Final Report for LNC00-172
Project Information
[Note to online version: The report for this project includes tables that could not be included here. The regional SARE office will mail a hard copy of the entire report at your request. Just contact North Central SARE at (402) 472-7081 or ncrsare@unl.edu.]
Dairy producers typically feed 20% more phosphorus (P) than is necessary to their lactating dairy cows. This results in manure that is high in P, and a form of manure P that is especially vulnerable to runoff into lakes and streams. Bone is an important reservoir for P in the body, and this study confirmed that long term feeding of P at NRC recommended levels (.35-.38% P in diet dry matter) maintained healthy bone structure. This study also demonstrated that feeding P in excess of .37% of diet dry matter did not improve reproductive performance of dairy cows. This research, along with published research, provides a strong justification for removing excess P from dairy diets.
Introduction:
Dairy producers routinely feed phosphorus (P) in excess of the cows' requirement. This is costing the U.S. dairy industry $80-100 million dollars annually, and is resulting in manure that is high in soluble P, a form that is particularly vulnerable to surface runoff into lakes and streams. Since P is the limiting nutrient for algae growth, P is responsible for algae blooms in fresh water lakes and streams. One aspect of understanding the P status of lactating cows is to know when bone is being called upon to supply P. Cows can mobilize up to about 1 kg of bone P when there is demand for P. Normally a cow mobilizes bone P in early lactation, and then restores it later in lactation. It is important that a cow restores her bone P by the end of lactation so she is ready to begin the next lactation.
Many dairy producers feed high levels of dietary P in early lactation to compensate for reduced levels of dry matter intake in early lactation, not recognizing that cows can readily mobilize P from bone to meet a temporary P shortage. Another important reason why dairy producers feed excess supplemental P is the widely held notion that high dietary P will improve reproductive performance of the dairy herd. There is evidence that extremely low dietary P can reduce reproductive performance, but that is likely due to impaired growth of rumen microbes under severe dietary P deficiency. Reduced rumen microbial activity reduces digestibility of the diet, thus diminishing the energy supply. With less microbial activity there is also less protein synthesized by the rumen microbes.
A reduced supply of both energy and protein can interfere with reproductive performance. However, it takes extremely low levels of dietary P before rumen microbes are inhibited, and such low levels are simply not reached with modern dairy diets.
1. To determine if feeding lactating dairy cows a low P diet for 2-3 years affects bone strength and P content.
2. To demonstrate to producers via a field study that lactating dairy cows can be fed 20 25% less P without affecting milk production or reproductive performance. NOTE: This objective was modified after we were more than half way into the project. We started working with five dairy producers, collecting reproductive information on their herds. It was during this time that a popular estrus synchronizing program (Ov-Synch) was being adopted by producers. Three of the five producers that we chose to work with wanted to use the Ov-Synch program. This interfered with our field study objectives, so we decided to conduct an experiment at the U.S. Dairy Forage Research Center research farm which would generate the kind of information needed to persuade dairy producers that they indeed could feed less P without impairing reproductive performance of their dairy herd. Our objective remained the same, but we developed the necessary information differently than we originally planned.
