Inbred corn line PHHB4

According to the invention, there is provided an inbred corn line, designated PHHB4. This invention thus relates to the plants and seeds of inbred corn line PHHB4 and to methods for producing a corn plant produced by crossing the inbred line PHHB4 with itself or with another corn plant. This invention further relates to hybrid corn seeds and plants produced by crossing the inbred line PHHB4 with another corn line or plant.

FIELD OF THE INVENTION 
This invention is in the field of corn breeding, specifically relating to 
an inbred corn line designated PHHB4. 
BACKGROUND OF THE INVENTION 
Plant Breeding 
Field crops are bred through techniques that take advantage of the plant's 
method of pollination. A plant is self-pollinated if pollen from one 
flower is transferred to the same or another flower of the same plant. A 
plant is cross-pollinated if the pollen comes from a flower on a different 
plant. 
Corn plants (Zea mays L.) can be bred by both self-pollination and 
cross-pollination techniques. Corn has separate male and female flowers on 
the same plant, located on the tassel and the ear, respectively. Natural 
pollination occurs in corn when wind blows pollen from the tassels to the 
silks that protrude from the tops of the incipient ears. 
The development of a hybrid corn variety involves three steps: (1) the 
selection of plants from various germplasm pools; (2) the selfing of the 
selected plants for several generations to produce a series of inbred 
lines, which, although different from each other, breed true and are 
highly uniform; and (3) crossing the selected inbred lines with unrelated 
inbred lines to produce the hybrid progeny (F.sub.1). During the 
inbreeding process in corn, the vigor of the lines decreases. Vigor is 
restored when two unrelated inbred lines are crossed to produce the hybrid 
progeny. An important consequence of the homozygosity and homogeneity of 
the inbred lines is that the hybrid between any two inbreds will always be 
the same. Once the inbreds that give a superior hybrid have been 
identified, the hybrid seed can be reproduced indefinitely as long as the 
homogeneity of the inbred parents is maintained. 
The objective of commercial maize inbred line development programs is to 
develop new inbred lines that combine to produce high grain yields and 
superior agronomic performance in hybrid combination. The primary trait 
breeders seek is yield. However, other major agronomic traits are of 
importance in hybrid combination and have an impact on yield or otherwise 
provide superior performance in hybrid combinations. Such traits include 
percent grain moisture at harvest, relative maturity, resistance to stalk 
breakage, resistance to root lodging, grain quality, and disease and 
insect resistance. In addition the lines per se must have acceptable 
performance for parental traits such as seed yields, kernel sizes, pollen 
production, all of which affect ability to provide parental lines in 
sufficient quantity and quality for hybridization. Traits have been shown 
to be under genetic control and many if not all of the traits are affected 
by multiple genes. Thus, to be selected as an inbred line, the inbred must 
be able to combine such that the desired traits are passed to the hybrid 
and also be able to satisfy production requirements as a parental line. 
Pedigree Breeding 
The pedigree method of breeding is the mostly widely used methodology for 
new inbred line development. 
In general terms this procedure consists of crossing two inbred lines to 
produce the non-segregating F.sub.1 generation, and self pollination of 
the F.sub.1 generation to produce the F.sub.1 generation that segregates 
for all factors for which the inbred parents differ. An example of this 
process is set forth below. Variations of this generalized pedigree method 
are used, but all these variations produce a segregating generation which 
contains a range of variation for the traits of interest. 
Example 1. 
Hypothetical example of pedigree breeding program 
Consider a cross between two inbred lines that differ for alleles at five 
loci. 
##STR1## 
the F.sub.1 from a cross between these two parents is: 
##STR2## 
Selfing F.sub.1 will produce an F.sub.2 generation including the following 
genotypes: 
##STR3## 
The number of genotypes in the F.sub.2 is 3.sup.6 for six segregating loci 
(729) and will produce (2.sup.6)-2 possible new inbreds, (62 for six 
segregating loci). 
Each inbred parent which is used in breeding crosses represents a unique 
combination of genes, and the combined effects of the genes define the 
performance of the inbred and its performance in hybrid combination. There 
is published evidence (Smith, O.S., J. S. C. Smith, S. L. Bowen, R. A. 
Tenborg and S. J. Wall, TAG 80:833-840 (1990)) that each of these lines 
are different and can be uniquely identified on the basis of 
genetically-controlled molecular markers. 
It has been shown (Hallauer, Arnel R. and Miranda, J.B. Of. Quantitative 
Genetics in Maize Breeding, Iowa State University Press, Ames Iowa (1981)) 
that most traits of economic value in maize are under the genetic control 
of multiple genetic loci, and that there are a large number of unique 
combinations of these genes present in elite maize germplasm. If not, 
genetic progress using elite inbred lines would no longer be possible. 
