A study was conducted to determine the effect of increasing levels of magnesium oxide on intake of a self-limited, monensin-containing energy supplement for growing steers grazing winter wheat pasture. Forty eight steers were assigned to four winter wheat pastures, and were given free access to a milo-based self-limited energy supplement that contained 4% salt, 75 mg monensin/lb and .25, .75, 1.25 or 1.75% magnesium oxide (as-fed basis). Individual animal supplement consumption was measured using pinpointer feeders. Daily gains were not affected by increasing levels of magnesium oxide. Supplement consumption averaged 1.49 lb/day, with animals spending an average of 15.7 minutes/day eating supplement and an average of 2.7 visits to the feeder/steer/day. Increasing levels of magnesium oxide did not appear to decrease supplement intake.

(Key Words: Wheat Pasture, Growing Cattle, Supplementation.)

Self-limited monensin-containing energy supplements, when consumed at a targeted intake level of 2 to 3 lb/hd/d, have been shown to increase live weight gains of growing steers grazing winter wheat by approximately .5 lb/d over unsupplemented cattle (Horn et al. 1990, 1992; Beck et al. 1993). Self-limited energy supplements, while improving performance, can also provide an ionophore or bloat preventative to decrease death loss due to bloat. Predictable supplement intake is essential in providing adequate bloat protection while maximizing utilization of wheat forage. The addition of monensin decreased daily supplement intake from 5.03 to 1.44 lb/steer while maintaining similar weight gains (Paisley et al. 1996), but the magnitude of the effects of salt and magnesium oxide remain unclear. The objective of this study was to determine the effects of increasing levels of magnesium oxide on intake of a self-limited supplement.

Study Site. Studies were conducted at the wheat pasture research facility located at Stillwater, OK. The research facility consists of four 20 acre pastures planted to hard red winter wheat and equipped with automatic waterers and pinpointer feeders designed to measure supplement intakes on an individual animal basis. Cattle were allowed to graze from November 8 through December 20, 1996 with large round bales of bermudagrass hay offered free-choice during the study. Steers were allowed access to pinpointer supplement feeders beginning November 13 with electronic measurement of individual supplement intake beginning November 22. Forage mass available for grazing in each of the four pastures was monitored during the study by hand-clipping wheat forage to ground level inside nine 2 ft² quadrants systematically selected across each pasture.

Supplement. Supplements used in this study were milo-based energy supplements containing 4% salt and 75 mg monensin/lb (as-fed basis) formulated to contain .25, .75, 1.25 or 1.75% magnesium oxide (BayMag; Table 1). Supplements were fed in meal form, with samples taken during sacking to determine actual levels of magnesium and monensin being fed (Table 2). Additional feed samples were also taken each time feed was added and samples were composited across days within each pasture.

Cattle. Forty eight head of spring-born Angus X Hereford crossbred steers (avg wt 550lb, Table 3) were obtained from one of the OSU beef cow herds. Steers were initially weighed November 8 and assigned to one of four pastures. Cattle were fitted with pinpointer collars on November 13 and allowed to adapt to feeders for 8 days prior to intake measurement. Final weights were taken December 20, with both initial and final weights recorded after a 14 h shrink.

Statistics. Steer weights and ADG were analyzed as a completely randomized design with animal as the experimental unit. Supplement intakes were analyzed using two models. Model I contained Treatment, Steer(Treatment), Day, and Treatment X Day as sources of variation, and was used to analyze all 1344 intake observations (28 d x 48 steer). Treatment X Day was used as the error term. For Model II, individual supplement intakes were averaged across the 28 d intake period, and the 48 observations were analyzed with Treatment as the only independent variable. Pre-planned orthoganal linear, quadratic, and cubic contrasts were used to interpret the effect of increasing levels of magnesium oxide on supplement intake.

Final weights and daily gains of steers were not affected (P>.11; Table 3) by increasing levels of magnesium oxide. Supplement intake was affected by magnesium oxide level, with both Model I and II indicating that increasing magnesium oxide levels resulted in a linear increase (P<.03; Table 4) in supplement intake. Orthogonal contrasts for both models indicate a quadratic effect (P=.01) of magnesium oxide level on time spent in the feeder. As magnesium oxide concentration increased, time spent in the feeder initially decreased, followed by a large increase in time, with the most time spent in the feeder occurring with steers receiving 1.75% MgO. Although not significant (P>.05), visits/day appear to follow the same trend as minutes spent in the feeder for Model I and II (P=.09 and .06, respectively).