Research
Objective 1
Thirty seven multiparous Holstein cows were used in a 308 d lactation trial. Diets containing 0.31, 0.39, or 0.47% P (dry matter basis) were assigned to groups of 10, 14, and 13 cows at parturition. Uneven group size resulted in part from attrition of cows from treatment groups initiated 1 to 2 yr before this part of the long term study. Molasses and beet pulp were included as diet ingredients because of their low P content; this enabled formulation of a basal diet containing 0.31% P. Diets containing 0.39 and 0.47% P were obtained by adding monosodium phosphate to the low P diet. Feed intake, milk production, reproduction records, and health records were part of the study reported in the Journal of Dairy Science Vol. 84 pgs. 1738-1748. The objective in this project was to make bone measurements on cows that were at the end of their second or third lactation during which time they were fed one of the three dietary P levels mentioned above. At the end of lactation, or in some cases after cows were removed from the experiment but still milking and receiving the same diet (320 days in milk, SD 26 d), surgery was conducted to remove part of the 12th rib bone. The surgery area on the right side of the cow was clipped with a number 40 blade, scrubbed with Betadine solution, and rinsed with water. The area then was rinsed with 70% alcohol, followed by a final rinse with 7% strong tincture of iodine. Local anesthesia was applied, and an incision (~25 cm) was made over the 12th rib through the skin, fascia, and muscle down to the bone. A periosteum elevator was used to separate periosteum from the rib. A gigli wire was used to saw proximally and distally to remove a piece of rib measuring 20 cm long. After the bone was removed, the periosteum and the muscle layers were sutured separately with catgut, and the skin edges were closed by interlocking sutures. A sulfa urea powder was sprinkled on the opening, followed by a final suture. The closed wound was sprayed with an antibiotic solution. Immediately after removal, bone samples were trimmed of adhering pieces of soft tissues, wrapped in damp paper towels, and placed in air tight Ziploc plastic bags (within 3 min of removal); bone samples in the bags were kept on ice and later frozen at -20° C. Before testing for strength, bone was thawed and brought to room temperature (22° C). Bone strength was tested mechanically (MTS Universal Testing Machine, model 5/G, MTS Systems corp., Raleigh, NC) by a double shear block apparatus, according to the ASAE standard (pg 568 in ASAE standards, 1998) and the procedure described by Combs et al. (J. Anim. Sci; 69:682, 1991). The force was loaded at 5 mm/min over a section at each end of the bone (~5 cm from the end); an average result was used for one bone sample. The test resulted in maximum sheer force, shear stress, and fracture energy, which were electronically recorded. After the test, the wall thickness of the bone was measured at three places by a digital caliper with a precision of 0.025 mm, and the measures were averaged to obtain an overall wall thickness. The unfractured part of the bone was sawed into approximately 4 cm chunks, and separated chunks were used to determine DM (100° C) and total ash content (at 600° C until a consistent white ash was obtained). The ash was analyzed for P content. Bone pieces were weighed in air and water for the calculation of specific gravity.
Objective 2
We initiated a field study with five cooperating dairy farms and were well underway with the field study when a new technology became available for synchronizing estrus (Ov-Synch). Three of the five producers wanted to use this technology, and proceeded to do so. Use of this management tool was not compatible with our research objective, so we stopped the field trial, and instead implemented a large scale study at the U.S. Dairy Forage Research Center research farm. Since this was a research herd it enabled a more detailed reproductive study. In the final analyses, we learned more from the substitute study than we could have with the cooperating dairy farms because of the detailed measurements we were able to make.
One hundred twenty eight primiparous and one hundred nineteen multiparous Holstein cows were used in an eighteen month feeding trial at the U.S. Dairy Forage Research Center. Transition diets (Table 1) containing 0.37 or 0.57% P (DM basis) were assigned alternately to cows at parturition. Cows were switched to lactation diets containing the same respective P content after three weeks. Uneven group size resulted in part from attrition of cows from treatment groups due to injury or illness. The 0.37% P diet was formulated without supplemental P. The 0.57% P diet was obtained by adding monosodium phosphate.
Cows were housed in a tie stall barn during the transition period and offered a TMR ad libitum (5 to 10% refusal) and one block of alfalfa hay (~1.7 kg, as fed) once daily. Actual amounts of TMR offered and refused by individual animals were recorded daily to monitor feed intake. Cows were moved to a free stall barn after wk 3 of lactation and fed as two groups. A trace mineral/salt mix (Vita Plus Corp., Madison, WI) was available free choice at all times. Milking was at 0500 and 1700 h. Milk yields were recorded at each milking until day 165. All cows were administered bST (Posilac; Monsanto Co., St. Louis, MO) every 2 wk, beginning at d 63 of lactation.
Cows were fitted with a radiotelemetric transmitter (Heatwatch DOx®) at day 50 post partum, and were bred to natural estrus from day 50 to day 100. After day 100, cows were bred to synchronized estrus. Weekly ultrasonography was performed from day 50 postpartum until pregnancy. Cows were removed from the experiment when confirmed pregnant 60 days after insemination or if they failed to become pregnant by 200 days post partum.
