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On-farm observations help better predict the flow of nitrogen and phosphorus from beef cattle open lots.

June 16 2021 12:10 AM

Homolka is a student, and Erickson and Koelsh are professors, all at the University of Nebraska, Lincoln

Manure collected from open lot animal housing systems varies due to weather conditions, management of cattle and pens, and other factors resulting in substantial changes in manure characteristics. Understanding variability is important for planning nutrient management systems.

Planning often relies upon standard values published by the USDA Natural Resource Conservation Service (NRCS) and American Society of Agricultural and Biological Engineers (ASABE) for open lot beef cattle manure quantities and characteristics. However, results from a University of Nebraska study suggest significant challenges associated with planning based upon standard values for estimating manure characteristics. This means estimates using site and time specific manure analysis are critical for open lot beef systems.

A look at open lots

Defining the characteristics of manure and runoff from open, earthen lots creates unique challenges compared to animals housed under a roof. Some variables include:

• Impact of climatic conditions

• Distinguishing between compacted soil and manure

• Management practices (such as frequency of manure collection)

• Diets fed due to the ability of ruminant animals to utilize a variety of by-products, forages, and crop residues

To quantify the differences of open lot systems, data was collected from cattle finishing trials conducted at the Eastern Nebraska Research and Extension Center (ENREC) beef feedlot facility. Over a 15-year period, 416 unique pen observations were evaluated for the impacts of a broad range of weather conditions, dietary treatments, feedlot management practices, and nitrogen and phosphorous conservation practices (for animal performance data, see the 2021 Nebraska Beef Report). A pooled analysis of manure and nutrient characteristics from the pen data was performed.

Trial methods followed common procedures for estimating animal performance, nutrient intake and excretion, as-removed manure, and runoff quantities. Losses of nitrogen (N) and phosphorus (P) were estimated using a mass balance comparison of nutrient inputs and known outputs with the difference representing losses or unaccounted P (P remaining in the lot after cleaning).

Standard methods were followed for harvesting manure and determining mass. Representative samples were collected and frozen at minus 4°C until analysis. When rainfall occurred, runoff was collected, sampled, and quantified.

The nitrogen balance

Nitrogen entering a pen as feed will exit the pen in one of four ways: in the marketed animal (retained); as runoff holding pond water; as removed manure; or as N loss, occurring predominantly as ammonia volatilization (Figure 1). The evaluation of the independent pen measurements at ENREC suggests that N retained by the marketed animal (approximately 13% of N in feed) are consistent between winter and summer feeding periods. On the other hand, nitrogen retained in the manure and runoff and the amount that is lost is significantly different for winter and summer periods.

Nitrogen loss ranged from 65% to 44% of fed N for summer and winter, respectively. As a result of changes in loss during these feeding periods, the manure retained 0.11 pound per head per day for summer periods and 0.20 pound per head per day for winter periods (of the 0.54 and 0.50 pound per head per day of N intake as feed). Season (summer versus winter) is an important factor influencing N recovered from open lot systems.

Intake provides some explanation for observed variability of as-removed manure N and N loss for manure harvested following a summer feeding period, but it offers less explanation for the winter feeding period. (See the 2021 Nebraska Beef Report for a summary of statistically significant variables.)The data set suggests that an increase of dietary intake of 1 pound results in approximately a 0.30-pound bump in as-removed manure N during the summer. There was no relationship during the winter.

This review also showed an increase of dietary intake of 1 pound is responsible for a 0.61-pound and 0.84-pound rise in N loss for summer and winter feeding periods, respectively. A better correlation was observed between N intake and N lost for the summer months [R2 = 0.54 (P < 0.01)] and for the winter feeding period [R2 = 0.37 (P < 0.01)].

The correlation between organic matter and N in the manure (Table 1) suggests a strong relationship (R2 = 0.85 for summer and 0.70 for winter, P < 0.01 for both). Improving manure’s organic matter also appears to reduce N losses. Management practices that elevate manure organic matter will impact planning for as-removed manure N and may moderately reduce N emissions as well.

Measuring phosphorus

Phosphorus entering a feedlot pen as feed will exit the pen in the marketed animal (retained), as runoff holding pond water, or as manure (Figure 1). Again, the fraction of P retained by the animal remained relatively constant for summer and winter periods.

The as-removed manure P was substantially greater in the winter than summer, exceeding the winter estimate of excreted P. The manure P for summer and winter represented 47% and 95% of fed P, respectively. The runoff P during the summer was double that observed during the winter feeding period (6% versus 3% of fed P).

