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Estimating fecal crude protein excretion in goats

A. L. Adams1, J. E. Moore2, A. L. Goetsch1, and T. Sahlu1

1E (Kika) de la Garza Institute for Goat Research, Langston University, Langston, OK
2Department of Animal Science, University of Florida, Gainesville

As part of an overall goal to estimate protein requirements of goats using the factorial method, equations were developed and tested to estimate fecal CP excretion. Data from 54 trials (n = 189)on CP intake and apparent digestibility in goats were compiled. There were 78 unsupplemented roughages (14 grass hays, 12 legume hays, 4 grain silages, 4 straws, 9 mixed roughage sources, and 35 browse and mixed browse/hay diets) and 111 mixed roughage/concentrate diets in the database. Hypotheses were 1) metabolic fecal CP as a proportion of DMI is constant and 2) true digestibility of CP is constant, and were tested by regressing apparent digestible CP (DCP, % diet DM) on total CP (% DM). Excluding extreme outliers, the equation was: DCP = .899 (CP) - 3.14; n = 179; r2 = .96; intercept = metabolic fecal CP excretion (% DM); and slope = true digestibility of CP. Coefficients from the regression equation were used to compute expected fecal CP (EFCP, g/d): EFCP = .0314 (TDMI) + [1- .899 (CP/100)(TDMI)], where TDMI = total DM intake (g/d). To evaluate the EFCP equation, the main database was divided into subsets for equation development (n = 107) and evaluation (n = 68). Subsets were balanced for TDMI, CP intake, CP digestibility, dietary forage % (FORG), presence or absence of browse in the diet (BRWZ; 1 vs 0, respectively), animal age, breed, and initial BW. Regression of actual fecal CP excretion (AFCP, g/d) on EFCP using the development set yielded the equation: AFCP = 1.160 (EFCP) -2.30; r2 = .89, RMSE = 11.4. The slope of the equation was significantly different from 1 (P < .0001). Multiple regression analysis conducted on the development set, using factors chosen with the stepwise selection option of PROC REG, gave the following equation: predicted fecal CP (PFCP, g/d) = -9.79 + 1.181 (EFCP) + 12.32 (BRWZ) + .0522 (FORG); r2 = .92; RMSE = 9.9. Testing this equation with the evaluation subset showed that the regression of AFCP on PFCP was close to ideal (i.e., intercept = 0, slope = 1, r2 = .93, RMSE = 9.3). In conclusion, when predicting fecal CP excretion by goats, forage concentration and presence or absence of browse in the diet should be considered in addition to dietary CP concentration and TDMI.


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