Cows were dropped from the experiment when they developed significant health problems or were sold as culls. Consequently, 55 cows were removed from the experiment after having been assigned to treatment groups but before completing 165 DIM.
Diets were formulated to contain 0.37% or 0.57% P on a dry matter basis and were adjusted several times during the course of the experiment in response to changes in forage quality and P content of feed ingredients. Total mixed rations and feed refusals were sampled daily, frozen, and composited weekly. Alfalfa silage, corn silage and high moisture shelled corn were sampled weekly. Concentrates (roasted soybeans, molasses, yeast and soybean meal), alfalfa hay and minerals were sampled monthly. Pre-mixes were sampled monthly during the first five months of the experiment and weekly for the remaining six months that they were used during the experiment. Dry matter content of weekly samples was determined by oven drying at 60° C for 48 h. Diet formulations (as fed basis) were adjusted weekly for changes in DM content of the ingredients.
Milk samples were collected monthly under the routine DHIA program and analyzed by AgSource Milk Analysis Laboratory (Menomonie, WI) for fat, crude protein and SCC, using an infrared spectrophotometer with a B filter (Fossmatic 605; Foss Technology, Eden Prairie, MN).
Blood samples were obtained on day 50 and day 100 to determine serum inorganic phosphorus. Approximately 10 ml of blood were obtained from the tail vein and allowed to clot before chilling. Blood samples were centrifuged at 1600 x g for 15 min to obtain serum. Serum for P analysis was refrigerated until analyzed.
All dried feed samples were ground through a Wiley mill with a 1 mm screen (Arthur H. Thomas, Philadelphia, PA). Ground monthly concentrate, alfalfa hay, pre mix, mineral and selected weekly silage samples (17 per silo) were analyzed for P. The ground monthly concentrate samples were composited quarterly. These composite samples, ground 4 wk composite pre mix samples and ground weekly silage samples were analyzed for dry matter (105° C) crude protein (LECO FP 2000 Nitrogen Analyzer, Leco Instruments, Inc., St. Joseph, MI), neutral detergent fiber (NDF) (heat stable amylase and Na2S03 were used) and acid detergent fiber (ADF) (Robertson and Van Soest, 1981). The ANKOM200 Fiber Analyzer incubator (ANKOM Technology, Fairport, NY) was used for sequential NDF and ADF analyses.
For analysis of P, ground samples were processed as described by Nelson and Satter (1992), increasing the amount of concentrated HCl to 15 ml. Samples were analyzed for P content by direct current plasma (DCP) emission spectroscopy by adapting the procedure described by Combs and Satter (1992). A certified commercial P solution (VHG Labs, Inc., Manchester, NH) was used to mix calibration standards. Accuracy of the analysis was assured by referring to additional commercial standards (Standard Reference Material 1570a, spinach leaves, and 8436, durum wheat flour; National Institute of Standards and Technology, Gaithersburg, MD). Serum was analyzed for inorganic P concentration by Marshfield Laboratories (Marshfield, WI) according to AOAC (1980).
Chemical analyses of feeds were based on dry matter measurements made at 105° C. Weekly alfalfa silage and corn silage samples were selected for analysis based on the time frame during which that particular silage was fed. Eleven different alfalfa silages and 10 different corn silages were used during the course of this experiment. Means for nutrient composition of each silage (n=17) were used to calculate nutrient composition of the diet ingredients. Nutrient content of the TMR was computed from the average nutrient content of the individual diet ingredients analyzed as indicated above.
Daily milk yield was reduced to weekly means, data on body condition scores and the data on milk component percentages were analyzed by the Proc Mixed procedure of SAS, with a statement for repeat measures. Data on blood serum P concentrations were compared by t-test.
