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Title: Assessment Of The Calcium, Phosphorus, And Oxalate Components Of Kikuyu Grass Pasture Based Diets In Hawaii And Their Relationship To Calcium, Phosphorus, And Oxalate Intake And Excretion By Horses. 
Author: Gusman, Veronica M.
Date: 2004-05
Abstract: Skeletal abnormalities or mineral deficiencies in horses occur more often from a lack of or imbalance of calcium (Ca) and phosphorus (P) or the ingestion of high oxalate grasses (>0.5% of dry matter (DM)). Kikuyu grass (KIK), known for its high oxalate content, is the major pasture grazed by horses in Hawaii. This research was conducted to survey common Kikuyu pasture based horse diets (20 horses of various breeds and ages at four sites each on both Maui and Hawaii) and determines levels of Ca, P, and oxalate intake and excretion. Samples of KIK, dietary supplements, and soils were collected during the four seasons. Soil pH generally ranged from slightly acid (>6.0) to neutral (7.0). Soil extractable Ca levels ranged from 1792 to 10350 mg kg-1. There were no significant differences in seasons or islands for any of the extracted cations, however significant season*island effects were found for Ca (p=0.05). Calcium and P levels in kikuyu grass ranged from 0.25-1.05% (µ=0.41, SD=0.18) and 0.18%-0.36% ( µ=0.29, SD=0.28), respectively. Calcium: P ratios ranged from 1.1 to 1.9 and was in the range of NRC requirements of>1: 1 and <6: 1 for both islands and seasons. The Ca and P levels of both KIK and feces did not differ (p>.05) with season or location. Oven dried and freeze dried water soluble oxalate levels ranged from 0.07-0.84% ( µ=0.33, SD=0.19) and 0.04-.54% ( µ=0.4, SD=O.13) respectively. Oven dried and freeze dried insoluble oxalate levels ranged from 0.04-2.06% ( µ=0.39, SD=0.38) and 0.02-0.77% ( µ=.54, SD=0.53) respectively. Total oxalate and soluble oxalate levels did not differ significantly by island, season, or season on each island. However insoluble oxalate levels (p = 0.04), and IV total to soluble oxalate ratios (p=0.03) differed among seasons on each island. Calculated NRC requirements for horses are as follows 9.38% crude protein, 0.29% Ca, 0.20% P, 0.11% Mg, and 0.36% K. Alfalfa cubes, alfalfa hay, timothy hay, and guinea grass supplements showed higher levels of Ca (0.32%-1.78%) and lower «0.5% total oxalate) concentrations of oxalates (0.11-0.52% total oxalate, 0.02-0.24 % soluble oxalate, and 0.09-0.47% insoluble oxalate) than the kikuyu grass pasture. A negative trend (p= 0.09) was seen between diet Ca and diet insoluble oxalate concentrations. Mean K1K Ca and P were 0.43 and 0.29%, and mean fecal Ca and P were 0.79 and 0.48% of DM. No differences (p>0.05) were found for season or location. Overall K1K mean values were 0.79% TO, 0.26% SO, and 0.53% insoluble oxalate (IO). Trends were found between K1K Ca and Mg with insoluble oxalate components. Fecal 10 was negatively correlated with diet Ca (r=-.30), and positively correlated with fecal Ca: TO (r=0.31). Oxalate impact on Ca and P balance in horses varies with season, location, and type/level of supplementation. Supplementation influence tends to increase Ca and P and can alter the diet consumed by horses; however it is also important to remember that the oxalate concentrations can still playa factor in binding of minerals in the diet.
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