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Reducing phosphorus inputs for grazing Holstein steers¹

A. M. Brokman^*, J. W. Lehmkuhler^*,2 and D. J. Undersander

^* Department of Animal Sciences, University of Wisconsin, Madison 53706 Department of Agronomy, University of Wisconsin, Madison 53706

	Abstract

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
A 2-yr study was conducted to confirm that managed pastures can provide Holstein steers adequate P to meet their daily requirement. Treatments offered were trace mineralized salt with or without additional P. In the first year, 80 Holstein steers (248 kg of BW) were assigned to 4 grazing groups. Treatments were trace mineralized salt only or a 67:33 mixture of trace mineralized salt and dicalcium phosphate. Steers rotationally grazed a cool-season grass/legume mixture for 137 d. Fecal bags were placed on 3 steers from each grazing group (n = 12) over a 4-d period for estimation of forage DMI and forage contribution to daily P intake twice during the grazing season. Analyzed pasture samples contained 3.28 mg of P/g of DM. During the second year, 72 Holstein steers (297 kg of BW) were blocked into 2 BW groups and subsequently assigned to 1 of 4 pasture groups. Steers rotationally grazed the same forage base as the first year for 126 d. Pasture samples contained 3.27 mg of P/g of DM. No significant differences (P > 0.10) were detected for BW, ADG, or free-choice supplemental mineral intake. Forage provided 126% of the recommended NRC P requirement. Thus, supplemental phosphorous was not required for Holstein steers grazing mixed, cool-season, grass/legume pastures.

Key Words: cattle • grazing • Holstein • mineral • phosphorus

	INTRODUCTION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Soil P concentrations above crop requirements are becoming more of a concern with respect to animal agriculture. Soil P accumulation may increase the risk of sediment runoff containing high P concentrations which can contaminate surface waters and represent an economic loss to producers (CAST, 2002). Recent research investigating the P requirement of finishing cattle on high grain diets suggests the requirement is near 0.14 to 0.16% of diet DM and below current recommended levels (Erickson et al., 1999, 2002). Phosphorus supplementation of grazing beef cattle is often performed without consideration to soil fertility or forage P concentrations. A recent survey of Iowa graziers reported that only 33% of respondents soil sampled at least every 5 yr and yet 77% reported fertilization as a method to improve pastures (Ellis et al., 2007). Forage P concentrations from central Iowa were reported to vary between 0.12 to 0.25% (Haan et al., 2007). Fertilizing according to recommendations based on soil sampling produced forages with P concentrations averaging 0.25% (Coffey et al., 2005), which is greater than the suggested P requirement for steers gaining 1.0 kg/d. Additionally, a common recommendation for supplementing P to grazing beef cattle is to mix dicalcium phosphate with salt (Church, 1988), or a commercial mineral supplemental is routinely offered that contains 4 to 12% P. Lastly, P supplementation needs of grazing Holstein steers has not been thoroughly evaluated. Based on the available information, P supplementation of grazing Holstein steers does not appear to be warranted. Therefore, a trial spanning 2 yr was conducted to confirm that the removal of P from the mineral supplement offered to grazing Holstein steers would not be detrimental to performance.

	MATERIALS AND METHODS

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
These trials were conducted under the approval of the Animal Care and Use Committee of the College of Agricultural and Life Sciences, University of Wisconsin, Madison. Cattle grazed pastures at the University of Wisconsin Lancaster Agricultural Research Station during 2002 and 2003.

Animals

Holstein steer calves were purchased from backgrounding operations. After arrival at the research station, steers were ear-tagged and administered an intranasal vaccine for protection against parainfluenza type 3 and infectious bovine rhinotracheitis (TSV-2, Pfizer Animal Health, New York, NY). Steers were boostered for parainfluenza type 3 and infectious bovine rhinotracheitis and vaccinated for bovine viral diarrhea and bovine respiratory syncytial virus (Bovishield, Pfizer Animal Health) and clostridium diseases 4 d after arrival and were dewormed with pour-on eprinomectin (Ivomec Eprinex, Merial, Duluth, GA) 13 d after arrival. Sulfamethazine was administered via drinking water for 5 d postarrival to prevent shipping-related illness. Hay was offered ad libitum, along with approximately 2.3 kg/d of whole-shelled corn for 30 d postarrival. After a 21-d training period to individual Calan electronic gates (American Calan, Northwood, NH) calves were administered tags embedded with pyrethrin and dewormed again.