Studies by Duvick and Russell (Duvick, D. N. Maydica 37:69-79 (1992); 
Russell, W. A. Maydica XXIX:375-390 (1983)) have shown that over the last 
50 years the rate of genetic progress in commercial hybrids has been 
between 1 and 2% per year. 
The number of genes affecting the trait of primary economic importance in 
maize, grain yield, has been estimated to be in the range of 10-1000. 
Inbred lines which are used as parents for breeding crosses differ in the 
number and combination of these genes. These factors make the plant 
breeder's task more difficult. Compounding this is evidence that no one 
line contains the favorable allele at all loci, and that different alleles 
have different economic values depending on the genetic background and 
field environment in which the hybrid is grown. Fifty years of breeding 
experience shows that there are many genes affecting grain yield and each 
of these has a relatively small effect on this trait. The effects are 
small compared to breeders' ability to measure grain yield differences in 
evaluation trials. Therefore, the parents of the breeding cross must 
differ at several of these loci so that the genetic differences in the 
progeny will be large enough that breeders can develop a line that 
increases the economic worth of its hybrids over that of hybrids made with 
either parent. 
If the number of loci segregating in a cross between two inbred lines is n, 
the number of unique genotypes in the F.sub.2 generation is 3.sup.n 
(Example 2) and the number of unique inbred lines from this cross is 
{(2.sup.n) -2}. Only a very limited number of these combinations are 
useful. Only about 1 in 10,000 of the progeny from F.sub.2 's are 
commercially useful. 
By way of example, if it is assumed that the number of segregating loci in 
F.sub.2 is somewhere between 20 and 50, and that each parent is fixed for 
half the favorable alleles, it is then possible to calculate approximate 
probabilities of finding an inbred that has the favorable allele at 
{(n/2)+m} loci, where n/2 is the number of favorable alleles in each of 
the parents and m is the number of additional favorable alleles in the new 
inbred. See Example 2 below. The number m is assumed to be greater than 
three because each allele has so small an effect that evaluation 
techniques are not sensitive enough to detect differences due to three or 
less favorable alleles. The probabilities in Example 2 are on the order of 
10.sup.-5 or smaller and they are the probabilities that at least one 
genotype with (n/2)+m favorable alleles will exist. 
To put this in perspective the number of plants grown on 60 million acres 
(approximate U.S. corn acreage) at 25000 plants/acre is 
1.5.times.10.sup.12. 
Example 2. 
Probability of finding an inbred with m of n favorable alleles. 
Assume each parent has n/2 of the favorable alleles and only 1/2 of the 
combinations of loci are economically useful. 
______________________________________ 
no. of no. favorable 
no. additional 
Probability 
segregating 
alleles in favorable alleles 
that genotype 
loci (n) Parents (n/2) 
in new inbred 
occurs* 
______________________________________ 
20 10 14 3 .times. 10.sup.-5 
24 12 16 2 .times. 10.sup.-5 
28 14 18 1 .times. 10.sup.-5 
32 16 20 8 .times. 10.sup.-6 
36 18 22 5 .times. 10.sup.-6 
40 20 24 3 .times. 10.sup.-6 
44 22 26 2 .times. 10.sup.-6 
48 24 28 1 .times. 10.sup.-6 
______________________________________ 
*Probability that a useful combination exists, does not include the 
probability of identifying this combination if it does exist. 
The possibility of having a usably high probability of being able to 
identify this genotype based on replicated field testing would be most 
likely smaller than this, and is a function of how large a population of 
genotypes is tested and how testing resources are allocated in the testing 
program. 
At Pioneer Hi-Bred International, a typical corn research station has a 
staff of four, and 20 acres of breeding nursery. Those researchers plant 
those 20 acres with 25,000 nursery rows, 15,000 yield test plots in 10-15 
yield test sites, and one or two disease-screening nurseries. Employing a 
temporary crew of 20 to 30 pollinators, the station makes about 65,000 
hand pollinations per growing season. Thus, one of the largest plant 
breeding programs in the world does not have a sufficiently large breeding 
population to be able to rely upon "playing the numbers" to obtain 
successful research results. Nevertheless, Pioneer's breeders at each 
station produce from three to ten new inbreds which are proposed for 
commercial use each year. Over the 32 Pioneer research stations in North 
America, this amounts to from about 100 to 300 new inbreds proposed for 
use, and less than 50 and more commonly less than 30 of these inbreds that 
actually satisfy the performance criteria for commercial use. 
