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* Center for Animal Health and Productivity, School of Veterinary Medicine, University of Pennsylvania, Kennett Square 19348
Dept. of Dairy Science, Cornell University, Ithaca, NY 14853
Dept. of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061
Dept. of Animal and Avian Science, University of Maryland, College Park 20742
|| Dept. of Plant and Soil Science, University of Delaware, Newark 19717
# Dept. of Dairy and Animal Science, Pennsylvania State University, State College 16802
Corresponding author: Z. Dou; e-mail: dou{at}cahp.vet.upenn.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: dairy farm • phosphorus • diet modification
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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A viable and cost-effective approach to reduce P surplus on dairy farms is to minimize excess P fed to animals through diet modification (CAST, 2002). By balancing P supply and intake against cow requriements, excess P in diets may be reduced without impairing cow performance and in the meantime decreasing P excretion in manure. This has been demonstrated by a number of studies in recent years. Results from research trials indicated that dietary P for lactating cows may be reduced by 20 to 50% from levels commonly fed on farms without sacrificing milk yield (e.g., Valk and Sebek, 1999; Wu and Satter, 2000; Knowlton and Herbein, 2002). Decreases in excess dietary P not only lower the excretion of total P in manure, but more importantly, reduce water-soluble P substantially (Dou et al., 2002). Water-soluble P content in manure correlates significantly with P loss in runoff (Sharpley and Moyer, 2000; Kleinman et al., 2002). As expected, P management via diet manipulation also decreases land acreage requried for spreading manure (Powell et al., 2001).
According to Satter (2003), a typical dairy cow fed with a dietary P concentration of 4.80 g/kg of feed DM would generate 24.7 kg of P in manure per lactation (305 d). This would require approximately 0.81 ha of cropland for spreading manure on a P-based application plan, assuming a P removal of 12.1 kg/ha per year for typical crop yields in the Midwest. In constrast, if the cow is fed at 3.50 g of P/kg of feed DM, an adequate level according to NRC and numerous research projects, the amount of manure P excretion would be reduced to 15.8 kg per lactation and the land requirement reduced to 0.53 ha for the Midwest crops, a 35% reduction. The potential of manure P reduction by eliminating unnecessary P in diets is obvious when this estimation is aggregated to regional or national levels. For example, in New York and Pennsylvania, ranked 3rd and 4th in dairy production in the United States with a combined lactating cow population of 1,271,000 (NASS, 2003), manure P could be decreased by more than 11 million kg/yr. Nationwide, an annual reduction of over 80 million kg of P in manure may be achieved for the 9 million lactating cows (NASS, 2003).
The projected decrease in manure P excretion may not be beyond reach because dietary P concentrations near or above 4.80 g of P/kg of feed DM are fairly common on farms, as revealed by several recent surveys. Shaver and Howard (1995) tracked the feeding programs of 11 high-producing herds in Wisconsin and found an average of 5.70 g of P/kg of ration DM. The authors commented that TMR formulated at 5.00 g of P/kg of DM were common. Sink et al. (2000) reported 4.90 g of P/kg on 33 Virginia dairies surveyed. Satter and Wu (1999) reported an average of 4.80 g of P/kg through a telephone survey of university extension personnel, nutrition consultants, and feed industry personnel. Sansinena et al. (1999) reported 4.90 g of P/kg from a survey of dairy nutritionists in the mid-South region.
Various reasons why producers feed more P than is required have been discussed (Shaver and Howard, 1995; CAST, 2002; Wu, 2003). The fundamental concern with lowering dietary P is possible impairment of animal health and performance and associated economic loss. This concern must be adequately addressed in order to take advantage of diet modification and promote its wide adoption for enhanced environmental stewardship while sustaining animal production. Toward this end, information originated from commercial operations would be valuable for verifying the results derived from research feeding trials. Such field data may be more persuasive and better received by the farming community.
A project has been initiated recently, involving a large number of dairy farms in five states (New York, Pennsylvania, Delaware, Maryland, Virginia) in the Northeast and mid-Atlantic regions. One of the project goals is to determine the dietary P range adequate for satisfactory cow performance while minimizing manure P excretion using farm-based data. This article reports findings from the first phase of the project, including a questionnaire survey coupled with on-site feed and fecal sample collection and analysis results. The specific objective of the present work was to evaluate P feeding levels and to identify the critical control points pertaining to P management in dairy herds.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). In 2001, the lactating cow population in these states totaled 1.48 million, and milk production approximated 11.8 million t (NASS, 2003). The cow numbers and milk production accounted for 74 and 73% of those in the 14 states combined in the Northeast and Mid-Atlantic regions, or 15 and 16% of U.S. totals (NASS, 2003).