During the first grazing season, 80 Holstein steers (248 ± 24 kg of BW) were stratified by BW and were allocated to 1 of 4 groups, such that the groups had similar initial average BW. Half of the steers within each group were randomly assigned to trace mineralized salt (TM) only, and the remaining steers were assigned to trace mineralized salt plus dicalcium phosphate (TMD). Two-thirds salt and one-third dicalcium phosphate was mixed to attain a 6% P concentration in the TMD mineral mix. Bunks with Calan gates allowed the steers ad libitum access to the assigned treatments while grazing similar pastures.

Steers rotationally grazed pastures consisting of predominantly fertilized cool-season grasses and legumes for 137 d. Pasture size varied based on visual assessment of height and density relating to estimated forage availability. Animals were rotated twice weekly, resulting in 3- or 4-d intervals in a paddock. Fixed grazing areas were not defined for each group, and pasture locations and composition varied over the course of the grazing season. Initial and final BW were the average of BW obtained on 2 consecutive days.

Parallel to this experiment, 20 steers were divided into 2 grazing groups of 10. One group was offered TM and the other TMD for ad libitum intake. A conventional mineral feeder was placed in each pasture to observe mineral intake for a period of 20 d after turn-out to determine if the Calan gates affected mineral consumption.

During the second season (126 d), 72 Holstein steers (297 ± 30 kg of BW) purchased from Nebraska were implanted with 40 mg of trenbolone acetate and 8 mg of estradiol (Revalor G, Intervet Inc., Millsboro, DE). Steers were vaccinated as described above, as well as being vaccinated for Moraxella bovis. Steers were blocked by BW into light (273 ± 19 kg) and heavy (321 ± 18 kg) blocks consisting of 18 animals per group. During both seasons, steers were treated for pinkeye throughout the grazing season, as necessary.

Sample Collection

Weather data, including maximum and minimum temperature as well as precipitation, were recorded at the University of Wisconsin Lancaster Agricultural Research Station. These data were averaged by month.

Mineral disappearance was measured every 3 to 4 d or after a rain event. The amount of mineral remaining and added was recorded to calculate the average consumption at each move or when necessary. Due to excessive precipitation entering the feeders during rain events, mineral intakes were calculated from time periods that excluded these observations.

Forage samples were obtained every 2 wk from 4 random quadrats (0.23 m²) per grazing group paddock and dried in a forced-air oven at 55° C for forage quality analysis (n = 40 for season 1 and n = 32 for season 2). All 55° C-dried forage samples were ground with a cross-beater Retsch SM 100 grinder through a 1-mm screen (F. Kurt Retsch GmbH and Co. K. G., Germany). Samples were composited by pasture for similar dates and analyzed in duplicate to determine the P concentration (AOAC, 1980). Nitrogen (LECO FP 528 Nitrogen Analyzer, Leco Instruments, Inc., St. Joseph, MI), NDF with -amylase and Na₂SO₃ addition and sequentially ADF (ANKOM²⁰⁰ Fiber Analyzer, ANKOM Technology Corporation, Fairport, NY), and IVDMD (ANKOM D200 Daisy II Incubator, ANKOM Technology Corporation) were determined for forage samples. The DM content was determined by drying samples in a 100° C oven for 24 h (AOAC, 1990). Ash content was determined after incineration at 500° C for 24 h in a muffle furnace. Data were reported by period for each season. During season 1, periods corresponded to the following time periods: period I = d 1 through 28, period II = d 29 through 61, period III = d 62 through 81, and period IV = d 82 through 137. Periods during season 2 were as follows: period I = d 1 through 33, period II = d 34 though 67, period III = d 68 through 103, and period IV = d 104 through 126. During season 2, no forage samples were collected during period IV, but mineral intake measurements continued.