This is a result of plant breeders using their skills, experience and 
intuitive ability to select inbreds having the necessary qualities. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a novel inbred corn line, 
designated PHHB4. This invention thus relates to the seeds of inbred corn 
line PHHB4, to the plants of inbred corn line PHHB4, and to methods for 
producing a corn plant produced by crossing the inbred line PHHB4 with 
itself or another corn line. This invention further relates to hybrid corn 
seeds and plants produced by crossing the inbred line PHHB4 with another 
corn line. 
DEFINITIONS 
In the description and examples that follow, a number of terms are used 
herein. In order to provide a clear and consistent understanding of the 
specification and claims, including the scope to be given such terms, the 
following definitions are provided. ABS is in absolute terms and % MN is 
percent of the mean for the experiments in which the inbred or hybrid was 
grown. 
BAR PLT=BARREN PLANTS. The percent of plants per plot that were not barren 
(lack ears). 
BRT STK=BRITTLE STALKS. This is a measure of the stalk breakage near the 
time of pollination, and is an indication of whether a hybrid or inbred 
would snap or break near the time of flowering under severe winds. Data 
are presented as percentage of plants that did not snap. 
BU ACR=YIELD (BUSHELS/ACRE). Actual yield of the grain at harvest in 
bushels per acre adjusted to 15.5% moisture. 
DRP EAR=DROPPED EARS. A measure of the number of dropped ears per plot and 
represents the percentage of plants that did not drop ears prior to 
harvest. 
EAR HT=EAR HEIGHT. The ear height is a measure from the ground to the 
highest placed developed ear node attachment and is measured in inches. 
EAR SZ=EAR SIZE. A 1 to 9 visual rating of ear size. The higher the rating 
the larger the ear size. 
EST CNT=EARLY STAND COUNT. This is a measure of the stand establishment in 
the spring and represents the number of plants that emerge on a per plot 
basis for the inbred or hybrid. 
GDU SHD=GDU TO SHED. The number of growing degree units (GDUs) or heat 
units required for an inbred line or hybrid to have approximately 50 
percent of the plants shedding pollen and is measured from the time of 
planting. Growing degree units are calculated by the Barger Method, where 
the heat units for a 24-hour period are: 
##EQU1## 
The highest maximum temperature used is 86.degree. F. and the lowest 
minimum temperature used is 50.degree. F. For each inbred or hybrid it 
takes a certain number of GDUs to reach various stages of plant 
development. 
GDU SLK=GDU TO SILK. The number of growing degree units required for an 
inbred line or hybrid to have approximately 50 percent of the plants with 
silk emergence from time of planting. Growing degree units are calculated 
by the Barger Method as given in GDU SHD definition. 
GRN APP=GRAIN APPEARANCE. This is a 1 to 9 rating for the general 
appearance of the shelled grain as it is harvested based on such factors 
as the color of the harvested grain, any mold on the grain, and any 
cracked grain. High scores indicate good grain quality. 
MST=HARVEST MOISTURE. The moisture is the actual percentage moisture of the 
grain at harvest. 
PLT HT=PLANT HEIGHT. This is a measure of the height of the plant from the 
ground to the tip of the tassel in inches. 
POL SC=POLLEN SCORE. A 1 to 9 visual rating indicating the amount of pollen 
shed. The higher the score the more pollen shed. 
POL WT=POLLEN WEIGHT. This is calculated by dry weight of tassels collected 
as shedding commences minus dry weight from similar tassels harvested 
after shedding is complete. 
It should be understood that the inbred can, through routine manipulation 
of cytoplasmic factors, be produced in a cytoplasmic male-sterile form 
which is otherwise phenotypically identical to the male-fertile form. 
PRM=PREDICTED RM. This trait, predicted relative maturity (RM), is based on 
the harvest moisture of the grain. The relative maturity rating is based 
on a known set of checks and utilizes standard linear regression analyses 
and is referred to as the Comparative Relative Maturity Rating System 
which is similar to the Minnesota Relative Maturity Rating System. 
RT LDG=ROOT LODGING. Root lodging is the percentage of plants that do not 
root lodge; plants that lean from the vertical axis at an approximately 
30.degree. angle or greater would be counted as root lodged. 
SCT GRN=SCATTER GRAIN. A 1 to 9 visual rating indicating the amount of 
scatter grain (lack of pollination or kernel abortion) on the ear. The 
higher the score the less scatter grain. 
