Phenology

The phenological stages were recorded in days after sowing (dds), because the measurement was visual (^{Solorzano, 2007}): vegetative (V-1= emergence, V-2= third ligulate leaf, V-3= fifth ligulate leaf, V-4= eighth ligulate leaf, V-5= visible flag leaf) and reproductive (R-6= extended panicle within the flag leaf sheath, R-7= flowering, R-8= grain soft doughy, R-9= grain hard doughy), data were recorded when 50% of the population had the aforementioned characteristics.

Growth variables

They were recorded by periodic and destructive evaluations of one tiller per experimental unit; in measurements plant height (AP), number of leaves (NDH) and number of nodes (NDN), the measurement was made in the main stem, AP was cut at ground level, the stem and was measured with graduated rule, from the basal part until the last leaf or inflorescence. NDH were counted at the time of removing leaves and pods to expose the stem knots (NDN) and perform counting. NDT was performed when cutting the tiller. All measurements were made at 25, 55, 85, 115 and 145 dds; IAF, DAF, TCC and TAN were also calculated.

The yield of dry matter (RMS, t DM ha^-1), was obtained by cutting at ground level, then the plant material was placed on work tables and separated into its morphological components and the leaf blades were passed through an integrator of leaf ares LI-COR 3100, then each component was placed in paper bags properly labeled and remained for 72 h in forced air circulation oven, at 70 °C, then removed and weight in analytical balance and transformed t DM ha^-1

The calculations were made in cut-off intervals: 0-25, 26-55, 56-85 and 86-115 dds, including IAF, DAF, TAN and CBT; for which, the formulas of ^{Escalante and Kohashi (2015}) were used: IAF=[(AF/NP)*(DP/10 000 cm²)], DAF=[(IAF₂-IAF₁)/(T₂-T₁)], TCC=[(PS₂-PS₁)/(T₂-T₁)] and TAN={[(PS₂-PS₁)/(AF₂-AF₁)]/[(InAF₂-InAF₁)/(T₂-T₁)]}; where: AF= leaf area, NP= number of plants, DP= population density, T= time, 1 and 2= start and end of interval, IAF= leaf area index, PS= weight of DM, InAF= natural logarithm of the foliar area.

Elements of the climate and phenological stages

In conditions of tip of irrigation, the minimum temperature fluctuated from 2.3 to 12.6 °C and the maximum from 23.9 to 31.7 °C, the accumulated precipitation was 630 mm, where 42% (269 mm) occurred in the vegetative phase and 58% (361 mm) in reproductive stage (Figure 1).

Figure 1 Climatic conditions and phenological stages in four varieties of forage sorghum, in Montecillo, State of Mexico, 2013. 

In the rainy season, the minimum temperature fluctuated from 2-12.8 °C and maximum 24-28.5 °C, on the other hand, the cumulative rainfall was 534 mm, where 84% (444 mm) occurred in the vegetative phase and 16% (90 mm) in reproductive stage. In this regard ^{Cordoves and Grizel (2009}), report that sorghum requires 450-650 mm of precipitation per crop cycle, therefore, the precipitation was adequate for the development of the crop in both sowing periods; however, in Texcoco, State of Mexico, the average maximum temperature in both periods was 26 °C, with cold mornings (8 °C), which limited the development of sorghum, given that the optimum temperature has a range of 27- 29 °C and C4 plants, require minimum growth temperature of 10 °C.

Despite variations in humidity and temperature, the phenology was similar for both sowing periods, the reproductive stage started at 13 weeks; the early cultivar was Esmeralda, which reached physiological maturity in 25 weeks and the later cultivar (Fortuna) in 28 weeks. The interval between phenological stages of sorghum is determined by genotype and environment, where temperature and precipitation have greater influence, which can advance or lengthen the life cycle of plants (^{Solorzano, 2007}).

Morphological components

They are the result of the environmental, nutritional and genetic expression of the varieties (^{Solorzano, 2007}), where the character of greater economic value is the yield of dry matter. In this sense, the highest yield was slightly higher in the irrigation period (Table 1). The variety that accumulated the largest amount of biomass in less time was sweet cane both in irrigation period (15.7 t DM ha^-1) and temporary (Table 2), 15.5 t DM ha^-1) at 145 dds in temporary we could cut from 115 dds, because the increase in dry matter is minimal, and we avoid deterioration by lignification.

Differences were observed (p≤ 0.001, Table 1 and 2) between varieties, sowing periods, AP, NDT, NDH, NDN and RMS, they increased as the crop cycle progressed at the beginning, the growth was slow, reflecting in small values of AP, NDT, NDN and RMS. Regarding ^{Afzal et al. (2012)}, evaluated yield of fertilized forage sorghum with 75 kg N ha^-1 and indicate that the plant grew 180 cm, as a response to fertilization increase the number of knots and distance between them, report 43 stems per tiller.

In the present work, sweet cane at 145 dds in both sowing periods, presents higher values in AP and similar NDT, in irrigation period. The average number of leaves and knots at 145 dds, in both planting methods was similar; In this regard, ^{Kapanigowda et al. (2013)}, when evaluating 48 lines of sorghum, the number of leaves varied by one per plant.

The varieties of sorghum presented their phase of accelerated growth after 55 dds, which covers from the phenological stage V-3 to R-8. In the stage V-3 the elongation of internodes was given, as reported by ^{Afzal el al. (2012)}, this is reflected at 85 dds with higher AP, NDT, NDH, NDN and RMS (Table 1 and 2). Caña dulce presented the best morphological parameters with respect to the other varieties studied, because it is adapted to edaphoclimatic conditions of the Central High Plateau of Mexico.

