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Using Net Energy for Gain and Maintenance in a Quadratic Equation to Calculate Beef Cattle Diets¹

University of Alaska², Palmer 99645

Abstract

Use of net energy to balance beef cattle diets for specific intakes and average daily gains is difficult because feedstuffs are utilized differently for maintenance and gain. Diets balanced on net energy for gain (NEg) only, on the average of NEg and net energy for maintenance (NEm) or on the weighted average of NEg and NEm are approximations which may be erroneous. In this paper, coefficients were derived for use in a quadratic equation that precisely calculated percentages of two feedstuffs needed to meet required NEg and NEm. The percentage of the first feedstuff was solved directly by the quadratic formula and the percentage of the other feedstuff was solved by subtraction. The following variables were used; I=intake (kg total dry matter/d); M=NEm required (Mcal/d); N=NEg required (Mcal/d); M₁=NEm (Mcal/kg dry matter) feedstuff one; M₂=NEm (Mcal/kg dry matter) for feedstuff two; N₁=NEg (Mcal/kg dry matter) for feedstuff one; and N₂=NEg (Mcal/kg dry matter) for feedstuff two. The coefficients A, B and C for the quadratic equation were: A=I(M₁–M₂)(N₁–N₂), B=(IN₂–N)(M₁–M₂) + (IM₂–M)(N₁–N) and C=N₂ (IM₂–M)–NM₂. Comparison of diets balanced with the quadratic method to NEg only, total digestible nutrients (TDN), NEg and NEm averaged or a weighted average of NEm and NEg showed the quadratic to be the only method to give precise dietary formulations. Total digestible nutrients and the average of NEg and NEm provided accurate formulations for only a few dietary cases. Using only NEg provided more accurate formulations than TDN or the average of NEg and NEm. Weighted average was the best approximation to the quadratic, but required accurate estimates of feed intake and of intake above maintenance relative to maintenance. The quadratic form required only an accurate estimate of intake and provides precise feedstuffs formulations for the net energy system.

¹ Contribution from the University of Alaska Agr. and For. Exp. Sta. Published with the approval of the Director as paper no. J-163.

² Palmer Res. Center.