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). The survey questionnaire, sent out in January 2002, consisted of 18 queries, which were a mix of open- and closed-ended questions with multiple choices when applicable. Information sought in the questionnaire included: 1) herd demographics such as breeds, herd size, milk yield, etc., 2) management practices regarding herd nutrition and reproduction, and 3) dietary P levels and related concerns. A draft questionnaire was previewed by local dairymen as well as university personnel who have frequent contact with producers. Feedback was incorporated into the final version.
Survey responses totaled 617 by the end of April 2002, approximately 25% of the sampling pool (Figure 1). Data included in the current report were based on 612 valid returns. Some participants chose not to respond to every question in the survey; hence the percentage, when reported, was the percentage response to the individual question. Also, some questions allowed multiple answers and thus data may not total 100%.
On-site Sample Collection
A total of 92 farms were selected for on-site sample collection during March to August, 2002 (Figure 1), including 35 from the survey response farm pool and 57 solicited from existing extension and education contacts. The selection criteria were based on the requirements of the follow-up study as part of the larger project. As a key element of the selection criteria, the farms must have production record systems in place (e.g., DHIA services) and must utilize professional nutrition and veterinary services. Also, only confined feeding operations were selected. The follow-up (and currently on-going) study involves evaluation of dietary P concentration and variability based on seasonal on-farm sampling, and identification of an adequate dietary P range derived from comprehensive analyses of milk production and reproduction records.
On-site sample collection included feed and fecal samples from lactating cows only. Feed samples consisted of TMR for each lactating cow group or individual feed ingredients if the lactating herd was on component feeding. Fecal grab samples were collected from 10 to 20 lactating cows per farm, with the number of samples varying depending on herd size and number of feeding groups. Uniform sampling protocols were used to minimize discrepancies among different sampling personnel from five participating institutions. Feed and fecal samples were collected on the same day on individual farms. Total mixed ration was collected at the feed bunk at feed delivery, mixed by hand, and randomly subsampled. Grains and silages had 5 to 10 grab samples taken at the silo, bin, or fresh trench face. Fecal samples were taken rectally from healthy lactating cows from each feeding group and stage of lactation. Approximately 0.5-kg bulk samples were taken per cow, hand-mixed, frozen, and forwarded to the University of Pennsylvania for laboratory analysis.
Laboratory Analysis
Feed samples were analyzed at a commercial laboratory using established procedures for standard feed quality parameters (e.g., CP, soluble proteins, NDF, etc.), including total P concentrations. Dietary P concentrations were derived either directly from the feed analyses for TMR samples or, in the case of component feeding, calculated based on the P concentrations of individual feeds in the laboratory reports and ration formulation information obtained from the farm.
Fecal samples were frozen upon arriving at the laboratories of the participating institutions and kept frozen until analysis. All fecal samples were analyzed at the Environmental Studies Lab at the University of Pennsylvania. Samples were thawed, homogenized by hand, and split into subsamples for determination of: 1) DM in a convection oven at 65°C, 2) total P by microwave-assisted acid digestion (Walter et al., 1997) with P concentration analyzed using the phosphomolybdate blue method of Murphy and Riley (1962), and 3) water-soluble P by extracting 2 g of wet fecal sample in 98 mL of deionized water (reciprocal shaker for 1 h, filtered through Whatman 42 paper), with aliquots of the water extract analyzed for inorganic P by the phosphomolybdate blue method and total P by ICP spectrometry.
Data Analysis
Analyses of variance and mean separation by Duncan’s multiple range test at a significance level of 5% were performed using the GLM procedure in SAS (SAS Inst., Inc., Cary, NC). A logistic regression model, developed with the SAS logistic program (Allison, 1999), was used to evaluate the association of the dependent dichotomous responses from the survey as a function of herd size (lactating cow number). Concentrations of dietary P and fecal P were reported on a DM basis and as means for the respective farms unless noted otherwise.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The survey farms had a mean of 84 lactating cows averaging 27.9 kg/d of milk per cow. The farms selected for the follow-up study had larger herds with a mean of 281 lactating cows averaging 32.9 kg/d of milk per cow. The herd size distribution pattern for farms responding to the survey was similar to that of the five-state area, but for the selected farms it deviated toward larger herds (Figure 2). Although herd size was not a major consideration during farm selection, larger operations are generally more amenable than smaller ones to participating in research projects or other activities, as indicated by the logistic regression analyses (Table 1). Larger herds also had a higher tendency toward adopting various services, except professionally administered AI (Table 1); the latter apparently preferred handling by their own employees.