Leaf area index (IAF)

In general, the four varieties showed an increase as the crop cycle progressed, Silo honey presented a higher IAF at 114 dds with 5 on the tip of irrigation and 6.8 in the rainy season (Figure 2).

Figure 2 Leaf area index (IAF), in four varieties of sorghum under irrigated and seasonal conditions, in Montecillo, State of Mexico, 2013. 

Subsequently, it decreased when reaching the physiological maturity of the crop, due to leaf senescence and translocation to reproductive organs (^{Smart, 1994}). Similar results were found by ^{Soto and Hernandez (2012}), when evaluating Sorghum bicolor (L.) Moench cultivate ISIAP Dorado, where the maximum value was 5.6 to 80 dds, under irrigation conditions. The variety Silo miel presented higher IAF in irrigation tip (6 to 120 dds) and rainy period (7, to 120 dds) and Fortuna, presented lower value in tip of irrigation (4.5, to 125 dds) and period of rains (2.9, to 145 dds).

This was mainly due to the fact that Silo honey presented leaves of greater length and width, while Fortuna, presented smaller leaves.

Duration of leaf area

The maximum DAF was presented in the Silo miel cultivar with 114 dds at irrigation point and in the rainy season; this is due to the fact that in this last period it had the highest IAF and the canopy obtained the highest radiation intercepted (Figure 3) (^{Rodríguez and Leihner, 2005}), while Fortuna had a lower DAF at 110 days at irrigation point at 80 days under conditions of rains.

Figure 3 Duration of leaf area (DAF), in four varieties of sorghum under irrigated and seasonal conditions, in Montecillo, State of Mexico, 2013. 

The IAF is as important as the DAF, as it is more effective over time, implies greater efficiency in the use of solar radiation; which manifests itself in greater growth and accumulation of dry matter during the crop cycle (^{Hernández and Soto, 2013}).

Growth rate of the crop (TCC)

In general, for the four varieties an increase in emergence was observed at 25 dds (6.2 g m^-2 d^-1, Figure 4).

Figure 4 Growth rate of the crop (TCC), in four varieties of sorghum under irrigated and seasonal conditions, in Montecillo, State of Mexico, 2013. 

The TCC reached a maximum in the interval 56-85 dds (15.4 g m^-2 d^-1) irrigation tip; in the rainy season, in the emergency interval at 25 dds, he had CBT of 4.3 g m^-2 d^-1 and maximum in the range of 56-85 dds (16.95 g m^-2 d^-1), later he decreased in both planting periods, due to the decrease in plant growth, leaf senescence and reduction of photosynthesis (^{Gutierrez et al., 2005}). The Caña dulce cultivar showed higher TCC, with 24 g m^-2 d^-1 in the interval 56-85 dds in irrigation tip and 26 g m^-2 d^-1 in the rainy season. Fortuna had lower TCC with 7.5 g m^-2 d^-1 (irrigation tip) and 8 g m^-2 d^-1 (rainfall) in the range 26-55 dds. ^{Carrillo and Ruiz (2004}), obtained similar results when evaluating six varieties of forage sorghum under irrigation conditions, the cultivar Dine a Mite reached higher TCC with 27.9 g m^-2 d^-1 and lower the cultivar Grazer with 24.42g m^-2 d^-1 to 58 dds.

Net assimilation rate (TAN)

The TAN in four varieties of sorghum, under two conditions of production, it was observed that the highest values occurred at the beginning of the emergency in the range of 0 to 54 dds and decreased as the crop development cycle progressed, until reaching physiological maturity, when lower values were presented, this was due to competition for light, nutrients, increase of the vegetal canopy, greater shade and senescence of lower leaves (^{Naresh and Singh, 2001}; Figure 5).

Figure 5 Net assimilation rate (TAN), in four varieties of sorghum under irrigated and seasonal conditions, in Montecillo, State of Mexico, 2013. 

In the irrigated tip condition, the Caña dulce cultivar presented the highest TAN of the emergency period at 25 dds (19.1 g m^-2 d^-1), 26-55 dds (7.3 g m^-2 d^-1) and 56-85 dds (5.4 g m^-2 d^-1). In the rainy season something similar occurred, the sweet cane cultivar presented the highest values of the emergency period at 25 dds (19 g m^-2 d^-1), 26-55 dds (7 g m^-2 d^-1) and 56- 85 dds (5.4 g m^-2 d^-1).

The cultivar Fortuna had lower TAN, in all the intervals and both sowing periods (irrigation tip and rainy season), similar values reported ^{Saberi and Siti (2013}), levels of (6 g m^-2 d^-1) at 50 dds. While ^{Carrillo and Ruiz (2004}), reported TAN to 58 dds in six varieties evaluated, with values ranging from (5.17 g m^-2 d^-1) in the cultivar Beefbuilder-T to (8.12 g m^-2 d^-1) in the Grazer cultivar under irrigation conditions.

It is a crop tillering capacity, Sorghum bicolor distributes annual total forage production in more than one cut, so planting at the tip of irrigation has the advantage of harvesting forage, before the physiological maturity of the grain and taking advantage of the suitable temperatures for growth from August to October, to produce a second cut; the above, does not occur in traditional highland crops such as corn. Sorghum bicolor is a good option to obtain fresh forage in two harvests, when using sowings at the tip of irrigation.