Objective 1
Bone strength and content
No differences were found among treatments in the sheer stress of the bone endured before rupture or the amount of energy required to deform the bone to the point of fracture (fracture energy) (Table 1). Wall thickness of the bone was ~5.1 min for all treatments. Bone specific gravity tended (P<O. 1) to be lower for the 0.31% P treatment than for the other two treatments, with the difference being about 4%. The ash content of the bone, expressed on dry weight, wet weight, or wet bone volume, was slightly lower (P<0.06 to 0.13) for the 0.31% group. The P content of bones was similar among treatments when expressed on an ash or dry weight basis, averaging 17.6 (SE 0.3) and 9.5% (SE 0.2), respectively. When expressed on a wet weight or volume basis, however, P content was lower (P<O.06 to 0.13) for the 0.31% P treatment compared with the 0.47% P treatment. The average decrease in ash and P contents (based on measurements in dry weight, wet weight, and wet bone volume) was 4.8 and 6.0%, respectively, between the 0.31% and 0.47% P treatments.
Means for the ash and P contents of dry bone were higher (48.6 and 7.5%, respectively) than those of tail bones reported by Brodison et al. (1989), who showed no differences in bone mineral measurements in lactating cows fed 0.36 or 0.44% P for 1 to 2 yr. The ash percentages on a dry basis were smaller (~59%) than those of right and left ribs (ribs 9, 10, 11, and 12) of younger cattle (6 to 36 mo, of age) reported by Beighle et al. (1993). The percentages of P in ash, dry bone, or wet bone (Table 1) were also lower than those (18.3, 10.8, and 10.3%, respectively) reported in Beighle et al. (1993). Their study also showed no differences in ash or P measurements among these rib bones. The ash percentages in our study were lower (~67%) than those of etherextracted metacarpal bone of beef heifers reported by Williams et al. (1991), but the P percentages of bone ash were higher in our study (~16.3%) than those reported in that study.
The P status of cattle has been evaluated frequently with blood serum inorganic P concentration, which is, however, subject to the influence of recent P intake. Serum P is also influenced by P mobilization from bone. If serum P concentration begins to decline due to insufficient dietary P, P is mobilized from bone in an effort to maintain a normal serum P level.
Animals deplete their bone P reserves to support normal function and production. Thus, bone characteristics should be the ultimate measure P status.
Bone contains 80 to 85% of the total P in the body of mammals. Most of the P in bone exists in the form of calcium phosphate [Ca3(PO4)2], which is amorphoric, and hydroxyapatite [Ca10(PO4)6(OH)2], which has a crystalline form and is spatially anchored in collagen, a fibrous protein matrix. Cortical bone is composed of densely packed layers of mineralized collagen, which provides rigidity and is the major component of tubular bone. Trabecular bone is spongy, provides strength and elasticity, and constitutes the major portion of the axial skeleton.
Hydroxyapatite is the source of mobile P in bone for regulating blood P. Phosphorus is mobilized as hydroxyapatite crystals and these crystals are degraded by osteoclasts to release Ca and P (osteolysis). Bone also restores Ca and P (ossification) when surplus minerals are presented. This is fulfilled by osteoblasts, which form new bone in newly synthesized bone matrix on the surface of previously resorbed bone. If Ca and P are not present in adequate amounts, bone matrix will not be fully mineralized and osteoid tissue will form. By weight, the organic proportion of bone will increase (Shupe et al., 1988). This is the reason for measuring specific gravity of bone and expressing bone composition on a volume basis. Little and Ratcliff (1979) suggested that bone specific gravity is highly related to bone mineral storage and is a sensitive measure of P depletion.
Resorption and formation of bone can occur simultaneously, but net change in bone P storage in dairy cows may vary with stage of lactation. Cows undergo a net loss of both Ca and P from bone to help supply these elements during early lactation. This is reversed in later lactation. Ternouth (1990) suggested that up to 30% of bone can be removed during early lactation. Based on this estimate for beef cows, a dairy cow weighing 600 kg could mobilize 600 to 1000 g of P, which also suggests that the amount of P that needs to be restored during later lactation could be large and supports the notion that the concentration of dietary P need not be reduced as the animal moves from early to late lactation (Braithwaite, 1986; Wu and Satter, 2000b). Judkins et al. (1985) and Shupe et al. (1988) reported on recovery of bone P when lactation stress was removed in beef cows and concluded that bone P levels can be replenished following lactation without P supplementation. However, if P intake is too low over a prolonged period of time, resulting in severe loss of P from bone, the bone will eventually become weak and porous, and could be deformed or broken by stress placed on the bone. These changes are better demonstrated in vertebrae and ribs because trabecular bone is more readily resorbed than tubular bone (Simesen, 1980; Ternouth 1990).
