Table 4. Response surface regression analysis of silage corn production against soil temperature at planting and growing degree days at harvest

Silage corn maturity	Parameters	Coefficient of regression	p-value	R2
Early	Constant	−605.80	0.15	0.79
	STP	−105.07	0.51
	GDD	2.92	0.08
	STP×STP	−67.77	0.01
	GDD×STP	−0.65	0.06
	GDD×GDD	−0.01	0.09
Medium	Constant	−618.09	0.11	0.76
	STP	34.33	0.72
	GDD	1.89	0.05
	STP×STP	−98.75	< 0.01
	GDD×STP	−0.08	0.68
	GDD×GDD	−0.01	0.02
Late	Constant	−121.35	0.58	0.96
	STP	252.47	< 0.01
	GDD	1.23	0.10
	STP×STP	−76.75	< 0.01
	GDD×STP	0.25	< 0.01
	GDD×GDD	−0.01	< 0.01

STP, soil temperature at planting; GDD, growing degree day.