Parental single crosses of maize hybrids

Between parental single crosses of the hybrids H-44, H-52, H-66, H-68E, H-70, and H-153E there were significant differences (p≤0.01) for the beginning of male flowering, yield, and seed quality (Table 1). This was due to the genetic characteristics of each one of the lines that form the simple cross (^{Virgen-Vargas et al., 2014}) and to the response of the plant to the damage caused by the removal of the tassel (^{Espinosa-Calderón et al., 2010a} and ^b); mainly the difference in the number of self-pollinations between them (^{Espinosa et al., 2003}; ^{Ávila et al., 2009}; ^{Arellano et al., 2011}). These results are similar to those obtained by ^{Virgen et al. (2010)} and ^{Virgen et al. (2013)} when evaluating parental single crosses of the maize hybrids H-50, H-52, H-64, H-66, H-68, and H-70 in Estado de México and Tlaxcala.

The number of days until male and female flowering were, on average, 70 and 72 (Table 1). This indicates that the simple crosses showed floral synchrony, which is why the female parent must be detasseled within no more than two days to avoid self-pollination and maintain the genetic quality of the certified category seed (^{Vallejo et al., 2008}). In the simple cross CML-241 x CML-243, the number of days until the beginning of male and female flowering was equal, therefore the exact moment in which to remove the tassel must be defined carefully. In M-43 x M-44 the difference between the beginning of flowering was 4 d, indicating more time to begin detasseling. Another factor to consider is the uniformity of the flowering of the female parent (^{Martínez-Lázaro et al., 2005}); the more uniformity there is, the lower the number of detasseling steps and the lower the cost of this activity in comparison to other parent variables, the male flowering period of which is broad (10 to 20 d). Crosses CML-239 x CML-242 and CML-241 x CML-243 have a greater uniformity, since they are formed with lines of six to eight self-pollinations, in comparison to crosses M-43 x M-44, M-47 x M-46, and M-55 x M-54, which have more variability (^{Espinosa et al., 2003}; ^{Avila et al., 2009}; ^{Arellano et al., 2011}).

Average seed yield was 7.79 Mg ha-1 and 80 % of the parental single crosses of hybrids yielded between 7 and 9 Mg ha^-1 (Table 1). Crosses M-43 x M-44 and M-55 x M-54, parents of hybrids H-52, H-66, and H-70, had the highest yields, with 9.1 and 9.4 Mg ha^-1, and the lowest yield was in CML-241 x CML- 243, parent of the H-68E, with 4.79 Mg ha^-1. These results are similar to those reported by ^{Avila et al. (2009)} and ^{Espinosa et al. (2012)}. The results were obtained in experimental lots, as opposed to a hybrid seed detasseling lot, where yield is lower due to factors such as asynchrony in male and female flowering between parents, scarce control of weeds, pests, and diseases, and genetic quality, as well as losses during the cleaning of the seed. In this regard, ^{Barrón (2010)} pointed out that in the production of certified seeds of hybrids H-515 and H-516 they obtained 3 Mg ha^-1 of marketable seeds, and for the hybrid H-318 between 4 and 7 Mg ha^-1 were obtained (^{Vallejo et al., 2008}). The yields of seeds from female simple crosses of hybrids H-44, H-52, H-66, H-68E, H-70, and H-153E make seed production profitable for domestic businesses, since the cost-benefit is >1; it is 1.9 to produce a trilinear hybrid, indicating that for each peso invested, one can recover this plus another 1.9 pesos (^{Vallejo et al., 2008}; ^{Virgen et al., 2013}).

Simple crosses had, on average, 60, 27, and 13 % large, medium, and small seeds, respectively (Table 1). CML-239 x CML-242, M-43 x M-44, and M-55 x M-54 displayed percentages of large seeds (8 mm) of over 70 % and of small seeds (6 mm) of 5 to 8 %, since they have conical ears (^{Avila et al., 2009}; ^{Arellano et al., 2011}), whereas M-47 x M-46 and CML-241 x CML-243 had between 49 and 39 % large seeds, 30 to 46 % of small seeds (7 mm), and 19 to 21 % small seeds, indicating a conical-cylindrical ear. Seed size is an important characteristic for the sale and profitability for seed companies, since producers prefer large and medium seeds, although small seeds have good physiological quality. ^{Laynez-Garsaball et al. (2007)} and ^{Tadeo-Robledo et al. (2010)} did not find significant differences in the vigor of plantlets between sizes of maize seed varieties. However, at the beginning of growth of the small seeds there was a lower accumulation of dry matter than in large seeds (^{López-Santillán et al., 2004}).