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). The mean was 4.42 g of P/kg of feed DM. The corresponding NRC (2001) recommendation would be 3.3 g of P/ of kg for the mean milk yield (27.9 kg/d per cow). That is, on average, P was fed 34% above the NRC guidelines. The results seem to suggest some slight decreases in dietary P over time, as compared to previous survey findings of 4.8 g/kg or greater (Shaver and Howard, 1995; Sansinena et al., 1999; Satter and Wu, 1999; and Sink et al., 2000). Wu (2003) also found decreased dietary P based on survey responses of 33 producers and 38 nutritionists in Pennsylvania, with dietary P averaging 3.9 g/kg for low-producing groups and 4.4 g/kg for high-producing groups.
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To assess the reporting accuracy regarding P feeding levels, we compared the dietary P concentrations in survey responses with those from the on-site feed sample collection and analyses. There were 11 farms from which such paired data were available (Figure 4). The differences were relatively small, with an average deviation of 5.6% (absolute values). Apparently, dietary P levels reported by producers were relatively accurate as compared to the measured data.
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. For the survey farms, regression analysis even suggested a negative association between dietary P concentration and milk yield, with a decrease of 1.2 kg/d of milk per cow for every increase of 0.5 g of P/kg in diets (P = 0.017), although the coefficient of determination was small (R2 = 0.057). For the selected farms, no significant correlation between dietary P and milk yield was detected (P = 0.73).
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Higher P in Diets Led to Higher P in Feces
Higher dietary P led to higher P concentrations in feces; this is true for both acid digest total P and water-soluble P of fecal samples (Figure 6). The slope of the linear regression equations indicated that for each unit of dietary P increase (g of P/kg of feed DM), there was an increase in feces of nearly 1.89 g of total P/kg of fecal DM, including 1.00 g of water-soluble P/kg of fecal DM.
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There is an important environmental implication associated with the fact that water-soluble P accounts for a substantial portion of total P increase in feces at higher dietary P. Water-soluble P in manure represents the most vulnerable fraction concerning potential runoff loss once the manure is field applied (Dou et al., 2000; Sharpley and Moyer, 2000). Lowering dietary P concentrations may decrease P loss potential by several-fold, as measured in an agronomic plots setting (Satter, 2003). It is also noteworthy that even if dietary P is near the adequate level (4.00 g/kg of feed DM or less), water-soluble P still constitutes a significant part of total P in feces (detailed data not presented). This is consistent with findings in the previous study using samples from research feeding trials (Dou et al., 2002). Clearly, P contained in the organic feeds, either in the current study with diverse feed ingredients on many farms or in the previous study with the base diets carefully selected, was largely water soluble or readily digestible and thus highly available for cows.
Relevant Issues
Minimizing P supplementation is the key.
Modern dairy diets typically contain 3.5 to 4.0 g of P/kg of DM before mineral P is added (CAST, 2002). That is, diets using regular feedstuff without mineral P contain adequate or slightly more than adequate amounts of P to meet cow needs. High-P diets (4.5 g/kg of DM or above) are almost always made so by using mineral P supplements. In our survey, when asked if mineral P was used in lactating cow diets, 60% out of 596 respondents answered "yes," 14% replied "no," and 26% said they were "not sure." The excess P in diets added through the use of P minerals was unnecessary, resulting in higher feed costs and increasing the potential of P loss to the environment. Clearly, lowering or eliminating mineral P supplementation is the logical first step toward the reduction of excess dietary P.
Certain byproduct feeds are rich in P; for example, soybean meal contains 7.1 g of P/kg of DM; wheat middlings, 10.2 g; and fishmeal, 30.1 (NRC, 2001). Thus, diets consisting of large amounts of P-rich byproducts may have P concentrations above 4.00 g/kg of DM even without mineral P addition. Under this situation, reducing excess P in diets would involve a delicate balance between economic and environmental consequences because by-product feeds are often priced lower than other feeds. Using computer ration formulation software, it is possible to fine-tune the rations for minimal excess P while maintaining an acceptable feed cost. Of course, such ration fine-tuning would be a secondary measure compared to the importance and necessity of reducing or eliminating mineral P from diets.