During season 1, 3 steers were randomly selected from each of the 4 grazing groups for total fecal collections using fecal bags. Because selection of the steers was blind to treatment, the resultant observations per treatment were n = 9 TM and n = 3 TMD. Total fecal collections were used to estimate pasture intake, P intake, and apparent P digestibility for grazing Holstein steers. Ten days before the first collection, steers were adapted to fecal bags for 2 d. Total feces were collected for 4 consecutive days on d 52 through 56 and d 115 through 118, with the bags emptied and sampled twice daily. Fecal contents were thoroughly mixed, and a 20% subsample was retained and frozen for later analysis. Dry matter intake was calculated as: fecal output, g/DM indigestibility determined from IVDMD. Apparent P digestibility (%) was calculated as: [(total P intake, g/d – P fecal excretion, g/d)/total P intake, g/d] x 100. One fecal sample from season 1 collection in July was omitted because the sample quantity was insufficient for analysis. Forage samples with the closest dates to the fecal sampling period were utilized in apparent digestibility calculations. Dried fecal samples were processed similarly to the forage samples, with P, DM, and ash determined. Chemical analyses were corrected to a 100° C DM basis.

A total of 7 mineral samples from each mineral supplement were collected during the first grazing season and 11 were collected during the second season. Three or 4 consecutive samples were composited on a weight basis, resulting in 3 composited mineral supplement samples from each treatment for each grazing season, which were analyzed at the University of Wisconsin Soil and Plant Analysis Laboratory, Madison, WI. Samples were analyzed using inductively coupled plasma optical emission spectrophotometry (IRIS Advantage, Jarrel Ash, Franklin, MA).

The NRC computer model (NRC, 1996) was utilized to determine P balance based on the observed forage and mineral intakes and rates of BW gain. An average BW over the grazing seasons of 349 kg, with an ADG of 1.0 kg, was entered into the model. The NRC assumes a P maintenance and gain requirement of 16 mg/kg of BW daily and 3.9 g/100 g of retained protein, respectively. Using the retained energy equations of the NRC (1996), it was estimated that 153 g of protein was retained daily. Predicted P for maintenance was calculated to be 5.6 g/d and predicted P for gain was calculated to be 6.0 g/d. The absorption factor of 68% (NRC 1996) was utilized to determine total daily dietary P intake.

Statistical Analysis

Season 1 and 2 data were analyzed separately due to differences in experimental design. Data analysis was performed using the MIXED procedure (SAS Inst. Inc., Cary, NC), with group as the experimental unit (n = 4). Treatment effects on animal performance were tested for group and period interactions in season 1, with spatial power as the covariance structure. In season 2, block effect was included in the model, again using spatial power as the covariance structure. Forage quality was analyzed to allow investigation of period effects, with group included in the model as a random variable. Individual animal gains within treatment and grazing group were investigated for outliers. Outliers were determined as being greater than 1.5 times the interquartile range. Mineral supplements were analyzed using the GLM procedures of SAS, with year, treatment, and treatment within year included in the model. Results are reported as least squares means, with the largest SEM reported where unequal observations were analyzed.

	RESULTS

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
During both seasons, the average summer (June, July, and August) temperatures were near the long-term temperature mean of 20.8° C (Young, 2007), being 21 and 20.8° C for seasons 1 and 2, respectively. Above average precipitation was observed for the summer period (June, July, and August) during season 1, totaling near 42 cm compared with the long-term average of 31 cm. Drought conditions were observed during season 2 with summer precipitation being only 60% of the long-term average. These conditions during season 2 resulted in greater land area utilized to ensure adequate forage availability. This larger area may explain some of variability in forage nutrient concentrations described below.

The P content of the mineral supplement for TMD was 6.2 and 6.5% for seasons 1 and 2, respectively (Table 1). The TM mineral supplement contained minimal amounts of P, K, and Al. Samples of TMD supplement contained greater concentrations of Ca, Mg, S, Fe, and Al (P < 0.05). Concentrations of Na, Cu, Mn, and Zn appear to have been diluted by the addition of dicalcium phosphate when compared with TM.

