Beef Quality Corner -- Cattle Diet and Its Impact on E. coli Concentrations and Viability
Livestock Update, November 1998
Bill R. McKinnon, Extension Animal Scientist, Marketing, Animal and Poultry Sciences, Virginia Tech
The popular press has reported the results from a study at Cornell suggesting that changing the diet of cattle before slaughter might reduce the number of E. coli bacteria in the lower digestive tract of the cattle. With reduced numbers of the organisms in the colon of the cattle, logic would suggest that there might be a reduced chance of E. coli contamination of beef products.
The beef industry has been exploring various routes to reduce the number of human illnesses induced by bacterial contamination of beef. These methods have included preharvest management of the cattle, slaughter procedure, processing techniques, and storage and preservation methods. The Cornell study addresses the preharvest area and helps answer some questions related to diet and bacterial population shifts within the lower intestinal tract.
In cattle on high grain diets, starch escapes the rumen and passes into the intestines. Some starch fermentation occurs in the colon which tends to lower the pH (make more acid) and provides starch degradation products on which E. coli live. A survey of 61 cattle at Cornell indicated that grain supplementation did lead to an increase of E. coli bacteria in the colon. Samples of ingesta were taken from the colon of cattle fed only hay or pasture, cattle on a moderate grain (60% of dry matter) diet, and those on a high grain (above 80%) ration. Cattle on the pasture or hay diets had a total E. coli cell count of 20,000 per gram of ingesta. The E. coli populations in the cattle on the grain supplemented diets totaled over 6 million cells per gram.
The acid environment of the human stomach serves as a natural barrier to many food borne pathogens. As a second part of the study, the viability of E. coli in an in-vitro environment mimicking stomach acid was tested using samples from the 61 cattle survey above. Virtually all the E. coli from the forage diet cattle were killed by the acid shock of 1 hour at pH 2. With the samples from the moderate grain ration cattle, the majority of the E. coli cells were killed by the acid shock treatment, but the "acid resistant" cell count was greater than 25,000 viable cells per gram. The resultant E. coli count was 250,000 viable cells in the sample taken from the cattle on the high grain diet. The results seemed to indicate that E. coli can gain some resistance to pH of 2.0 if it is grown in a mildly acid environment.
In a second trial utilizing mature nonlactating Holstein cows, the relationship between grain level in the diet and an increased level of E. coli bacteria in the colon was again demonstrated. Hay fed cattle had less than 105 coliform bacteria per gram of ingesta in the colon. Cattle fed rations with either 45% or 90% grain possessed approximately 108 cells per gram. When the E. coli bacteria were subjected to an acid shock treatment mimicking stomach acid, 99.99% of the cells from the hay fed samples were killed. A much higher percentage of the E. coli cells from the grain supplemented cows survived the acid shock treatment.
The result of the study which gained so much national attention was the drop in the number of acid resistant E. coli cells as the cattle on a grain diet were switched to an all hay ration. With the switch to an all hay diet the number of acid resistant cells dropped after two days and by day 5, the E. coli population was nearly 100,000 fold lower.
At first glance, one might suggest that cattle on feed be put on a hay diet five days before they are shipped to the packer. There are several questions that must be resolved before cattle feeders adopt such a practice. How would quality grade, cost of gain, dressing percent and other profitability factors be impacted by a hay ration the last week of feeding? What would be the cost of delivering large quantities of hay to pens of heavy finished cattle? The results of the above study are still considered preliminary with some scientists questioning the methodology, especially the medium used as the acid shock treatment to identify "acid resistant" bacteria. There was no specific testing for the response of the E. coli O157:H7 organism, which has caused the much publicized disease problems. Addressing the E. coli problem at the pre-harvest level will ultimately be just one strategy in the industry wide effort to produce safe beef products.