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RESEARCH – for Jan. 5, 2009

Kenneth Eng recently presented an excellent review of research on residual feed intake (RFI), a measure of feed efficiency in cattle. RFI is defined as actual feed intake minus expected feed intake. A negative RFI is indicative of improved feed efficiency. Previous research has revealed that if cattle are segregated into low-, medium-, and high-RFI groups, there is no difference in gain, but a significant improvement in feed efficiency for the lower-intake (lower-RFI) cattle. Moreover, there are no differences in carcass traits, as shown in the following table (Fox et al., 2004).

A possible basis for the improved efficiency of the lower-RFI cattle has been suggested by other scientists (Nkrumah et al., 2006). They found that low-RFI cattle had significantly lower losses of feed energy via methane, urine and nitrogen losses and higher protein and fiber digestion compared to the other two groups (Source: Feedstuff smagazine).

Effect of

Prebreeding Body Weight on Beef

Heifer Performance

For years, traditional recommendations have advocated substantial energy inputs for replacement heifer development because pregnancy rates

Daniel Buskirk & Steven Rust

in heifers depend on the number displaying estrus early in the breeding season. Consequently, it has been commonly recommended that heifers be developed between 60 and 65 per cent of mature body weight (BW) before breeding. However, heifer development costs could be reduced if these standards were relaxed.

Univ. of Nebraska scientists randomly assigned a total of 261 heifers (505 lb.) to be developed to 55 per cent of mature BW (660 lb.) before a 45-day, breeding season (intensive, INT) or 50 per cent of mature BW (600 lb.) before a 60-day breeding season (relaxed, RLX). Overall pregnancy rate did not differ, but RLX heifers had 7-day later calving dates

and 11-lb. lighter calf weaning weights than INT heifers. Calf birth weight, calving difficulty, second-calf conception rates, and 2-yr.-old retention rate did not differ between systems.Netcost per pregnant 2-year-old cow was less for RLX than for INT ($577 vs. $594).

Of the heifers that failed to become pregnant, a greater proportion of RLX than INT heifers were prepubertal when the breeding season began. Therefore, a second 2-year experiment evaluated melengestrol acetate (MGA, 0.5 mg/day) as a means of hastening puberty in RLX heifers. However, the proportion of heifers pubertal before breeding was not affected by MGA.

The authors concluded that developing heifers to 50 or 55 per cent of mature BW resulted in similar overall pregnancy rates, and supplementing the diets of heifers developed to 50 per cent of mature BW with MGA before breeding did not improve reproductive performance (Martin et al. 2008. J. Anim. Sci. 86:451).

Feedlot

Performance Optimized with Ractopamine

Texas Tech Univ. researchers conducted two trials to evaluate the effects of the feed additive, ractopamine hydrochloride (RAC), on performance, intake patterns, and acid-base balance of feedlot cattle.

In the first trial, 360 crossbred steers (1,202 lb.) were used to determine the effects of dose (0, 100, or 200 mg/steer/day) and duration (28, 35, or 42 days) of feeding RAC on performance. As RAC dose increased, avg. daily gain, final body wt., and gain/ feed increased linearly. As duration of feeding increased, avg. daily gain, final body wt., and gain/feed peaked at 35 days. Compared to control steers, those fed RAC had increased ribeye areas and lower numerical yield grades. There were no differences among treatments in marbling or quality grade.

In the second trial, steers fed 200 mg of RAC/day for 30 days required more time to consume 50 and 75 per cent of their daily feed intake relative to control steers. This change in feeding behavior was not related, however, to changes in blood acid-base balance or urine pH.

The authors concluded that feeding RAC for 35 days at 200 mg/steer/day provided optimal performance, and that no effects on acid-base balance were noted with RAC fed at 200 mg/ steer over a 30-day period (Abney et al. 2007. J. Anim. Sci. 85:3090).

The authors are animal scientists at Michigan State University in East Lansing, Michigan, U. S. A.

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