Shupe et al. (1988) described various signs of osteoporosis and other related changes in the bone of beef cows fed extremely low P (6 to 12 g/d) for 2 yr. The vertebrae and rib bones were demineralized, and spontaneous fractures occurred in some animals. Osteoid tissue increased and trabeculae were thin and sparse. The specific gravity of the bones was low, ranging from 1.06 to 1.34, and the bones had low P content (1.55 and P content of 0.2 g/cc and showed no osteoporotic characteristics. Williams et al. (1991) reported that feeding 0. 12 compared with 0.20% P to beef heifers 7 to 8 mo, of age for 16 mo, decreased the third metacarpal breaking load, breaking stress, and ash and P contents. Little (1972) showed that the ash content of fresh rib bone decreased from 50 to 41% in yearling cattle after 6 wk of P depletion. Little and Ratcliff (1979) reported that the critical bone P concentration ranged from 0.12 to 0.15 g/cc. Little (1984) further suggested that P content of less than 5% in fresh rib is indicative of a low P reserve. Ternouth (1990) suggested the following minimum bone ash and P contents for cattle, below which bone demineralization should be considered significant: 40 and 7.5% on a wet weight basis or 0.69 and 0.12 g/cc on a volume basis, respectively. In our experiment, the lowest means for ash and P contents were 46.0 and 8.1% on a wet weight basis or 0.69 and 0.122 g/cc on a volume basis. Also, the lowest mean for specific gravity was 1.50, similar to that reported by Shupe et al. (1988) for beef cows with normal bones. No bone related abnormalities were observed on our experiment.
Considering that bone by nature is difficult to measure and measurements can vary, multiple measurements of bone strength and mineralization were made in this experiment. Several of the measurements suggest some loss of P from bone for the lowest P treatment (~6.0%), although the other measurements, particularly P content of ash would suggest that no P loss occurred with the low P treatment. Because the contents of Ca and P in bone ash should be relatively constant (36% for Ca and 17% for P), expressing P content per unit of ash content is not as descriptive as expressing it on a total weight basis. On the other hand, as mentioned earlier, specific gravity and composition measures on a volume basis are good indicators of bone changes. All things considered, we concluded that with the lowest P concentration (0.31%) fed for 2 yr that bone P content might have been slightly decreased but not to the extent of affecting bone strength. Feeding 0.38 to 0.40% P for 3 yr did not affect bone P content or bone strength. We should note that, in our experiment, rib bone was removed just before cows were dried off. Had the rib been removed later in the dry period, P content of ribs from the low P treatment might have been more fully restored.
OBJECTIVE 2
Dietary P and reproductive performance
The chemical composition of diets fed during this experiment are in Table 2. The only difference between the low and high P diets was that the high P diet had a small amount of corn replaced by sodium monophosphate.
There was no discernible difference in animal health or body condition score due to dietary P content. As expected, blood serum inorganic P content was slightly higher for cows receiving the high P diet (Table 3).
Milk production, milk composition and body condition score are reported in Table 4. There was no treatment effect on any of these measures.
The results from the reproduction measurements are in Tables 5,6 and 7. None of the reproductive measurements made were affected by dietary P content.
This project has contributed crucial information related to bone P and reproductive performance as they might be related to dietary P content. Results of the experiments reported herein bolster the conclusions reached in lactation studies conducted by ourselves and by others, namely that dietary P concentrations of ~.37% P are adequate to support very high levels of milk production over long periods of time. Adding P above this level is without effect on reproductive performance of lactating cows. This information supports the recently published NRC (2001) recommendations.
