Effects of supplement type and previous experience on variability in intake of supplements by heifers

Abstract

Intakes, and variability in intakes, of a range of supplements were examined in groups (n = 10–20) of cattle grazing tropical native pasture in 24-ha paddocks. Intakes of supplement by individual animals were measured using lithium sulfate as a marker. In Expt 1, heifers (n = 160) were offered 1 of 4 supplements consisting of (i) a restricted amount of cottonseed meal (CSM); or ad libitum amounts of (ii) molasses containing 74 g urea/kg (M8U), (iii) a loose mineral mix (LMM) containing (g/kg) salt 390, urea 300, CSM 150, calcium phosphate 150, and sulfur 10, and (iv) feed block supplements containing (g/kg) molasses 494, urea 99, calcium phosphate 62, salt 62, bran 62, calcium oxide 148, and magnesium oxide 74. After 5 and 10 weeks the variation in supplement intake among heifers within a group was lower (P < 0.05) for CSM and M8U (coefficient of variation (CV) 24–37%) than for the LMM or block supplements (CV 55–118%). All heifers offered CSM or M8U consumed at least some supplement, but up to 5% and 20% of heifers were non-eaters of LMM or block supplement, respectively. Both the per cent non-eaters of supplement and the variability in intake of these latter supplements tended (P < 0.10) to decline as the experiment progressed. In Expt 2 the same heifers were re-allocated to paddock groups and were offered ad libitum supplements of (i) M8U, (ii) molasses containing 107 g urea/kg (M12U), (iii) M8U mixed with monensin (M8U-M), or (iv) M8U mixed with meatmeal (M8U-MM). The CV of supplement intake ranged from 37 to 58%, and except in one paddock group offered M8U-MM, all heifers consumed at least some supplement. In Expt 3, paddock groups of heifers (n = 120) without, or with experience of LMM supplements during the previous dry season were offered LMM supplements containing either nil or 300 g CSM/kg. Voluntary intake of LMM supplement DM was increased (P < 0.001) by 93% by inclusion of CSM and decreased (P < 0.05) by 24% by previous experience of a similar LMM supplement. Neither variability in supplement intake (CV 66–150%) nor the per cent non-eaters was significantly (P > 0.05) affected by previous experience of the heifers with LMM supplements or inclusion of CSM in the supplement. However, the per cent non-eaters of LMM supplement was inversely related to the mean voluntary intake of the supplement by the paddock group; when the average voluntary intake of the supplement by the paddock group exceeded 0.2 g DM/kg LW.day, then ≤10% of heifers were non-eaters of supplement. In conclusion, within groups of heifers offered LMM and block supplements the variability in intake and per cent non-eaters of supplement were higher than for cottonseed meal or molasses-urea supplements, and variability appeared to be associated with voluntary intake of the supplements.