University of Nebraska Cooperative Extension MP 71

Summary

Yearling Angus steers averaging 889 pounds were fed 12.1 or 13.8 percent crude protein with two energy levels during the first half of finishing, followed by 12.3 or 13.8 percent crude protein with the higher energy level during the second half of finishing. In a 126-day trial, finishing performance and carcass traits were not improved by feeding more than 12.1 percent crude protein or more than 56 Mcal/cwt NEg during the first half of finishing or more than 12.3 percent crude protein in the second half. These results indicate increased corn silage levels during the first half of finishing may not reduce overall performance.

Introduction

Protein and energy levels are major inputs in formulating diets for finishing cattle. The goal is to have enough protein and energy in the diet for the age, type and condition of the animal so desirable performance and carcass traits are achieved for a competitive cost of gain. Feeding more roughage in the first half of finishing may be one way to cope with periods when grain prices are high relative to forage. Feeding more protein during the first half of finishing may seem logical when cattle may be gaining at a fast rate, but the additional expense must be covered by improved overall performance.

The objective of this study was to study protein and energy level combinations during first and second half finishing phases on overall performance and carcass traits in Angus yearling steers.

Procedure

Table 1. Composition of diets and calculated nutrient analyses.

Angus yearling steers averaging 889 pounds were randomly assigned to four pens of seven or eight steers on each of four treatment combinations of crude protein (CP) and net energy for gain (NEg) during the first half of finishing. All pens received the higher energy diet during the second half. The diets shown in Table 1 were combinations of two levels of CP and two levels of NEg in a 2x2 factorial arrangement as shown in Table 2. Diets were based on dry-rolled corn with variation in levels of CP or roughage achieved by varying the percentages of corn, corn silage, alfalfa hay or protein supplement. The CP levels assumed for the corn and corn silage in calculating the diets were 8.8 and 7.8 percent with NEg at 67 and 45 Mcal/cwt dry matter (DM), respectively. Dry supplements contained 58 or 40 percent CP (air dry basis) with about two-thirds of CP coming from urea. Monensin and tylosin were included in all diets at 29 and 9.7 grams/ton, respectively. All steers were implanted at the start of the trial with Synonex S. Carcass data were collected and final weights were calculated by dividing hot carcass weights by a common dressing percentage (62). Results were analyzed by using the General Linear Model in SAS.

Table 2. Performance and carcass results from diet protein and energy combinations.

Results

Steers fed 12.1 percent CP in diets with about 40 percent corn silage in the diet DM during the first half of finishing did not appear to gain as fast as those fed about 17 percent corn silage, but overall gains for the 126-day trial were similar for those first half treatments when followed with 17 percent corn silage and 12.3 percent CP in the second half or for those fed 17 percent corn silage throughout the trial with 13.8 percent CP. First half and overall daily gain and feed conversion appeared to be lower when 13.8 percent CP was fed with the lower energy diet during the first half, but that diet included alfalfa hay to meet CP and energy targets while the 12.1 percent CP, lower energy diet included only corn silage as the source of roughage. The calculated NEg levels, however, were the same for these diets. Carcass traits appeared to be unaffected, except for an apparent relationship to the lower gain in the group receiving 13.8 percent CP and lower energy during the first half. The results from this protein level comparison are consistent with earlier data from the University of Nebraska (1994 Nebraska Beef Report, pp. 35-36) in which 12 percent CP resulted in yearling performance unimproved by higher protein levels obtained by urea additions to dry-rolled corn diets containing 5 percent corn silage and 5 percent alfalfa hay as the source of roughage.

Finishing diets based on dry-rolled corn contribute substantial amounts of rumen bypass or rumen undegradable intake protein (UIP), often resulting in dietary metabolizable protein (MP) levels in excess of the amount needed by finishing yearling steers. Evaluating the diets with corn silage as the only source of roughage with the NRC metabolizable protein model showed MP levels more than adequate for finishing Angus yearling steers gaining an average of 3.45 pounds per day for 126 days. The rumen degradable intake protein (DIP) was about adequate in the 12.3 percent CP diet in which two-thirds of the supplemental protein came from urea.

Utilizing more corn silage in the diet (56 Mcal/cwt NEg) during the first half of finishing was not detrimental to overall daily gain, feed efficiency and carcass traits when fed with 12.1 percent CP and followed by 12.3 percent CP and 61 Mcal/cwt NEg in the diet. Using a lower energy level during the first half of finishing allows the flexibility of using more forage when available and priced to be economical on cost of gain. These results, showing higher corn silage levels can be used during the first half of finishing without sacrificing overall performance or carcass traits achieved with higher grain diets, may be especially useful when grain prices are high relative to forage prices.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Elbert C. Dickey, Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources.

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