Fertilization of Extremely Grazed and Moderately Grazed Mixed-Grass Prairie with Slow Release Phosphorus and Urea

Total production on the moderately grazed trial resulted in no treatment effects over the four years (Table 3). A year effect resulted in 2002 being significantly lower than 2000, 2001, and 2003. Cool season grasses, warm season grasses, forbs, and shrubs had similar treatment effects although year effects were prominent in each plant class. Phosphorus, being a limiting factor in plant growth, and the second most important nutrient behind nitrogen, should be a concern late in the grazing season in regards to the plants being able to meet the minimum cattle requirements of phosphorus. During the grazing season the control was deficient of phosphorus 37% of the time (Table 2). The 33 lb N/ac was deficient 44% of the time and 66 lb N/ac was deficient 49% of the time, versus the May 1 applications of 24 lb P/ac being deficient 12% and 48 lb P/ac 2% of the time. The June 20 applications of 24 lb P/ac were deficient 11% and 48 lb P/ac 9% of the time. It appears that the higher lbs/ac applied is more beneficial than the earlier date applied. This is opposite what was found on the extremely grazed trial where the earlier application seemed more beneficial than the higher pounds applied. Overall, the moderately grazed trial and the extremely grazed trial were similar in percent time phosphorus deficient from 2000 to 2002. The moderately grazed trial resulted in a phosphorus deficiency on the nitrogen treatments 47% of the grazing season. The phosphorus treatments were deficient 9%, of the season and the control was deficient 37% of the grazing season. The extremely grazed trial resulted in a phosphorus deficiency on the nitrogen treatments 37% of the grazing season. Phosphorus treatments were deficient 12% of the season and the control was deficient 42% of the grazing season. Forbs on the moderately grazed fertilization exclosure had a year effect with 2001 and 2002 having significantly lower forb diversity than 2000. A treatment effect was also observed with 33 lbs N/ac having higher forb diversity than 24 lbs P/ac June 20, 66 lbs N/ac June 20, control, and 48 lbs P/ac May 1 (Figure 2).

Table 2. Slow release fertilizer treatment application rates and percent time (from June-September) that plants did not meet the minimum phosphorus requirements of a 1,200 lb lactating cow with respect to fertilizer treatments on the extremely grazed and moderately grazed exclosures, Central Grasslands Research Extension Center, Streeter.
Fertilizer Type	Date Applied	Rate Applied	Extremely Grazed	Moderately Grazed
			Percent time P deficient
Control	None	None	42%	37%
Phosphorus	May 1	24 lb/ac	11%	12%
Phosphorus	May 1	48 lb/ac	7%	2%
Phosphorus	June 20	24 lb/ac	16%	11%
Phosphorus	June 20	48 lb/ac	13%	9%
Urea	June 20	33 lb/ac	31%	44%
Urea	June 20	66 lb/ac	42%	49%

Table 3. Above ground production (lb/ac) for fertilizer treatments on moderately grazed mixed grass prairie at Central Grasslands Research Extension Center, Streeter.
Fertilizer Treatment
	Year	Control	May 1 24 lb P/ac	May 1 48 lb P/ac	June 20 24 lb P/ac	June 20 48 lb P/ac	June 20 33 lb N/ac	June 20 66 lb N/ac
Cool-season grass	2000^y	2174^abcd	2736^abcd	3166^bcd	2512^abcd	2671^abcd	2034^abc	1936^abc
	2001^y	2656^abcd	3127^bcd	2520^abcd	2740^abcd	2131^abcd	2877^abcd	3291^cd
	2002^x	1787^ab	1868^ab	1717^a	2036^abc	1931^abc	1985^abc	2198^abcd
	2003^y	2447^abcd	2535^abcd	2522^abcd	2292^abcd	2337^abcd	2932^abcd	3435^d
Warm-season grass	2000	36	3	1	0	1	160	64
	2001	0	18	0	0	192	1	0
	2002	31	2	1	0	20	2	3
	2003	0	0	61	62	65	0	0
Forb	2000^z	676^ab	532^ab	918^abc	1008^abc	748^abc	1421^bc	1169^abc
	2001^y	973^abc	439^a	828^abc	649^ab	773^abc	620^ab	441^a
	2002^x	533^ab	263^a	325^a	306^a	364^a	529^ab	271^a
	2003^z	1040^abc	773^abc	1656^a	1220^abc	797^abc	656^ab	539^ab
Shrub	2000^z	367	573	69	281	541	449	390
	2001^xy	191	247	189	174	263	255	315
	2002^x	88	218	111	92	226	187	269
	2003^xy	265	263	166	411	279	246	224
Total	2000^y	3254	3844	4156	3801	3962	4066	3561
	2001^y	3821	3833	3538	3564	3362	3755	4048
	2002^x	2441	2352	2154	2434	2542	2705	2741
	2003^y	3753	3572	4407	3986	3479	3835	4199
¹Means in a plant class followed by a different letter differ (P<0.05). ²Means in the year column in each plant class followed by a letter differ (P<0.05).

NDSU Central Grasslands Research Extension Center

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