These results suggest that a P balance based upon these four inputs and outputs left some P unaccounted, approximately 0.02 pound per head per day in the summer (likely left on the lot surface or mixed in the soil) and minus 0.01 pound per head per day in the winter. Pen cleaning practices in fall following a summer feeding period (lot surfaces are drier and the soil-manure interface is more easily maintained) resulted in some excreted P not being removed from the pens.

Spring pen cleaning following the winter feeding period more likely involves muddy conditions (and less easily defined soil-manure interface) with more soil and additional P being removed beyond what is excreted. Differences in ash content appear to support this conclusion. These findings suggest that pen cleaning following the winter feeding period was removing P left behind during the cleaning at the end of summer.

Efforts to explain variability in manure P recovery based upon feed P intake demonstrated weak correlations (Table 1). However, planning procedures for managing manure P should recognize the significant differences between winter and summer feeding periods for as-removed manure P.

Evaluating the manure solids

Significant seasonal and individual feeding period variability in the amount of manure harvested was also observed. Variability in the amount of as-removed manure quantity occurs even when following predefined protocols for managing pen surfaces, as are used at the ENREC feedlot.

Total manure, total solids, total organic matter, and total ash were all significantly greater for the winter versus summer feeding period when expressed on a unit mass per head per day basis. For example, cleaning following the winter feeding period removed 47% more ash (most likely soil), 87% more organic matter, and 56% more total manure.

These observations of variation in manure solid characteristics between the winter and summer feeding period (and similar previous observations for N and P) suggest the need for characterizing and managing manure independently based upon the feeding period. Differences at the ENREC feedlot are due, in part, to a summer feeding period, which included higher N and P feed intake, a shorter feeding period, and larger animals entering the lot. Differences in weather conditions and pen surface conditions are likely important contributors to variability, commonly impacting the amount of soil contamination that occurs.

Compared to the standards

As animal performance, feeding program options, and other management practices evolve, standard methods for predicting feedlot manure characteristics and quantities are prone to greater errors. When ASABE assumptions and estimates are compared with field measures in this study, the following observations were made:

1. Greater total feed intake, higher average daily gains, and greater finishing weights were observed for the animals finished at the ENREC feedlot than the assumed values in the ASABE standard (originally published in 2004), better reflecting industry trends.

2. ASABE underestimates the dietary N intake and excreted N observed for the ENREC feedlot. Our observed P dietary intake and excretion was also greater than estimated by ASABE.

3. ASABE substantially underestimates total manure, dry matter, organic matter, and ash for winter feeding periods. For example, total as-removed manure averages for both winter (28.9 pounds per head per day) and summer feeding periods (20.5 pounds per head per day) were greater than that of ASABE standards at 16.5 pounds per head per day. The ENREC data set also suggests more ash in the manure than anticipated by ASABE.

4. As-removed manure N following the winter feeding period for the ENREC feedlot was similar to the ASABE estimate but substantially less after the summer feeding period. Manure P levels were substantially less than reported by ASABE (more than 50% less in the summer-feeding period).

In addition, ammonia volatilization from open lots is substantial. For every 1,000 head finished at the ENREC feedlot, the nitrogen loss is approximately 46,000 pounds and 38,000 pounds of N for the summer and winter feeding periods, respectively. This loss is an environmental risk and represents an annual economic loss of roughly $35,000 per 1,000 head for the ENREC feedlot. Experience would suggest that by doubling organic matter in the manure, one might expect to retain approximately two-thirds more nitrogen in the manure.

Standard values for estimating excreted and as-removed manure have historically been used for many planning and design procedures, including the development of nutrient management plans. These standard values have little to no validity in earthen, open-lot animal housing based upon these observations for the ENREC feedlot.

In commercial yards that harvest manure following each feeding period (or possibly more often), separately monitoring and managing manure for unique feeding periods is important. Base nutrient planning processes on manure sampling protocols that establish a history of feedlot-specific manure characteristics, including separate histories for manure removed following winter and summer feeding periods. Due to the high degree of variability in manure characteristics, individual year adjustments for manure and fertilizer rates are essential and should be based upon a current manure sample analysis.

Financial support for this research came from the Robert and Karla Baltzell Student Innovation Award at the University of Nebraska-Lincoln. This information was originally published in the 2021 Nebraska Beef Cattle Report

This article appeared in the May 2021 issue of Journal of Nutrient Management on pages 19-21.

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