The test weight and weight of 200 seeds were, on average, 64 kg hL^-1, and 72.33 g, the cross CML- 241 x CML-243 had the highest test weight (68.33 kg hL^-1) and the lowest weight for 200 seeds with 48.33 g, whereas crosses M-55 x M-54 and M-43 x M-44 had the lowest weight of 200 seeds with 88.64 and 85.59 g and the lowest test weight (62.88 and 62.66 kg hL^-1) (Table 1). These figures are similar to those obtained by ^{Zepeda et al. (2002)} with simple crosses H-14 and H-214. The seeds with the highest weight have the least number of seeds, and vice-versa. The number of seeds per kilogram is important when establishing a detasseling lot to determine the population density of both parents, and for the sowing ratio of male and female rows, as well as the greater amount of seeds of the male parent that can be used in case planting more rows is required to obtain more pollen or as border for isolation, in compliance with seed certification regulations (^{SAG, 1975}).

Effect of removing the tassel in the female parent (detasseling)

There were no significant differences (p>0.05) in yield and physical seed quality between levels of detasseling (Table 2), due probably to the fact that the tassel demands nutrients, and removing it did not alter the physiology of the plant, but rather contributed to photosynthates being directed at filling the seeds (^{Martínez et al., 2005}). There was no significant reduction (p>0.05) in yield when removing the tassel and flag leaf because the photosynthetic capacity of the plant was not affected, since the area of the flag leaf removed was less than in comparison to other leaves, which were enough to carry out photosynthesis. ^{De Brito (2011)} pointed out that reductions of over 41.01 % of the foliar area in maize plants jeopardize their productivity. Similar results were observed by ^{Espinosa-Calderón et al. (2010b)} when removing the tassel with or without a different number of leaves in maize hybrid H-49; and in hybrid H-47 was reduced seed yield of 26 % with the removal of the tassel (^{Espinosa-Calderón et al., 2010a}). But removing one or two leaves in both hybrids did not affect yield (^{Tadeo et al., 2013}).

In a hybrid maize seed production lot, it is crucial to remove the tassel of the female parent (^{Martínez-Lázaro et al., 2005}), which is why it is important to count the days until the beginning and male and female mean flowering in both parents. Simple crosses had an average of 70 d to begin male flowering, and 72 d for female flowering (Table 2). There is a period of two days to remove the tassel from the female parent, so as to maintain genetic quality and avoid self-pollination. It is recommendable to begin 2 to 4 d after the appearance of the first tassel (male flowe) and continue for 10 and 20 d every third day until all tassels have been removed, depending on the genetic characteristics of both parents, the effect of the environment and their interaction in a detasseling lot (^{Virgen et al., 2013}; ^{Virgen-Vargas et al., 2014}).

Seed yield increased 6.31 % (480 kg ha^-1) with the removal of the plant’s tassel in comparison to the control (tassel not removed). This could be due to the fact that by removing the source of demand, nutrients were translocated to the cobs and used for plant growth, and reduced the percentage of small seeds (10.5 %). Removing the tassel, and the tassel and flag leaf had no significant negative effects on the yield and quality of the seed. ^{Espinosa-Calderón et al. (2010b)} also observed a positive effect on yield when removing the tassel from the female parent. This indicates the possibility of reducing at least one step towards detasseling seed production lots or the posibility of callibrating machinery to make the cut in the point of insertion of the flag leaf. However, in each female parent it is important to know the effect of eliminating the tassel along with one, two, or three leaves, because in some parents, seed yield is affected (^{Espinosa-Calderón et al., 2010a}).

Effect of the environment (years of production)

Significant differences (p≤0.01) were observed between the spring-summer planting cycles of the years 2006, 2007, and 2008 for the beginning of male and female flowering, yield, and physical quality in seeds (Table 3). This indicated that at least one characteristic was different from one year to the next, perhaps due to the difference in temperature (average of 16 °C in each year) and amount and distribution of rainfall; the rainy season covered between June and September, and rainfall was 722.7 mm in 2007, and 20 and 32 % higher than in 2006 (583 mm) and 2008 (495.2 mm) (Figure 1, Agro-Weather Station of the ^{Colegio de Postgraduados, 2013}). These differences probably contributed to the differences in both days for the beginning of flowering, yield, and the physical quality in seeds for

Figure 1 Average monthly temperature and rainfall for 2006 and 2008 (Agro- Weather Station of the ^{Colegio de Post graduados, 2013}). 

Between years there was a difference of 4 d for the beginning of female flowering; 2006 displayed the most days, and 2007, the least (Table 3). This may be due to the fact that during the stages of germination, vegetative stage, and floral differentiation of the maize plant, temperatures fell between 2.4 and 0.5 °C, causing lower growth and development of the plant. Similar results were observed by ^{Zepeda-Bautista et al. (2009)} and ^{Virgen et al. (2013)} in parental single crosses of maize hybrids for the High Valleys of Mexico.

