Short communication: Use of fecal starch concentration as an indicator of dry feed digestion in preweaned dairy calves

Fecal starch (FS) has been used as a tool to evaluate starch and diet digestibility in lactating dairy cows and feedlot steers. Some on-farm advisors also use FS to evaluate calf starter digestibility in preweaned dairy calves. Our objective was to evaluate the influence of starter intake (SI), starch and organic matter digestibility, milk replacer (MR) feeding rate, and age on FS concentrations in preweaned dairy calves. Male Holstein calves (43 +/- 2.9 kg of body weight; n = 35) from a single farm were fed different amounts Of MR ranging from 0.44 to 1.10 kg of dry matter (DM) daily (27% crude protein, 17% fat) and weaned by 7 wk of age. Starter ingredient composition was 37% whole corn, 20% whole oats, 35% protein pellet, and 3% molasses and contained 43 +/- 1.9% starch. Fecal grab samples were taken at 3 (n = 20), 6 (n = 20), and 8 wk (n = 35) of age. Twelve fecal samples per calf were taken via rectal palpation over a 5-d period each week, frozen daily, combined on an equal wet -weight basis, and subsampled for analysis. Chromic oxide was used as an external digestibility marker at 3 and 6 wk (included in MR), whereas acid -insoluble ash was used as an internal marker at 8 wk. Milk replacer and starter intakes (offered and refused) were recorded daily during collection periods. Multiple and linear regression of organic matter digestibility (% of DM), total-tract starch digestibility (TTSD; % of DM), MR intake (kg/d), SI (kg/d), and age (week) versus FS (% of fecal DM) were determined using PROC REG of SAS (version 9.2, SAS Institute Inc., Cary, NC). Prior to weaning, SI, age, and MR rate explained 89% of the variation in TTSD, where TTSD = [19.7 x SI (+/- 4.25)] + [3.8 x age (+/- 0.79)] - [24.8 x MR (+/- 3.19)] + 56.2 (+/- 3.39). At 3 wk of age, TTSD increased (coefficient of determination = 0.53) and SI decreased (coefficient of determination = 0.20) with increasing FS. At 6 wk of age, TTSD and SI were unrelated to FS. In 8-wk-old calves (with 2 trials), SI, MR rate, FS, and trial explained 92% of the variation in TTSD, where TTSD = -[2.6 x SI (+/- 0.67)] [2.4 x MR (+/- 0.56)] [0.6 x FS (+/- 0.04)] + [1.1 x trial (+/- 0.33)] + 100.4 (+/- 1.02). Postweaning, TTSD decreased linearly as FS increased (coefficient of determination = 0.86), whereas FS and SI were unrelated, a relationship in contrast to the previously observed result in calves still consuming milk replacer. In the current study, FS was not a good estimate of TTSD or dry feed intake in the preweaned calf, but has potential for evaluating TTSD in calves after weaning.