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Previous research shows that dissolved P losses in runoff are well related to soil test P (STP), thus, various P loss prediction models incorporate the use of a STP vs. runoff DRP relationship. However, the relationship between STP and runoff DRP will vary based on soil type due to differences in soil properties. The purposes of the first two studies were to (i) investigate the effect of soil mineralogy on P sorption behavior and dissolved P in runoff and (ii) determine if any simple soil test extractions could indirectly take into account this effect of mineralogy. Nine soil types from the Virginia Piedmont, Coastal Plain, and Ridge and Valley were collected and used in a rainfall simulation study. Phosphorus retention among separated clay fractions and whole soils were related to Al bearing minerals such as hydroxy-interlayered vermiculite (HIV), gibbsite, and amorphous Al. Samples dominated by kaolinite retained very little P. Application of these results to runoff data showed that soil types with a HIV:kaolinite ratio > 0.5 caused significantly less DRP in runoff for a given soil WSP level compared to soils with a ratio < 0.5. The second study showed that the soil P vs runoff DRP relationships varied between physiographic provinces. Generally, the Coastal Plain soils resulted in a higher runoff DRP concentration for a given soil P level compared to Piedmont and Ridge and Valley soils. However, soil M3-P/Al resulted in one relationship with DRP for all three groups of soils. Results from the incubation study suggested that Al related P is more easily desorbed into solution compared to Fe related P. The final study demonstrated that phytase enzyme and high available P corn supplements in poultry diets can reduce manure WSP and total P. Results from the runoff study showed that DRP losses were related to sediment losses which consisted of > 90% manure particles. Manure particles were directly deposited into the collection container followed by desorption of P based upon the WSP content of that manure type. The results emphasized best management practices that prevent direct loss of manure particles from soil into surface waters. / Ph. D.

The objective of this dissertation was to assess the dynamics of calcium (Ca) and phosphorus (P) metabolism in dairy cattle. Hypocalcemia, or a drop in blood Ca, is a common condition near parturition. All cows experience some degree of hypocalcemia. Maintenance of blood Ca within the acceptable range of 8 to 10 mg/dl is a balancing act between the demand for Ca for milk production and the cow's homeostatic mechanisms to maintain blood Ca. These homeostatic mechanisms include bone resorption that is driven by Ca demand however both Ca and P are released when bone is resorbed. These times of bone resorption and bone mineral replenishment have not been accounted for in current mineral recommendations. For the first study, it was postulated that dairy producers could administer 25-hydroxyvitamin D₃ (25-OH) in the prepartum period to prevent hypocalcemia. Twenty-seven multiparous Jersey cows were randomly assigned to receive an oral bolus containing corn starch (control, CON) or corn starch plus 15 mg of 25-hydroxyvitamin D₃ (25-OH) or 15 mg of vitamin D₃ (D₃) at 6 d prior to expected parturition. Jugular blood samples were collected at -14, -13, -5, -4, -3, -2, -1 d prior to expected calving, on the day of calving, and 1, 3, 5, 7, 9, 11, 13, 28, 56, and 84 d with respect to calving. Samples were analyzed for 25-OH, Ca, P, magnesium, osteocalcin (OC), and parathyroid hormone (PTH). Blood Ca, P, and Mg decreased near the time of calving and then increased over time. Serum 25-hydroxyvitamin D₃ was higher for cows dosed with 25-OH (119.0 pg/ml) compared with those dosed with D₃ (77.5 pg/ml) or CON (69.3 pg/ml). Cows dosed with 25-OH tended to have lower serum PTH concentration, but treatments did not affect serum Ca, P, or Mg. Serum OC was higher in second lactation cows compared with cows entering their third or fourth lactation but OC was unaffected by treatment. Although results indicated a 60% increase in serum 25-OH due to a single oral dose of 25-OH prior to calving, the amount administered in this study apparently was not sufficient for initiation of any improvement in Ca homeostasis at parturition. Due to the intimate relationship of Ca and P in bone, it was postulated for the second study that dietary Ca would affect bone mobilization and Ca and P balance in the lactating dairy cow. Eighteen Holstein cows were blocked by parity and calving date and randomly assigned to one of three dietary treatments: high (1.03%, HI), medium (0.78%, MED), or low (0.52%, LOW) dietary Ca. Dietary P was 0.34% in all diets. Total collection of milk, urine, and feces was conducted 2 wk prior to calving and in wk 2, 5, 8, 11, and 20 of lactation. Blood samples were collected at -14 and -10 d prior to calving and 0, 1, 3, 5, 10, 14, 21, 28, 35, 56, 70, 84, 98, and 140 d after calving. Blood samples were analyzed for Ca, P, PTH, OC, and deoxypyridinoline (DPD). Rib bone biopsies were conducted within 10 d of calving and during wk 11 and 20 of lactation. Dietary Ca concentration affected Ca balance, with cows consuming the HI Ca diet in positive Ca balance for all weeks with the exception of wk 11. Interestingly, all cows across all treatments had a negative Ca balance at wk 11, possibly the result of timed estrous synchronization that occurred during wk 11. At wk 20, Ca balances were 61.2, 29.9, and 8.1 g/d for the HI, MED, and LOW diets, respectively. Phosphorus balances across all treatments and weeks were negative. Dietary Ca concentration did not affect P balance in the weeks examined for this study but there was a clear effect of parity on balance, markers of bone metabolism, and bone P. Regardless of dietary treatment, serum OC concentration peaked around d 35 of lactation. Simultaneously, DPD concentration began to decrease, which may indicate a switch from net bone resorption to net bone formation after day 35. This was not reflected in balance measures however, this information may help refine dietary mineral recommendations for lactating dairy cows and ultimately reduce P excretion into the environment. Ultimately from the first study it is clear that oral dosing with 25-OH at 6 d prior to expected calving is not justified. However, we learned that parity has an effect on bone formation with younger animals resorbing and forming more bone and that net formation appears to occur after 30 days in milk. Both of these points were corroborated in the second study. Additionally, the second study demonstrated that dietary Ca content has no effect on P balance from 2 to 20 wk of lactation. Finally, the rib bone does not appear to be a sensitive indicator of bone metabolism or at least not at the time points we measured. / Ph. D.

