Forage and Nutrient Utilization by Livestock Grazing Rotationally Stocked Pastures

Pastures are often the lowest cost option for ruminant livestock production. In much of the Northeast, pasture and aftermath grazed hay meadows are the largest portion of the agricultural acreage. University research provides guidelines on how to best manage pastures. However, how well livestock on farms use pastures is not often measured. This summary of on-farm pasture research provides a view of how well pastures are utilized when livestock graze rotationally stocked pastures.

Rotationally stocked pastures on farms in West Virginia and southwestern New York were sampled before and after grazing. Each of seven farms had four to eight paddocks, which were sampled two to three times between May and November over the course of two to three years. Farm locations ranged from 39 to 42 degrees North latitude, at elevations from 1,000 to 2,600 feet above sea level. Depending on the farm, pastures were grazed by beef cow-calf pairs, yearling steers or heifers, or lactating dairy cattle. Grasses in the pastures varied across farms and included orchardgrass, tall fescue, timothy, smooth bromegrass, bluegrass, bentgrass and quackgrass. The legumes in the pastures included white clover, red clover, birdsfoot trefoil and alfalfa.

In each pasture, 15 random forage samples were taken before and after grazing. Forage samples were clipped at ground level to recover as much of the live and dead material as possible. Forage mass (FM) is reported as pounds of dry matter (DM) per acre per grazing event. Pre-grazing initial forage mass (IFM) and post-grazing residual forage mass (RFM) samples were analyzed for crude protein (CP), neutral detergent fiber (NDF) and total digestible nutrients (TDN), reported as percentage of DM. The forage mass grazed (FM _Grazed) off by livestock was calculated as the difference between IFM and RFM for the grazing event. Using CP as an example, apparent intake (AI) of forage components was calculated as:

  CPAI = ((IFM x IFMCP) - (RFM x RFMCP)) / (IFM – RFM)

Each data point represents a pasture sampled during one grazing event. A total of 80 grazing events provided data on the utilization of IFM and 66 grazing events provided data on the utilization of forage nutritional components. There is much variability in sampling pastures due to diversity in plant species and livestock management. Therefore, 5% of the pastures having measurements extremely far from regression means (i.e., greater than two standard deviations) were considered outliers and removed.

Pasture Utilization

The forage mass removed by grazing cattle was directly related to IFM (Figure 1). Above a base forage mass of 700 pounds DM per acre, 54% of the forage was removed by the grazing livestock. This is in line with the old saying “take half and leave half.” The 700 pounds DM per acre is significant in that it represents a residual height of about 2 inches in an average pasture. This residual forage is tough and resistant to grazing and contains much of the carbohydrate reserves in grasses, such as orchardgrass. After accounting for IFM, grazed FM did not differ between farms. Two-thirds of the samples ranged within 311 pounds of the grazed FM trendline (Figure 1). In these on-farm projects, we had defined pre-grazing and post-grazing heights. However, due to the relationship between livestock, forage and human managers, there was a consistent pattern in grazed FM across farms.

Figure 1. Forage mass grazed off pastures (FM _Grazed, pounds DM per acre) was 54% of the forage above a 700 pounds DM per acre initial forage mass (IFM) [FM _Grazed = 0.54 (IFM – 700)]. 

Pasture Nutritive Value on Offer

As pasture IFM increased, the CP content decreased (Figure 2A), the NDF content increased (Figure 3A) and the TDN content decreased (Figure 4A). After accounting for IFM, there were no differences between farms in these measurements.

Forage Quality Consumed by Grazing Livestock

The apparent intake of CP, NDF and TDN in the forage mass grazed off the pastures increased as these nutritive components increased in the IFM (Figures 2B, 3B and 4B, respectively). These relationships accounted for 73% to 80% of the variability in apparent intake.

