Evaluation of Sunn Hemp as a Cover Crop for No-Tillage Corn

S. A. Ebelhar and M. P. Plumer

• Introduction
• Results
• Summary

Introduction

Sunn Hemp (Crotalaria juncea) is a tropical legume cover crop with tremendous potential for growth and dry matter accumulation when planted after wheat as a cover crop for corn the following year. It is being evaluated in southern Illinois as a possible substitute for hairy vetch. The cover crops are being evaluated for growth potential, N content and effects on subsequent corn crops.

Sunn hemp was planted at three locations (Dixon Springs Agricultural Center, Ewing Field, and Ralph Upton, Jr. Farm in Springerton) at two different planting dates (mid-July and mid-August) following wheat and at three planting rates (20, 30 and 40 lb/acre). Hairy Vetch was included in the mid-August planting and was drilled at a seeding rate of 20 lb/acre.

Dry matter samples of sunn hemp were taken in mid-November approximately one week after a killing frost. Hairy vetch samples were taken just prior to burndown herbicide application, approximately ten days prior to corn planting.

Results

Dry matter and N content of cover crops. The mid-July planting of sunn hemp had the highest dry matter yield and N content compared to the other covers and planting dates (Table 1). When averaged across the three locations, this planting date produced twice the dry matter of the hairy vetch cover and nearly four times what the mid-August planting date of sunn hemp produced. Cover crop planting rate had no effect on dry matter production (data not shown). At Dixon Springs the hairy vetch and early planted sunn hemp produced about the same amount of dry matter, but because of the higher N concentration of the hairy vetch, the vetch treatment had significantly higher N contents. There was a hairy vetch cover crop failure at Ewing in 1998 when excessive soil wetness cause the vetch to die out during the winter. Excessive wetness also caused reduced vetch growth at Springerton in 1999. The spring at Springerton was very wet with the plots remaining waterlogged for extended periods of time. This would account for the low production from the hairy vetch at this location since hairy vetch does most of it's growing in the late spring. The conditions at all three locations were conducive to grow sunn hemp when planted in mid-July. Only the Dixon Springs location seems to favor hairy vetch over the sunn hemp in terms of N production.

The percent N in the hairy vetch material was about 3.5% whereas the sunn hemp had percent N in the range of 1.5 to 1.75% (data not shown). This indicates that the vetch had a lower C:N ratio and that the N from the vetch would be more readily available to corn than the N from the sunn hemp.

Ear-leaf N concentrations. The hairy vetch cover treatment increased the N concentration in corn ear leaves at each N rate compared to no cover treatment (Table 2). The largest effect of the hairy vetch is with the zero N rate where the leaf N increased from 1.74 to 2.36%. The sunn hemp treatments also increased the ear-leaf N concentrations compared to the no cover treatment, but to a lesser extent than the hairy vetch. This is an indication that the N released from the vetch more readily available than the N in the sunn hemp. At the higher N rates, neither sunn hemp treatment appeared to effectively increase the N levels.

Grain yields. The hairy vetch cover treatment had the highest grain yields, especially at the lower N rates, even though it did not produce the highest dry matter yield and N content. There was a linear response to increasing N rates up to the 120 lb/acre rate, with the greatest benefit occurring at Dixon Springs (Table 3). The hairy vetch with no fertilizer N had much higher yields than the other cover crops with no N. Hairy vetch still appears to be the cover crop choice for no-till corn production in southern Illinois.

There was a significant increase in yields as N rates increased but there was no significant interaction between cover treatments and N rates. There did appear to be a significant increase in yield with hairy vetch covers over the other covers and low N rates, but for the most part fertilizer N was still needed to achieve optimum yields with each of the covers..

Economics. When averaged across N rates the hairy vetch produced an average of 20 bu/acre over the no cover treatments at Dixon Springs. At a cost of about $20 to establish the vetch cover, this was the only economical treatment. At the other locations, the vetch did poorer such that the yield benefits were less and therefore probably not economical. The sunn hemp seed was costly and even at the 20 lb/acre seeding rate would cost $40 or more to establish. Economically, the use of sunn hemp is probably not justified even though it probably did contribute to organic matter production and some improvement in soil quality.

Summary

Planting of the Sunn Hemp to mid-July (after wheat harvest) proved to be quite beneficial in producing high dry matter and N contents. Economically the 20 lb/acre seeding rate of Sunn Hemp is sufficient. Hairy vetch still performs better for grain yields in this area, especially at Dixon Springs. The apparent niche for Sunn Hemp may be where hairy vetch winter kills or grows poorly in the spring because Sunn Hemp produces much of its growth in the late summer (after wheat harvest) and early fall (prior to a killing frost).

Table 1. Summary of dry matter and N contents of cover crops by location, 1998-2000.