Phosphorus (P) losses due to overfeeding of the mineral to livestock can contribute to surface water degradation. The objective of this study was to examine the impact to supplementing various levels of mineral P to grazing beef cattle. A producer survey and a research trial were conducted to examine the effects of supplementing mineral phosphorus (P) to grazing cattle. In the first study, mineral tags, producer surveys, and fecal, forage, and soil samples were collected from beef cattle operations in Virginia's Chesapeake Bay watershed. Samples (n=166) were collected from 120 producers in 11 counties. Soil test P results were based on Virginia Cooperative Extension soil test guidelines characterized as low (12 %), medium (37 %), high (35 %), and very high (16 %). Pasture grab samples contained 0.34 ± 0.12% P and forage P concentration increased (P < 0.01) across soil P categories going from low to very high. Fecal total phosphorus (TP) was lowly correlated (R2 = 0.18, P < 0.01) to forage P concentration. Mineral supplements were categorized as nil (<1.0% P), low (1.0-<3.0% P), medium (3.0-<6.0% P), and high (>6.0% P). Fecal TP and inorganic phosphorus (Pi) concentration increased (P < 0.01) with mineral P levels. Fecal TP and Pi were lower (P < 0.01) when nil and low P mineral were supplemented as compared to medium and high P mineral. Soluble P (defined as fecal Pi/fecal TP*100) also increased (P < 0.01) with increasing mineral P content going from nil to high. All farms surveyed required little or no P supplementation in regard to beef cattle P requirements. The majority (82%) of producers were receptive to modifying mineral P supplementation practices based on forage P levels. A 56-d study was also conducted with eight yearling Hereford steers (261±30 kg) grazing cool-season grass fall re-growth to determine the effects of varying levels of P supplementation on fecal P excretion. Treatments consisted of dicalcium phosphate supplemented at 0 (D1), 10.0 (D2), 20.0 (D3), or 30.0 (D4) g/d in a randomized 4x4 replicated Latin square design. These treatments provided an additional 0, 1.9, 3.7, and 5.6 g/d of P respectively. Two esophageally cannulated steers were used to collect forage samples for nutrient analysis. Forage P content was analyzed from hand collected samples. Forage P concentrations averaged 0.49% of dry matter (DM) across all periods. Chromic oxide (Cr2O3) was administered twice daily via gelatin capsule at 0630 and 1830 to serve as an external marker for determination of fecal dry matter excretion (DME). Indigestible NDF (iNDF) was used as an internal marker to determine dry matter intake (DMI). Due to the high forage P content, average P intake was in excess of National Research Council (NRC) requirements for all diets (D1 = 281%; D2 = 297%; D3 = 323%; D4 =348%). Orthogonal contrasts were performed to assess the relationship between treatment and P excretion. A linear response (P < 0.01) in daily inorganic P (Pi) excretion (0.054, 0.052, 0.062 and 0.063 g/kg of BW ± 0.003 for D1, D2, D3 and D4, respectively) was observed across treatments. Daily total P (TP) excretion increased linearly (P < 0.01) across treatments (0.080, 0.079, 0.092 and 0.093 g/kg of BW ± 0.003 for D1, D2, D3 and D4, respectively. When forage P content is sufficient to meet the requirement of grazing cattle, increasing P supplementation results in greater P excretion without additional benefits to growth or nutrient digestibility. / Master of Science

Includes bibliographical references. / xviii, 171 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, 1994

Includes bibliographical references.