Responses of Growing Beef Cattle to Liquid Molasses-Based Supplements in a Temperate Grazing System

Abstract

<p>While temperate pastures provide a low cost feed-base for northern New South Wales livestock enterprises, productivity of growing beef cattle is constrained, with growth rates of <0.5 kg/d commonplace. This study investigates the growth responses of beef cattle to molasses-based supplements while grazing unimproved temperate pastures, typical of northern New South Wales. While growth responses have been investigated in tropical environments there is limited work conducted in a temperate grazing environment, particularly concerning the inclusion of added mineral and rumen modifiers in the Molasses-based supplement. Supplementation of this type (molasses-based with inclusion of minerals and rumen modifiers) is a standard management practice for many beef producers in the northern New South Wales temperate region. Thus, the focus of the work reported in this thesis is to contribute new understanding in relation to beef cattle supplement responses, specific to this region. </p> <p>The work reported in this manuscript is comprised of two sets of investigations, being: i) two experiments conducted under commercial conditions on a temperate-zone property near Kentucky in Northern NSW; and ii) an intensive animal-house study using rumen-fistulated cattle. In the commercial on-farm investigations, two field trials using a total of 301 animals were conducted with growing beef cattle in a temperate grazing environment. Liveweight changes were compared for groups of cattle offered various molasses-based supplements. When growing cattle were offered molasses-based supplements providing 60g/d crude protein inclusion, they grew faster than cattle that were not offered the supplement. These findings have not been reported previously for cattle grazing in a temperate zone.</p> <p>The housed animal study compared the effect of supplementing a wheat straw based diet alone (control) or together with a legume, or with molasses-based supplement as nitrogen sources. The effects on intake, digestibility, straw degradation, nitrogen balance, rumen ammonia-nitrogen, microbial protein production, volatile fatty acid, protozoa number, rumen mean retention time and methane emission of cattle were evaluated. Twelve rumen-fistulated steers were used in a completely randomised design for a total period of 35 days, including 17 days for adaptation to the dietary treatment and 18 days for collection. The cattle were blocked by liveweight in each of three treatments: i) straw only; ii) straw + legume, and iii) straw + molasses/urea based supplement. Total dry matter intake of cattle offered supplements (14.6 g/kgLW.d, straw + legume or 14.1 g/kgLW.d straw + molasses/urea) were higher (<i>P</i><0.05) than control cattle (12.3 g/kgLW.d). The straw + legume supplemented cattle consumed the most organic matter daily (<i>P</i><0.05). Molasses-based supplemented cattle had a higher nitrogen intake and nitrogen digestibility than the control group. Both treatment groups increased total VFA concentration and the number of protozoa was reduced by 25% for Legume + Straw group and 21% for molasses-based supplement group, relative to unsupplemented cattle. The estimation of microbial crude protein production (MCP) showed that the purine absorption and MCP production by molasses- based supplement group was higher (<i>P</i><0.01) than in the straw group. The efficiency of MCP synthesis (EMCP) was also higher (<i>P</i><0.05) in molasses–based supplement group than in the straw-fed group. Methane yield of the Legume+ Straw group (23.1 L/kg DMI) and molasses–based treatment group (25.4 L/kg DMI) was significantly lower (<i>P</i><0.01) than for the straw treatment group (27.7 L/kg DMI). This work has identified production benefits from molasses-based pasture supplements for grazing cattle in temperate regions which, despite broad industry acceptance, have not been previously documented. This work has also contributed new insights into why these benefits occur. Specifically, the reduction in rumen protozoal numbers and increased daily production of microbial crude protein provide explanation for increased growth seen in the on-farm study.</p>