In summary, magnesium oxide was originally believed to affect intake of this self-limited energy supplement . In this study, increasing levels of magnesium oxide appeared to increase supplement intake, however, differences were small and supplement consumption for all levels of magnesium oxide remained slightly below the targeted 2 to 3 lb/steer/d as previously reported. This study suggests that feeding magnesium oxide at levels less than 1.75% does not decrease supplement intake. In addition, including all daily supplement intake data (Model I) slightly reduced the standard error of the means for supplement intake, but the use of Model I increased standard errors for minutes spent eating as well as number of visits/d. The daily variability in supplement intake may discourage the use of daily observations in analyzing supplement intake for cattle grazing high quality forages.

Beck, P.A. et al. 1993. Okla Agr. Exp. Sta. Res. Rep. P-933:256.

Horn, G.W. et al. 1990. Okla. Agr. Exp. Sta. Res. Rep. MP-129:209.

Horn, G.W. et al. 1992. Okla. Agr. Exp. Sta. Res. Rep. MP-136:301.

Paisley, S.I. et al. 1996. Okla. Agr. Exp. Sta. Res. Rep. P-951:218.

This material is based upon work supported by the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture, under Agreement No. 93-34198-8410. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

 
 

Table 1. Feedstuff and nutrient content of energy supplements (as-fed basis).
Ingredient
Milo, Ground, %	62.8
Wheat middlings, %	21.0
Molasses, sugarcane, %	4.8
Limestone (38%), %	4.0
Dicalcium Phosphate, %	2.6
Fine Mixing Salt, %	4.0
BayMag (Magnesium Oxide), %	++a
Rumensin 80 Premix	(851 grams/ton)b
Calculated nutrient content
Dry matter, %	88.7
NEm, Mcal/cwt	61.3
NEg, Mcal/cwt	37.9
Crude protein, %	9.17
Calcium, %	2.2
Phosphorus, %	.9
Magnesium, %	++a

^aAdded to achieve .25, .75, 1.25, or 1.75% magnesium oxide (as-fed basis).

^bTo result in monensin concentration of 75 mg/lb of supplement.

 
 

Table 2. Calculated and actual monensin and magnesium concentrations in supplements (as-fed basis).
	.25% MgO	.75% MgO	1.25% MgO	1.75% MgO
Monensin, mg/lb
Calculated	75	75	75	75
Actual	80	85	82	85
Magnesium, %
Calculated	.43	.70	.98	1.26
Actual	.49	.85	1.17	1.56

 

 
 

Table 3. Effect of energy supplementation with increasing levels of magnesium oxide on performance of steers grazing winter wheat.
		Level of magnesium oxide, %							Contrasta
Item	.25		.75	1.25	1.75	SEb		L		Q	C
BW Nov. 8, lb	562		556	548	533	15.5		.18		.80	.96
BW Dec. 20, lb	632		637	616	605	16.2		.16		.62	.62
ADG, lb/d	1.71		1.98	1.67	1.74	.132		.74		.45	.11

^a Observed significance level for linear (L), quadratic (Q), and cubic (C) contrasts.

^bStandard error of the means.

 
 

Table 4. Intake of a self-limited monensin-containing energy supplement with increasing levels of magnesium oxide.
		Level of magnesium oxide, %							Contrasta
Item	.25		.75	1.25	1.75	SEb		L		Q	C
Model I
Intake, lb× hd-1× d-1	1.31		1.45	1.27	1.93	.138		.01		.07	.07
Min. eating suppl.	15.5		13.5	13.7	20.2	1.44		.03		.01	.55
Visits to feeder	2.66		2.53	2.52	3.13	.217		.16		.09	.62
Model II
Intake, lb× hd-1× d-1	1.31		1.45	1.27	1.93	.162		.03		.12	.13
Min. eating suppl.	15.5		13.5	13.7	20.2	1.37		.02		.01	.53
Visits to feeder	2.66		2.53	2.52	3.13	.190		.11		.06	.57

^a Observed significance level for linear (L), quadratic (Q), and cubic (C) contrasts.

^bStandard error of the means.