Average seed yield was 7.79 t ha^-1; in 2008 it was 17.12 % higher, and in 2006, 33.24 % lower (Table 3). This may be due to the greater avaiability of water in the stages of flowering and grain filling, due to rainfall; in 2008, rainfall was highest in September (week 40, 53.50 mm) in comparison to 2006 (week 40, 12.40 mm) (Figure 1). This confirms the effect of the environment on the productivity of female parental single crosses in maize hybrids (^{Virgen et al., 2010}; ^{Virgen et al., 2013}), which is useful to determine the location, date, and how to establish a seed production lot. ^{Avila et al. (2009)} observed a similar environmental effect when increasing the female simple cross of the hybrid H-52 in the Texcoco Valley; production was between 9 and 10 Mg ha^-1 with 70 % of large seed, and in Tlaxcala, 6.5 Mg ha^-1 in yield for registered seed. In 2008 a greater seed size was observed than in 2006 (Table 3), in which large seeds along with the el test weight and the weight of 200 seeds were 64.94, 8.42, and 42.32 % smaller.

Genotype x environment interaction

The interactions parental single crosses x years of evaluation were significant (p≤0.01) for the beginning of male flowering and seed yield, as well as the percentage of large, medium, and small seeds, test weight and the weight of 200 seeds (data not shown), since at least one simple cross had a different behavior in each agricultural year. This was due to their genetic characteristics and agroclimatic conditions that determine the production and quality of the hybrid seed (^{Virgen et al., 2013}; ^{Virgen-Vargas et al., 2014}).

Days until the beginning of male flowering

Between years, a change in the number of days until the beginning of male flowering was observed in 100 % of the simple crosses (Figure 2). This parameter is important to define the moment in which to remove the tassel of the female parent and the synchrony of male and female flowering, for pollination and formation of the seed. Similar results were observed by ^{Virgen et al. (2010)} after evaluating parental single crosses of hybrids in locations in Tlaxcala. The simple cross M-55 x M-54, female parent of the hybrid H-70, displayed a difference of 2 d until the beginning of male flowering (67 to 69 d). CML- 239 x CML-242 displayed a higher variation in days until the beginning of male flowering. In 2007 it flowered at 68 d and 5 d later in 2006. This difference causes problems when establishing a detasseling lot, since it may cause synchronicity problems, and in consequence, a lack of seed formation, the loss of genetic quality due to pollution of foreign pollen, and self ’pollination of the female due to badly-timed pollination.

Figure 2 Genotype x environment interaction (years) until beginning of male flowering in parental single crosses in male hy brids. Texcoco, Estado de México (2006-2008). 

Seed yield

In all the parental single crosses between years, different seed yields were obtained, since it is a trait determined by many genes and influenced by the environment (^{Poehlman, 1979}; ^{Márquez, 1988}). The simple cross CML-241 x CML-243 had the lowest variation in seed yield: in 2007 and 2008, it produced an average of 5.16 Mg ha^-1, which could be due to the higher rainfalls during flowering and grain filling (Figura 1); in 2006 yield was 21.45 % lower (1.11 Mg ha^-1). The cross CML-239 x CML- 242 had its highest yield (10.28 Mg ha^-1) in 2008, and its lowest, 14.19 and 70.55%, in 2007 and 2006 (Figure 3).

The simple cross M-43 x M-44, female parent of hybrids H-52 and H-66 (^{Avila et al., 2009}; ^{Arellano et al., 2010}), displayed its highest yield (11. 24 Mg ha^-1) in 2008. In 2007 and 2006, they were lower by 14.11 and 42.32 % (Figure 3). ^{Avila et al. (2009)} observed seed yield of 9 to 10 Mg ha^-1 for the cross M-43 x M-44 in a detasseling lot located in the Texcoco Valley, at altitudes of 2300 masl; and the Valle de Mexico Experimental Field, at 2250 masl. During the spring-summer 2011 cycle 3.79 and 3.52 Mg ha-1 were obtained of the direct (M-43 x M-44) and reciprocal crosses (^{Virgen et al., 2013}). Simple cross M-55 x M-54 displayed a lower variation in seed yield, in 2007 and 2008, it yielded an average of 10.75 Mg ha^-1, and in 2006, yield was 36.77 % lower (3.95 Mg ha^-1). ^{Virgen et al. (2013)} produced, in the Valle de Mexico Experimental Field, in the spring-summer 2011 cycle, 1.55 and 3.19 Mg ha^-1 of the direct (M-54 x M-55) and reciprocal (M-55 x M-54) crosses. This shows the unstable behavior of the female parents, which as a direct effect on yield and seed quality, crucial for success in production, economic and technical profitability in domestic businesses (^{Vallejo et al., 2008}; ^{Virgen-Vargas et al., 2014}).

Figure 3 Genotype x environment interaction (years) for seed yield in parental single crosses of maize hybrids. Texcoco, Estado de México. 2006-2008.