	Dixon Springs			Ewing			Springerton			Ave
	98	99	00	98	99	00	98	99	00
	----------------------- lb/acre --------------------------
Dry Matter		&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp
Early Planted Sunn Hemp	-	2739a	4388a	-	7299a	5039a	-	5689a	3767a	4820
Late Planted Sunn Hemp	1366b	725c	1010c	219a	3823b	1195c	757b	1730b	1242c	1341
Hairy Vetch	3679a	1704b	3522b	0b	2880c	2574b	2945a	1224b	1878b	2267
LSD	262	403	426	80	707	565	139	614	567	&nbsp
N Content		&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp
Early Planted Sunn Hemp	-	55a	90b	-	112a	83b	-	79a	70a	82
Late Planted Sunn Hemp	25b	11b	30c	5a	56c	32c	22b	25b	40b	27
Hairy Vetch	129a	63a	159a	0b	93b	104a	103a	32b	71a	84
LSD	4.1	18.4	16.4	1.7	9.2	18.1	5.4	13.6	18.6	&nbsp
MEANS	DS	DW	SP	&nbsp	&nbsp	&nbsp	DS	DW	SP	&nbsp
Dry Matter				N Content
Early Planted Sunn Hemp	3564	6169	4728	Early Planted Sunn Hemp			73	98	75
Late Planted Sunn Hemp	1034	1746	1243	Late Planted Sunn Hemp			22	31	29
Hairy Vetch	2968	1818	2016	Hairy Vetch			117	66	69

 

Table 2. Summary of cover and N effects on leaf N concentrations by location, 1998-2000.

Cover	N Rate	Dixon Springs			Ewing			Springerton			Ave
		98	99	00	98	99	00	98	99	00
	&nbsp	----------------- % N in Corn Ear-leaves ----------
Check	0	1.37	1.82	1.76	Data not collect-ed	1.73	1.92	1.84	Data not collect-ed	1.48	1.74
	60	1.30	2.95	2.38		3.01	2.16	1.85		2.04	2.53
	120	1.89	3.32	2.78		3.38	2.48	1.60		2.51	2.89
	180	1.72	3.38	2.80		3.51	2.70	1.57		2.79	3.04
Early Sunn Hemp	0	-	2.09	1.69		2.26	1.81	-		1.67	1.90
	60	-	2.96	2.50		3.19	1.88	-		2.19	2.54
	120	-	3.15	2.91		3.28	2.33	-		2.62	2.86
Late Sunn Hemp	0	1.15	1.81	1.57		2.11	1.84	1.42		1.66	1.80
	60	1.43	3.04	2.28		3.01	2.12	1.75		2.25	2.54
	120	1.73	3.38	2.94		3.47	2.46	1.68		2.62	2.97
Hairy Vetch	0	1.88	2.53	1.98		3.05	2.26	1.83		1.99	2.36
	60	1.53	2.98	2.46		3.39	1.93	1.72		2.60	2.67
	120	1.62	3.41	2.78		3.58	2.67	2.10		2.63	3.01
Contrasts	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp
N Rate linear	&nbsp	**	**	**	n/a	**	**	*	n/a	**	&nbsp
N Rate quad.	&nbsp	**	**	NS	&nbsp	**	*	NS	&nbsp	NS	&nbsp
Cover x N Rate	&nbsp	NS	NS	NS	&nbsp	**	NS	NS	&nbsp	NS	&nbsp

10%, * and ** refer to significance at the 10%, 5% and 1% levels, respectively. NS = non-significant.

Table 3. Summary of cover and N effects on yields by location, 1998-2000.

Cover	N Rate	Dixon Springs				Ewing				Springerton				Ave
		98	99	00	Ave	98	99	00	Ave	98	99	00	Ave
	&nbsp	------------------- bu/acre ---------------------
Check	0	76	81	60	71	38	58	52	55	112	48	66	57	61
	60	83	123	107	115	53	98	80	89	116	78	87	83	96
	120	118	124	128	126	87	114	96	105	133	63	117	90	107
	180	122	129	148	139	90	108	98	103	131	129	129	129	124
Early Sunn Hemp	0	-	88	60	74	-	71	59	65	--	42	71	57	65
	60	-	118	108	113	-	100	78	89	--	55	104	80	94
	120	-	134	135	135	-	106	87	97	--	53	117	85	105
Late Sunn Hemp	0	78	79	71	75	45	66	68	67	118	36	68	52	65
	60	102	130	107	115	62	98	89	94	132	52	108	80	96
	120	119	130	130	130	96	117	95	106	134	56	120	88	108
Hairy Vetch	0	100	109	95	102	34	110	48	79	112	55	103	79	87
	60	115	133	134	134	55	116	58	87	114	24	129	77	99
	120	145	135	138	137	91	115	72	94	134	68	120	94	108
	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp
Contrasts	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp	&nbsp
N Rate linear	&nbsp	**	**	**	&nbsp	**	**	**	&nbsp	*	**	**	&nbsp	&nbsp
N Rate quad.	&nbsp	NS	**	**	&nbsp	10%	**	NS	&nbsp	NS	NS	*	&nbsp	&nbsp
Cover x N Rate	&nbsp	NS	NS	NS	&nbsp	NS	*	NS	&nbsp	NS	NS	NS	&nbsp	&nbsp

10%, * and ** refer to significance at the 10%, 5% and 1% levels, respectively. NS = non-significant.

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