SDG VGR=SEEDLING VIGOR. This is the visual rating (1 to of the amount of 
vegetative growth after emergence at the seedling stage (approximately 
five leaves). A higher score indicates better vigor. 
SEL IND=SELECTION INDEX. The selection index gives a single measure of the 
hybrid's worth based on information for up to five traits. A corn breeder 
may utilize his or her own set of traits for the selection index. One of 
the traits that is almost always included is yield. The selection index 
data presented in the tables represent the mean value averaged across 
testing stations. 
STA GRN=STAY GREEN. Stay green is the measure of plant health near the time 
of black layer formation (physiological maturity). A high score indicates 
better late-season plant health. 
STK CNT=NUMBER OF PLANTS. This is the final stand or number of plants per 
plot. 
STK LDG=STALK LODGING. This is the percentage of plants that did not stalk 
lodge (stalk breakage) as measured by either natural lodging or pushing 
the stalks and determining the percentage of plants that break below the 
ear. 
TAS BLS=TASSEL BLAST. A 1 to 9 visual rating was used to measure the degree 
of blasting (necrosis due to heat stress) of the tassel at time of 
flowering. A 1 would indicate a very high level of blasting at time of 
flowering, while a 9 would have no tassel blasting. 
TAS SZ=TASSEL SIZE. A 1 to 9 visual rating was used to indicate the 
relative size of the tassel. The higher the rating the larger the tassel. 
TAS WT=TASSEL WEIGHT. This is the average weight of a tassel (grams) just 
prior to pollen shed. 
TEX EAR=EAR TEXTURE. A 1 to 9 visual rating was used to indicate the 
relative hardness (smoothness of crown) of mature grain. A 1 would be very 
soft (extreme dent) while a 9 would be very hard (flinty or very smooth 
crown). 
TILLER=TILLERS. A count of the number of tillers per plot that could 
possibly shed pollen was taken. Data is given as percentage of tillers: 
number of tillers per plot divided by number of plants per plot. 
TST WT=TEST WEIGHT (UNADJUSTED). The measure of the weight of the grain in 
pounds for a given volume (bushel). 
TST WTA=TEST WEIGHT ADJUSTED. The measure of the weight of the grain in 
pounds for a given volume (bushel) adjusted for percent moisture. 
YLD=YIELD. It is the same as BU ACR ABS. 
YLD SC=YIELD SCORE. A 1 to 9 visual rating was used to give a relative 
rating for yield based on plot ear piles. The higher the rating the 
greater visual yield appearance. 
MDM CPX=Maize Dwarf Mosaic Complex (MDMV=Maize Dwarf Mosaic Virus & 
MCDV=Maize Chlorotic Dwarf Virus): Visual rating (1-9 score) where a "1" 
is very susceptible and a "9" is very resistant. 
SLF BLT=Southern Leaf Blight (Bipolaris maydis, Helminthosporium maydis): 
Visual rating (1-9 score) where a "1" is very susceptible and a "9" is 
very resistant. 
NLF BLT=Northern Leaf Blight (Exserohilum turcicum, H. turcicum): Visual 
rating (1-9 score) where a "1" is very susceptible and a "9" is very 
resistant. 
COM RST=Common Rust (Puccinia sorghi): Visual rating (1-9 score) where a 
"1" is very susceptible and a "9" is very resistant. 
GLF SPT=Gray Leaf Spot (Cercospora zeae-maydis): Visual rating (1-9 score) 
where a "1" is very susceptible and a "9" is very resistant. 
STW WLT=Stewart's Wilt (Erwinia stewartii): Visual rating (1-9 score) where 
a "1" is very susceptible and a "9" is very resistant. 
HD SMT=Head Smut (Sphacelotheca reiliana): Percentage of plants that did 
not have infection. 
EAR MLD=General Ear Mold: Visual rating (1-9 score) where a "1" is very 
susceptible and a "9" is very resistant. This is based on overall rating 
for ear mold of mature ears without determining specific mold organism, 
and may not be predictive for a specific ear mold. 
ECB DPE=Dropped ears due to European Corn Borer (Ostrinia nubilalis): 
Percentage of plants that did not drop ears under second brood corn borer 
infestation. 
ECB 2SC=European Corn Borer Second Brood (Ostrinia nubilalis): Visual 
rating (1-9 score) of post flowering damage due to infestation by European 
Corn Borer. A "1" is very susceptible and a "9" is very resistant. 
ECB 1LF=European Corn Borer First Brood (Ostrinia nubilalis): Visual rating 
(1-9 score) of pre-flowering leaf feeding by European Corn Borer. A "1" is 
very susceptible and a "9" is very resistant.