Based on total fecal collections from season 1, no treatment differences (P > 0.05) in DMI or total P intake were observed for mid or late-season time points (Table 4). Forage P intake ranged between 23 and 32 g/d and did not differ by treatment (P > 0.05). Steers assigned to TMD had increased P fecal excretion (P = 0.01) for the midseason collection but no differences were detected in the latter collection. Additionally, no differences in apparent P digestibility were detected with estimates ranging between 44 and 59%.

	DISCUSSION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Apparent P digestibility values are similar to those found by Wu (2005) in dairy cattle consuming a diet with a P concentration of 0.33%. When P is fed above requirements, apparent P digestibility decreases (Karn, 2001; Wu et al., 2000; Ternouth et al., 1996; Challa and Braithwaite, 1989). Betteridge et al. (1986) suggested that urinary P excretion is less than 1 g/d in adult cattle consuming 30 g of P/d. However, genetic variation could contribute significantly to urinary P excretion (Siebert and Saunders, 1976; Field et al., 1984) in turn having an impact on P retention estimates. Urinary P excretion was not measured in this study and may have contributed significantly to endogenous P loss (Field et al., 1984). In recent research involving feedlot cattle consuming diets with various inclusion rates of distillers grains and concentrations of dietary P, Meyer et al. (2006) illustrated a curvilinear relationship between P intake and total, fecal, and urinary excretion of P. Based on P intake observed in these grazing steers, estimates of urinary excretion using their equations would suggest urinary excretion losses of 4 to 6 g/d. Using these figures, the P retention for our grazing Holstein steers would be similar to their feedlot cattle. Estermann et al. (2002) reported P retention levels of 9 to 23 g/d for calves and 11 to 15 g/d for cows. The P retention levels appear quite similar between classes of cattle and this warrants further investigation.

Although P intakes were similar between TM and TMD steers, TMD steers had significantly higher fecal P excretion in the first fecal collection. This may be a result of the P in the mineral being more biologically available than forage P. Because forage supplied P in excess of the recommended NRC (1996) requirement, an increase in the biological availability of P could have increased plasma P, thereby stimulating greater endogenous losses (Ternouth, 1989; Coates and Ternouth, 1992). Individual animal variation could have skewed fecal P excretion estimates as well due to the number of observations in this trial. High Ca:P ratios may also depress phosphate absorption (Schneider et al., 1985). The Ca:P ratio of the harvested forage was 1.5:1, and TMD contained close to a 1:1 Ca:P ratio, so Ca:P ratio was not considered to be a confounding factor for apparent P absorption. Additionally, pasture selectivity or variation in forage consumed may have impacted these results. It is not clear what main factors contributed to the observed P excretion level, and this warrants additional investigation.

Mixed results have been reported in the literature regarding supplementing cattle with P. Phosphorus supplementation has improved live BW gains (Preston and Pfander, 1964; Winks et al., 1977; Little et al., 1978). Black et al. (1943) observed a BW increase of 57 kg in heifers 18 mo of age fed supplemental P. However, Karn (1995) reported that BW gains of replacement heifers were not affected by P supplementation. Cohen (1972) observed Australian yearling steers grazing pasture with a P concentration of 0.043 to 0.11% did not respond to P supplementation over a 1-yr period. These levels are much lower than that observed in our study and, thus, based on these earlier results, one would not anticipate a response in animal performance to additional supplemental P in the present study.

Responses to P supplementation are typically observed when cattle graze forages grown on P deficient soils resulting in low forage P concentrations; this was the condition for much of the earlier research. This early work is limited in its applicability in many areas of the United States today. Wisconsin’s average soil P concentration for the state was reported to be 52 mg/kg based on samples from 1995 through 1999 (Combs and Peters, 2000), much higher than the Northern Plains region of 10 to 18 mg/kg (Johnston, 2006), which subsequently may result in higher forage P concentrations in Wisconsin pastures leading to lower P supplementation needs. Thus, P supplementation strategies for grazing beef animals needs to account for regional soil fertility differences.

