Effects of Low-Moisture Molasses Block Supplements on Tissue Concentrations of Trace Elements and Growth Performance of Forage-Fed Beef Cattle

Feed manufacturing processes, particularly those involving extremes of heat or pH, can alter bioavailability of nutrients. Similarly, form of supplements (pellets, loose minerals, liquids, or blocks) can influence supplement consumption patterns, which may in turn impact nutrient utilization. This study was conducted to evaluate changes in tissue concentrations of Cu, P, and Zn in forage-fed heifers in response to supplementation with plain salt (S), a free-choice mineral supplement (M), or a low-moisture process block supplement (B). Heifers (n=360, 253.9 ± 6.7 kg) were weighed, blocked by initial BW, and randomly allocated to treatments consisting of ad libitum access to salt (S); loose mineral (M); or low-moisture molasses block plus salt (B). The M and B treatments were manufactured using the identical mineral mixture, adding salt as a diluent to make M, or by incorporating the mineral mixture into a hot, pliable evaporated molasses/oil mixture to make B. Heifers were housed in 24 dirt-surfaced pens (15 animals/pen, 8 pens/treatment) and fed a basal diet consisting (DM basis) of 70% ground brome hay and 30% corn silage for 100 d. Blood and liver biopsy samples were collected on d 0 and 70 from 4 randomly selected heifers per pen to determine changes in concentrations of Cu in liver and Zn and P in plasma. No treatment differences were detected for DMI, or G:F, but ADG was greater for M than for C or B. No differences were detected for changes in plasma P or Zn (P > 0.10). Liver copper concentration was different across treatments (114, 428, and 266 mg/kg tissue DM for S, M, and B, respectively; P < 0.05). The percent of heifers identified as having sufficient tissue Cu (i.e., > 87.5 mg/kg tissue DM) was less for S than for M and B (59.4, 100, and 100%, respectively; P < 0.05). In conclusion, tissue mineral content was proportionate to mineral intake, but differences in mineral availability between loose mineral and block supplements were not evident.

Item	Salt	Loose Mineral	Block	SEM	P-value
ADG, kg/day	0.55a	0.58b	0.55a	0.013	0.03
DMI, kg/day	6.94	7.06	6.92	0.216	0.54
G:F	0.079	0.082	0.079	0.0018	0.11
Cu intake, mg/d	61a	601b	496c	27.3	< 0.01
P intake, g/d	55a	73b	65c	1.6	< 0.01
Zn intake, mg/d	112a	2021b	1296c	89.9	< 0.01

^{a, b, c} Means without a common superscript letter are different, P < 0.05.