This study provides the last set of evidence needed to make a convincing case to dairy producers that they can reduce dietary P levels from the .44-.45% range where they currently are to .35-.38% of dietary P. If dairymen adopt this recommendation, manure P content will drop by ~25%, and perhaps more importantly, the solubility of manure P will be reduced by half. This has great potential for lessening the potential damage to lakes and streams from P leached from dairy manure. In addition to the environmental benefits, there will be a dollar savings to dairy producers, as they will not need to purchase as much P supplement. This can reach $80 million annually in the United States.
Economic Analysis
The potential dollar savings was mentioned in the previous section. The risk associated with reducing dietary P from .44-.45% down to .35-.38% is extremely low. A growing number of elite dairy herds have reduced P content of their lactating cow diets without effect. This augments the experimental evidence that has accumulated. The convincing evidence is now in hand to persuade dairy producers to remove the excess P from their dairy diets.
Farmer Adoption
Based on several surveys (Sansinema, et al., 1999, Bertrand et al., 1995, Satter, unpublished information) conducted in 1998 1999, it appears that dairy producers were being advised to include P in their lactating cow diets at an average concentration of .48%. The surveys were in good agreement on this. Based on a limited survey conducted with Wisconsin dairy producers, and based on conversations with nutritionists around the U.S., this level of feeding has dropped some, perhaps to the .44-.45% range. We are optimistic that this level will keep dropping, particularly as we are now able to provide convincing evidence that feeding high levels of dietary P does not improve reproductive performance.
If the nation's dairy producers reduce P content of their lactating cow diets from .44-.45% P (approximate current level) to .40-.42% P in the next five years, we can rejoice in a remarkable piece of progress. The ultimate goal will be to reduce P content of lactating cow diets to .35-.38%. At this level, little or no supplemental P will be fed.
Educational & Outreach Activities
Participation Summary:
Published:
1. Wu, Z., L.D. Satter, A.J. Blohowiak, R.H. Stauffacher, and J.H. Wilson. 2001. Milk production, estimated phosphorus excretion, and bone characteristics of dairy cows fed different amounts of phosphorus for two or three years. J. Dairy Sci. 84:1738 1748.
2. Satter, L.D. 2001. Management in dairy production systems. Proceedings of the 3rd Babcock Institute Technical Workshop "Nutrient Management Challenge in Livestock and Poultry Operations: International and National Perspectives", University of Wisconsin, Madison. August 21-24. Pp. 38-53.
3. Lopez, H., F.D. Kanitz, V.R. Moreira, M.C. Wiltbank, and L.D. Satter. 2002. Effect of dietary phosphorus concentration on reproductive performance of lactating diary cows. J. Dairy Sci. 85 (suppl. 1): 365 (Abstract).
4. Satter, L.D. 2002. Reducing dietary phosphorus for dairy cows reduces land required for spreading manure. Proc. of the American Forage and Grassland Council Annual Conference. Special symposium on nutrient management in forage ruminant systems. Pp 21-29.
5. Klopfenstein, T., R. Angel, G.L. Cromwell, G.E. Erickson, D.G. Fox, C. Parsons, L.D. Satter, and A.L. Sutton. 2002. Animal Diet Modification to Decrease the Potential for Nitrogen and Phosphorus Pollution. CAST issue Paper. Number 21. June 16 pgs.
In Press
1. Satter, L.D. 2002. Meeting phosphorus requirements of ruminants in an environmentally responsible way. Proceedings of the Minnesota Nutrition Conference. September 17-18, 2002. Eagan, MN. In Press.
2. Satter, L.D. 2002. What goes in must come out. Phosphorus balance on dairy farms. Proceedings of the American Association of Bovine Practitioners. September 28, 2002. Madison, WI. In Press.
3. Satter, L.D., T.J. Klopfenstein and G.E. Erickson. 2002. The role of nutrition in reducing nutrient output from ruminants. J. Anim. Sci. (In Press) (14 pgs).
Presentations not involving a manuscript
1. "Managing the Nutrient N and P: What we do and don't know." World Dairy Expo. Madison, WI October 5, 2001 (primarily dairy producers).
