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Study of the starch losses in Brazilian beef cattle and use of starch as a management tool in the feedlot The number of animals finished in the feedlot system is rapidly growing in Brazil. At the same time there is an increase in the starch content of the diets. The expansion of grain production and lower costs of digestible energy from grain compared to roughages should accelerate this process. The objectives of this work were: a) evaluate starch losses in commercial feedlots; b) develop the NIRS methodology for fecal starch content; c) study the relationships between fecal parameters (%starch, dry matter and pH) with performance efficiency of Nellore and crossbreed cattle receiving diets of medium/high concentrate; d) evaluate the effect of time at fecal sample collection and animal breed on fecal starch content. The fecal starch content was determined on 2,003 samples. Fecal starch content was affected by time of fecal sample collection both in the controlled experiment and in commercial feedlots (P < 0.01). There was higher fecal starch content in animals 100% zebu compared to B. taurus crossbreds (P < 0.01). There is a need to standardize the time of collection and genetic group to use fecal starch as a management tool. There was an effect of experiment (P < 0.0001), sex (P = 0.006) and genetic group (P = 0.0466), with higher fecal starch content found for males, Nellore, when compared to females and crossbreds. The correlations between fecal starch and metabolic mid weight (kg), dry matter intake (kg/day) and feed efficiency (kg/kg) were low (r < 0.20). There was no correlation between fecal starch content and residual feed intake for Nellore heifers or bulls, suggesting starch content is not a good tool to determine efficient animals fed medium concentrate diets. The correlation between the level of fecal starch and fecal pH for animals in commercial feedlots showed a negative correlation of r = -0.57 and -0.51 when corn and sorghum were the sources of starch. Fecal pH was different (P = 0.0006) depending on the source of the dietary starch, 6.55 and 6.04 for corn and sorghum respectively, suggesting lower efficiency of starch use with sorghum. However there was no difference in fecal starch percentage for the two starch sources evaluated (P = 0.29). For the fecal DM, there was an effect of source of starch (P = 0.06), lower for corn (20.5%) when compared to sorghum (23.1%). The database obtained with the 1,985 lab analysis of fecal starch from both controlled experiments and commercial feedlots were used to develop a NIRS calibration equation for fecal starch content. Part of the analysis was used to independently validate the equations. Equations were developed from three different sets of samples: a) all samples regardless the starch source; b) samples where corn was the main source of starch c) samples were sorghum was the main source of starch. All equations had good predictive ability, with high regression coefficients (R > 0.92) and low bias (< 2%) indicating high accuracy with independent data. Results suggest a single equation can be used to estimate the fecal starch regardless the source of starch in the diet.

Estudo das perdas de amido em confinamentos brasileiros e do uso do amido fecal como ferramenta de manejo de bovinos confinados