Responses to P supplementation of cattle grazing P deficient pasture may also be confounded by limited protein. Little et al. (1978) observed that P supplementation does not affect live BW gain if N is limiting. In our trial, the N concentration in the harvested forage provided CP in excess of the daily requirements for Holstein steers at the observed rates of gain and would not confound responses to P supplementation.

Consumption of free-choice mineral supplements by grazing cattle was erratic in season 1 and season 2 (data not shown). Mineral intakes per animal ranged from 12 to 77 g/d. Pasture quality, composition, and weather may have affected mineral consumption and contributed to the variation observed. These variations in intake are supported by Coppock et al. (1972) who measured individual mineral consumption by lactating dairy cows to range from 0 to greater than 1,000 g/animal daily. However, Tait et al. (1992) reported mineral consumption of grazing Holstein steers (350 kg) to range between 60 and 330 g/d. Most trace mineral and complete mineral supplements have targeted intake levels of 57 to 113 g daily. Average consumption of season 1 and 2 steers was 46 g/d. Therefore, these data are considered near the normal range for steers grazing similar pastures.

Variation of forage quality among treatments and periods was expected. Pasture composition varied among groups and throughout the grazing season due to soil characteristics and weather. Forage quality was highest during period I and II which would correspond with early season vegetative growth and advancement into the reproductive stage of the forage. As the plants matured, forage cell wall content increases resulting in lower digestibility and quality. With the pasture management employed, large forage quality differences were not anticipated because the forages would be allowed rest and regrowth periods that would sustain similar quality. Forage NDF, ADF, and CP values were similar to those reported by Martz et al. (1999) for samples obtained from mixed cool-season grass and legume pastures.

Forage availability can influence animal performance as well. Calculated DM consumption was 644 kg for 20 steers grazing a paddock for 4 d based on observed intakes. A paddock size of 0.40 ha provided 774 kg of forage DM exceeding projected intake levels. It was concluded that visual assessment of forage availability and subsequent paddock size was not a limiting factor in steer performance.

The pastures grazed in these trials were managed such that they resulted in high quality forage that provided adequate protein (19%) and P (3.27 mg/g of forage DM). These values were above the recommended requirements and a gain response to P supplementation would not be expected. A total of 55 hay samples taken in 2003 and 2004 from beef cattle operations in 17 Wisconsin counties had an average P content of 2.5 mg/g of forage DM (J. W. Lehmkuhler, unpublished data). It is speculated that these hay samples reflect similar forage species and soil types that would be utilized during the grazing period. These hay samples would therefore suggest that many areas across the state would not require additional supplemental P as the forage P would provide adequate P intake for stocker steers.

Between the years 1997 and 2001, 607 pasture samples were collected in various regions of West Virginia (Rayburn et al., 2006). The pasture species composition of cool-season grasses and legumes in their samples was quite similar to the pastures grazed in our trials. The P content of the West Virginia forage samples was between 2.7 and 4.1 mg/g of DM. The forage P content was reported to be inadequate for high-producing lactating beef cows in only 10 to 15% of the pastures sampled. However, P levels in pasture samples were stated to be adequate for a 250-kg steer gaining 1.0 kg/d. Reid et al. (1990) reported P concentrations in hay of 4.4 and 2.3 mg/g forage DM from orchardgrass (Dactylis glomerata L.) or timothy (Phleum pretense L.) mixed with 25% red clover (Trifolium pretense L.), respectively. Thus, the data reported for our study fall within the range reported by others grazing similar forage types. These data would also suggest that P supplementation is generally not necessary for grazing stocker cattle in Wisconsin and likely in several regions across the Midwest because the forage alone contains adequate P concentrations to meet the requirements of growing steers.

	Footnotes

 
¹ Funded by USDA CSREES HATCH.

² Corresponding author: jwl{at}ansci.wisc.edu

Received for publication March 29, 2007. Accepted for publication November 30, 2007.

	LITERATURE CITED

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 


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