Ration formulation is the critical control point.
For most dairy operations, ration formulation is the first step in feeding management, which determines the type and amounts of diet ingredients, and thus, the concentration of P in the diet. Therefore, to lower excess dietary P, ration formulation is the critical control point and those who perform the ration formulation are at the critical control position. From our survey, 84% of the herds had their rations formulated by professionals, whereas 16% claimed to do their own (Figure 7). That is, the professionals rather than producers themselves are at the critical control position regarding the current presence of excessive P feeding and its future reduction.
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). Average deviation was 0.086 g of P/kg of DM (absolute values). Sixty-seven percent of the measured data had a deviation less than ± 10% from the formulated values, and 16% of the data had a deviation greater than ± 15%. Considering large variations for farm-origin data in general, it is fair to say that the majority of farms accurately implemented the formulated rations, in terms of P concentrations. Clearly, producers were feeding more P than cow requirements because they were guided by the rations formulated for them.
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). There were also written comments in the survey from a number of producers indicating that they needed to discuss the P feeding issue with their nutritionists or veterinarians.
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). Also, there was a generally positive attitude toward the need for managing P for both good animal performance and environmental stewardship, reflected in more than 80 written comments. Many of the comments sought additional information, whereas only a handful expressed grudges toward regulatory measures or blamed others for polluting the environment. It is possible that producers who responded to the survey were progressive and educated and their attitudes toward the issues were positive in general. Nevertheless, the written questionnaire assured confidentiality to all survey recipients and the responding farms were reflective of the overall operations in terms of herd size distribution (Figure 2).
Reproductive performance remains to be adequately addressed.
Reproductive performance, in addition to milk yield, is another key factor affecting decisions and practices regarding P feeding on dairy farms. There are long-held beliefs, particularly in the veterinary community, that P deficiency plays an important role in reduced fertility (Morrow, 1969; Steevens et al., 1971; Gerloff, 1996) and that feeding high-P rations improves reproductive performance. Because reproductive performance is economically important, attempts to lower excess P in dairy diets are often met with resistance from concerned producers and professionals, such as veterinary practitioners, alike.
Recent experiments found no negative impacts on reproductive parameters when cows were fed diets containing 3.10 to 4.00 g of P/kg of DM (Wu et al., 2001; Satter, 2003). However, individually, these experiments did not withstand rigorous statistical testing due to limited sample number or experiment duration. Indeed, it is difficult to assemble enough cows in research trials under controlled conditions to develop statistically sound data regarding cow reproductive performance (Wu and Satter, 2000). This type of study would require large numbers of cows for a long period of time at substantial expense.
Through the ongoing project, data are being collected from approximately 100 commercial dairies in the five states, totaling more than 25,000 lactating cows. The data and sample collections were planned for a total of 3 yr (2002 to 2004). Preliminary analyses based on the 1st yr of data indicated no significant differences in reproductive parameters (conception rate, heat detection rate, pregnancy rate, days to first breeding, and days open) between groups into which the farms were tentatively categorized (dietary P 
3.80, >3.80 4.50, and >4.50 g/kg of feed DM). Comprehensive analyses will be performed at the completion of the project to identify the dietary P range that is adequate for both milk production and reproductive performance, while at the same time minimizing P excretion in feces. It is anticipated that the results from this farm-based, long-term project would adequately address concerns over dietary P levels and satisfactory cow performance, and further promote a wide adoption of the diet modification strategy.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Phosphorus overfeeding on most farms was attributed to excessive P in formulated rations, provided mostly by nutrition service professionals rather than producers themselves. Clearly, to lower excess P in diets, ration formulation is the critical control point and minimizing the excessive use of P minerals is the logical first step.
Balancing P and other nutrients in diets as a front-end nutrient management approach has the unique advantage of not only lowering P surplus on farms and subsequently reducing potential environmental losses, but also of saving producers’ money in feed costs and preserving P minerals as nonrenewable resources. The amount of P minerals conserved could amass tens of millions of kilograms of P per year. To take advantage of this opportunity, concerted efforts are needed by all stakeholders, including nutrition service providers, feed supply industries, veterinarians, and producers. Toward this end, more data to adequately address concerns over dietary P levels and satisfactory cow performance, especially reproductive efficiency, are deemed necessary. Policies and incentive programs to encourage the adoption of diet modification as a front-end approach are essential.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication May 1, 2003. Accepted for publication June 17, 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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