EXTRA PROTEIN IS TOO FATTY FOR FINISHERS
Feeding extra protein to finishing steers or heifers may not be cost-effective. Robert Davies reports.
FEEDING extra protein to finishing steers does not affect silage intake or animal performance, but significantly increases carcass fatness.
These were the main findings of a series of eight trials at the Agricultural Research Institute of Northern Ireland, Hillsborough. Similar work was carried out on heifers and bulls. The trials tested the claim that, because beef cattle do not use silage protein well, any concentrates fed to finishing cattle should have relatively high protein contents.
The silages used for the steer trials were all well preserved, with protein contents ranging from 11 to 17%, and D values from 66 to 73. In each case silage was supplemented with 2.4 or 3.6kg of concentrate a head a day. The supplement was either mineralised barley, or mixtures of barley and soya bean meal, or barley and fish meal containing about 17% crude protein.
The trials ran for 98-155 days, and consistent effects were recorded in six of the eight (Table 1).
The increase in carcass fatness resulting from increasing protein intake was greatest in animals with the lowest growth potential. The two experiments in which increasing protein intake did not affect carcass composition involved high growth potential Continental steers. However, increasing the protein content above that of barley did not improve the growth rate of these animals. In six trials, subcutaneous fat depth and marbling score (assessed in the cut surface of the eye muscle at the 10th rib) increased significantly.
Another series of four experiments looked at the effects of increasing the protein content of barley-based concentrates fed to finishing heifers. The silages used were well preserved, and had D values ranging between 64 and 73. These were supplemented with 2 to 4.5kg a head a day of either barley, or mixtures of barley and soya bean meal (20% crude protein).
The results (Table 2) show that increasing the crude protein content from 9.6 to 19.4% did not affect the performance of the heifers, or carcass fatness as indicated by fat classification, subcutaneous fat depth, or marbling score. There was also no impact on the contents of saleable meat and fat trim when the carcasses were boned-out commercially.
However, increasing the protein content of the concentrate significantly reduced lean content, and increased the fat content of the fore-rib joint, as determined by total dissection into separable lean, fat and bone. The detrimental effects on carcass fatness of increasing protein were less pronounced and more variable than in steers. This was despite the lower growth potential and protein requirement of heifers.
In two similar comparisons Friesians and Continental x Friesian finishing bulls were fed 73 D silage supplemented either with barley, or a mix of 80% barley and 20% soya. These were 12 months old and weighed 409kg at the start. The results (Table 3) show that increasing protein did not affect performance in an all-housed system but increased carcass gain by 12% when cattle were housed after being on pasture. These had a poorer growth rate before the start of the trial and the improvement was probably a compensatory growth effect. Carcass composition was not affected by increasing protein on either system.
Other work suggested that beef cattle do not use the protein in poorly preserved silage as well as the protein in the silages used in the trials. Positive performance responses to extra protein have been recorded on low digestibility/low crude protein silage.
"Experimental evidence indicates that when finishing steers or heifers are given well preserved silages of medium to high digestibility, increasing the protein content of concentrates above 10% will not improve animal performance, and may increase carcass fatness," says researcher Raymond Steen. "With young bulls, or steers with exceptionally high growth potential, increasing the protein content of concentrates to about 16% is likely to produce some improvement in performance without affecting carcass composition."n
When grass silages are well preserved, increasing the protein content of concentrates above 10% will not improve animal performance and may increase carcass fatness, says Hillsborough researcher Raymond Steen.
EXTRA PROTEIN EFFECTS
• Increase in carcass fatness greatest in animals with lowest growth potential.
• Increasing crude protein from 9.6 to 19.4% does not affect heifer performance.
• Extra concentrate protein content reduced lean content.
Table 1: Effect of protein supplementation on steers
Protein in concentrates (%)
Silage DM intake (kg/day)5.65.6
Liveweight gain (kg/day)1.021.01
Carcass gain (kg/day)0.610.60
Carcass fat classification 390%60%
Subcutaneous fat (mm)6.67.4
Area of eye muscle (cm2)6665
Scale of 1 (leanest) to 8 (fattest) 2.93.4
Saleable meat content (%)7069
Fat trim (%)9.610.5
Table 2: Effect of protein supplementation on heifers
Protein in concentrates (%)
Liveweight gain (kg/day)0.970.96
Carcass gain (kg/day)0.580.57
Carcass fat classification 370%60%
Subcutaneous fat (mm)7.98.1
Saleable meat content (%)69.269.2
Fat trim (%)1010
Carcass lean meat content (%)63.763
Carcass fat content (%)20.521.3
Table 3: Effect of protein supplementation on bulls
Protein in concentrates (%)
Liveweight gain (kg/day)
Carcass gain (kg/day)
Carcass fat classification 330%30%
Subcutaneous fat (mm)4.84.8
Carcass saleable meat content (%) 72.071.7
Fat trim (%)7.27.9