen was able to construct a nonlinear logarithmic curve of the alkane marker, and with some advanced math, not only obtain forage intake but also measures of digesta kinetics such as passage rate of the forage, forage retention time, and relative size of the gastrointestinal tract. lot derived forage intake values from the National Research Council to a rangeland setting. We apply an activity adjustment (multiply calories needed by 1.3) and add in energetic costs for estimated milk production (based on weaning weights and day of calving) to come up a maintenance requirement. Using these adjustments, the actual intake obtained from this experiment was 97% of the estimated forage intake.

Forage digestibility and crude protein were estimated by chemical analyses of random forage plots (forage available) and by near infrared spectroscopy (NIRS) of daily fecal samples from each cow (forage consumed).

HOW MUCH DID THE COWS EAT?

The two-year-old cows in this project weighed approximately 1,020 pounds and consumed from 2.4 to 2.9% of body weight in forage. There was no difference in intake between efficient and inefficient cows. Forage quality declined from 59% to 54% digestibility from June to August and the forage retention time increased from 39 hours to 44 hours from June to August. Total grazing time (estimated using grazing collars with accelerometers) tended to increase from June (606 minutes/d) to August (643 minutes/d).

Oftentimes, producers may just attempt to apply the 2% of body weight rule to estimate forage intake. While this rule may have validity with animals in a confined system or a small pasture, it can vary greatly from rangeland conditions. Range livestock nutritionists such as myself attempt to “tweak” the feed-

Figure 4 illustrates the expected forage intake for a range cow in Idaho (March calving). The figure presented is based upon the standard Animal Unit Day used in range management for a 1,000 lb. cow and her calf. Larger cows should use the appropriate multiplier; for example a 1,300 lb. cow at a body condition score of 5.0 would require a multiplier of 1.3 for the values shown in Figure 4. If you provide a protein supplement during times of forage dormancy, intakes should increase approximately 7% and digestibility by 5% for cool season dominated forage. The values shown in Figure 4 illustrate the lag effect of peak forage intake which occurs at mid-lactation. Peak milk production is usually at 50 to 60 days of lactation.