2. "Management of Dietary Phosphorus in Dairy Production Systems" Phosphorus Roundtable Seminar. Madison, WI January 9, 2002. (primarily extension and research faculty)
3. "Phosphorus in Dairy Rations" County Extension Meeting. Clintonville, WI January 30, 2002 (primarily dairy producers)
4. "Phosphorus in Dairy Diets" County Extension Meeting. Manitowoc, WI April 5, 2002 (primarily dairy producers)
5. "Phosphorus in Dairy Diets" County Extension Meeting. Casco, WI April 5, 2002 (primarily dairy producers)
6. "Management of dietary phosphorus in dairy production systems" European MiniSymposium on Phosphorus in Dairy Systems. Lelystad, Holland June 5, 2002. (primarily researchers)
Project Outcomes
Areas needing additional study
The scientific evidence is pretty well in place now to support an all out push to reduce P in lactating cow diets to the .35-.38% range (NRC, 2001 recommendation). The major task ahead is to persuade nutritionists, veterinarians, the feed industry and dairy producers to take the final steps in eliminating excess P from dairy diets.
Beighle, D.E., P.A. Boyazoglu, and R. W. Hemken. 1993. Use of bovine rib in serial sampling for mineral analysis. J. Dairy Sci. 76:1047-1052.
Bertrand, J.A., J.C. Fleck, and J.C. McConnell, Jr. 1999. Phosphorus intake and excretion on South Carolina Dairy Farms. Prof. Anim. Sci. 15:264-267.
Braithwaite, G.D. 1986. Phosphorus requirements of ewes in pregnancy and lactation. J. Agric. Sci. (Camb.) 106:271-278.
Brodison, J.A., E.A. Goodall, J.D. Armstrong, D.I. Givens, F.J. Gordon, W.J. McCaughey, and J.R. Todd. 1989. Influence of dietary phosphorus on the performance of lactating dairy cattle. J. Agric. Sci. (Camb.) 112:303-311.
Judkins, M.B., J.D. Wallace, E.E. Parker, and J.D. Wright. 1985. Performance and Phosphorus status of range cows with and without phosphorus supplementation. J. Range Manag. 38:139-143.
Little, D.A. 1972. Bone biopsy in cattle and sheep for studies of phosphorus status. Aust. Vet J. 12:668-670.
Little, D.A. 1984. Definition of an objective criterion of body phosphorus reserves in cattle and its evaluation in vivo. Can. J. Anim. Sci. 64(Suppl.):229-231.
Little, D.A., and D. Ratcliff 1979. Phosphorus content of bovine reb. Res. Vet. Sci. 27:239-241.
Sansinena, M., L.D. Bunting, S.R. Stokes, and E.R. Jordan. 1999. A survey of trends and rationales for phosphorus recommendations among Mid South nutritionists. Pages 51-54 in Proc. Mid South Ruminant Nutr. Conf., Dallas, TX.
Shupe, J.L., J.E. Butcher, J.W. Call, A.E. Olson, and J.T. Blake. 1988. Clinical signs and bone changes associated with phosphorus deficiency in beef cattle. Am. J. Vet. Res. 49:1629-1636.
Simesen, M.G. 1980. Calcium, phosphorus, and magnesium metabolism. Pages 575-648 in Clinical Biochemistry of Domestic Animals. 3rd ed. J.J. Kaneko, ed. Academic Press, Orlando, FL.
Ternouth, J.H. 1990. Phosphorus and beef production in northern Australia. 3. Phosphorus in cattle a review. Tropical Grassl. 24:159-169.
Williams, S.N., L.A. Lawrence, L.R. McDowell, N.S. Wilkinson, P.W. Ferguson, and A.C. Warnick. 1991. Criteria to evaluate bone mineralization in cattle. 1. Effect of dietary phosphorus on chemical, physical, and mechanical properties. J. Anim. Sci. 69:1232-1242
Wu, Z., and L.D. Satter. 2000b. Milk production and reproductive performance of dairy cows fed two concentrations of phosphorus for two years. J. Dairy Sci. 83:1052-1063