Manhours per Acre and Yield in bushels per acre

Wheat	1800	1840	1860	1880	1900	1970
Total(mnhrs/ac)	56	35	28	20	15	3
Before harvest	16	12	10	8	7	2
Harvest	40	23	18	12	8	1
Yeild/acre(bu)	15	15	14	13	14	31
Man hr/100 bu	373	233	193	152	108	9

Corn	1800	1840	1860	1880	1900	1970
Total(mnhrs/ac)	86	69	58	46	38	5
Before harvest	56	44	36	28	22	29
Harvest	30	25	22	18	16	2
Yeild/acre(bu)	25	25	25	26	26	72
Man hr/100 bu	334	276	228	180	147	7

Cotton	1800	1840	1860	1880	1900	1970
Total(mnhrs/ac)	185	135	127	119	112	24
Before harvest	135	90	79	67	62	21
Harvest	50	45	49	52	50	3
Yeild/acre(bale)	147	147	168	188	189	438
Man hr/100 bales	601	438	371	303	284	26

one bale cotton = 478pounds

Two interesting aspects of this data:
First, data shows Cotton to be more labor intensive than wheat or corn, making it prone to a very cheap labor system like slavery. Second, the dramatic effect on productivity of modern methods and mechanization by 1970.

Source: Historical Statisitcs of the United States Colonial Times to 1970; U.S. Dept. Commerce; Bureau of the Census, 1976.

above from tables k445-485 p.500 part 1

M. Collie October 2008