Selective Grazing

Selective grazing is the ability of livestock to consume forage of a higher quality than the average quality in the pasture. Selective grazing for CP was uniformly positive across pastures at 20% above (1.2 times) the CP content in the IFM (Figure 2B). When the IFM contained 20% CP, apparent intake CP averaged 24%. Selective grazing for NDF was not uniform (Figure 3B). Low quality pastures, having a high NDF content in the IFM, provided little selective grazing. When the IFM contained 65% NDF, there was no selection and average NDF intake was 65%. On high quality pastures having 45% NDF in the IFM, livestock selectively grazed and average NDF intake was 35%, 10 percentage units lower (Figure 3B). Since TDN is a function of NDF, selective grazing for TDN also was not uniform. On pastures having 50% TDN in the IFM, average TDN intake was 50%. On high quality pastures having 65% TDN in the IFM, average TDN intake was 70%, five percentage points higher (Figure 4B).

Figure 2. A) As pasture initial forage mass (IFM, pounds DM per acre) increased, the crude protein (CP) content in the IFM (IFM_CP, %) decreased.  B) As the IFM_CP increased, the apparent intake of CP (CP_AI, CP% in forage mass grazed) increased.

Figure 3. A) As the initial forage mass (IFM, pounds DM per acre) increased, the neutral detergent fiber (NDF) content in the IFM (IFM_NDF, %) increased. B) As the IFM_NDF increased, the apparent intake of NDF (NDF_AI, NDF% in forage mass grazed) increased.

Figure 4. A) As the initial forage mass (IFM, pound per acre) increased, the total digestible nutrient (TDN) content in the IFM (IFM_TDN, %) decreased. B) As the IFM_TDN increased, the apparent intake of TDN (TDN_AI, TDN% in forage mass grazed) increased.

Figure 5. When the number of days animals were in a paddock (Days_Stay) was seven days or less, forage species and management determined forage mass utilization. When days stay was greater than seven days, forage mass utilization (Util_FM) decreased.

Pasture utilization ranged from 11% to 80% of the IFM. Across this range, percent utilization had little effect on selective grazing, impacting only CPAI. When percent utilization was included in the IFMCP vs CPAI relation, the average absolute error improved by only one percentage unit, so percent utilization was not included in the analysis.

Length of Stay in Pasture

The length of time animals are in a paddock is termed the occupation period or length of stay. When length of stay in pastures was seven days or less, as recommended by WVU Extension, there was not a significant impact of length of stay on percent forage utilization (Figure 5). In the pastures where length of stay was less than seven days, forage utilization was determined by forage species and management. For example, the site PE in Figure 5 was dominated by smooth bromegrass having large tillers, with little else in the stand. These pastures had high grazing efficiency compared to orchardgrass and tall fescue dominated stands having dense tiller bases in the lower canopy that livestock avoided. When length of stay was greater than seven days, average forage utilization decreased (Figure 5).

Conclusions

Across farms, pastures differed in IFM, RFM, FM grazed, percentage of CP, NDF and TDN in the IFM, and percentage of CP, NDF and TDN in the FM grazed. Pasture forage height was not descriptive of the CP, NDF or TDN content in the IFM. However, IFM did a good job of describing trends in CP, NDF and TDN content in pasture IFM across farms. As IFM increased, NDF content increased while CP and TDN content decreased. After accounting for differences in IFM, there were no differences between farms in the FM grazed off the pastures. The apparent intake of CP, NDF and TDN were highly and positively related to the content of the respective component in the IFM.  After accounting for CP, NDF, or TDN in the IFM, the apparent intake of these components followed the same pattern across all farms

Livestock displayed selective grazing across pastures with selection occurring in high quality pastures. This demonstrates that managing for high pasture quality is needed for high animal performance. Animals not needing high quality forage will do well on lower quality pasture that does not allow selective grazing. The CP and NDF content in a pasture’s IFM is dependent on legume content (legumes are higher in CP and lower in NDF than grasses) and maturity of the forage (younger forage is higher in CP and lower in NDF than older forage). In spring when plants are growing rapidly, regrowth intervals between grazing events of three weeks provide good forage growth and high forage quality. In midsummer when cool-season forages have slower growth rates, regrowth intervals of five to six weeks provide adequate regrowth of high-quality forage. As summer regrowth intervals exceed six weeks, forage quality decreases as leaves age and die. After accounting for IFM and quality of IFM, utilization of FM and apparent intake of CP, NDF and TDN was uniform across farms and forage species on these farms.

Author: Ed Rayburn, Retired WVU Extension Specialist – Forage Agronomy

Last Reviewed: November 2022

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