Effect of magnesium-mica during grazing and/or feedlot phases on performance of steers

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Introduction
Previous work at KSU-SEARC has shown a tendency for increased digestibility and increased rumen fermentation products from cattle fed magnesium-mica (MM).Carcass marbling scores and the percentage of steers grading Choice were higher from feedlot steers fed MM compared with steers fed our typical feedlot diet in another study.Our objective was to measure grazing and subsequent feedlot performance and carcass characte ristics of cattle fed magnesium-mica in the grazing and(or) feedlot phases.

Experimental Procedures
Seventy-two mixed-breed steers were weighed on April 6 and 7, allotted into eight groups of nine head each, and assigned randomly to one of eight 10-acre smooth bromegrass pastures.Half of the steers were fed 2.2 lb/day of a grain sorghum-based control supplement (PC), whereas half were fed 2.2 lb/day of a supplement containing MM to provide .075lb of MM/head daily.
Following a 112-day grazing period, steers were transported to the SEARC feedlot facility at Mound Valley, KS, blocked by previous pasture treatment, and assigned randomly to one of two finishing diets (80% ground grain sorghum, 15% corn silage, and 5% protein supplement on a dry matter basis).One group was fed the control supplement (FC), and the other group was fed a supplement with 10% of the wheat middlings replaced by MM (FMM; Table 1); both contained 50% CP.At the end of a 120-day finishing period, steers were slaughtered at a commercial packing plant, and carcass data were col lected following a 24-hour chill.

Results and Discussion
Although steers fed PMM gained faster during the pasture period (2.41 vs. 2.32 lb per day), the difference was not statistically significant.No significant pasture treatment feedlot treatment interactions were detected (P>.05) for any of the performance or carcass measurements.Therefore, feedlot data were pooled across the main effects of pasture treatment and feedlot tre atment.Neither pasture nor feedlot treatment affected feedlot gain, efficiency, or cost of g ain (Table 3).These data are in agreement with those from a previous SEARC study.
Feedlot supplements had no ef fect on any of the carcass measurements evaluated in this study.However, steers fed PMM had heavier (P=.11) hot carcass weights and higher (P<.06) dressing percentages than those fed PC.Fifty percent more steers graded USD A Choice in the groups fed PMM compared with those fed PC.These factors combined to produce a $19.92 higher (P<.06)net carcass value.
Considering these data with previous data, we conclude that MM fed at a level of 9-10 lb/ton of dry matter should have minimal effects on gain and efficiency of feedlot steers, but feeding MM during the grazing or feedlot period may have a s ignificant impact on carcass marbling score and, therefore, value of the cattle.

Table 1 . Composition of Feedlot Supple- ments Fed to Finishing Steers a
a matter.

Table 2 . Performance by Steers Grazing Smooth Bromegrass Pastures and Fed Magnesium-Mica (.0 75 lb/day) in a Grain Supplement a
a detected.

Table 3 . Performance and Carcass Characteristics of Finishing Steers Fed Magnesium- Mica in a Pasture or Feedlot Supplement a
magnesium-mica supplements during the feedlot phase.Differences between pasture supplements were detected (P=.11).Net carcass value is based on a base price of $113/cwt hot carcass weight with discounts of $6/cwt g for Select grade and $16/cwt for yield grade 4 or carcasses heavier than 950 lb.
eg Magnesium-mica levels were .075lb/day in the pasture supplement and 10 lb/ton DM in the feedlot a ration.PC and PMM = control and magnesium-mica su pplements during the pasture phase; FC and FMM b = control and c Calculated using actual unshrunk live weight.d Differences between pasture supplements were detected (P<.06).