Lucerne (Medicago sativa L.) is the legume most used for livestock feeding around the world, its importance lies in its yield and nutritional value¹. Its phenology and regrowth rate allows repeated crops². In the State of Oaxaca, the use is either green or marketed in bulk³.

The forage regrowth is considered as the time it takes the plant to recover the removed biomass after a defoliation, hence the importance of knowing the plant phenology to accumulate their biomass between crops⁴. With the advance of the maturity, physical and chemical changes of the forage cause a decreased digestibility and increased yield, reducing the leaf:stem ratio, which is linked with quality, since the leaves and stems contain different concentrations of protein and fiber^5,6; likewise, the regrowth depends on the morphogenesis of the plant for the production of leaves⁷.

A scheme of management based on a calendar resting days or by the stage of development of the crop, is a good choice for handling, reaching the widespread recommendation of harvest when there is a 10 % flowering or the presence of the leave flag, in legumes and grasses respectively; however, a recommendation cannot be universally applicable^8,9. Therefore, it is necessary to know the dynamics of biomass accumulation after defoliation, which varies by environmental factors. In this way, knowledge of seasonal changes in yield, allows to determine the best harvest frequency, to get more high quality forage production by area^10,11.

In this regard in the State of Mexico, a study¹² reported that lucerne presented a higher forage yield on May. Also¹³, in Oaxaca, assessing five lucerne varieties found that the greater dry matter (DM) yield was on July and August, with an average of 2.82 t DM ha^{- 1}, and presented the following descending order: summer 31 % > spring 27 % > fall 22 % > winter 20 %. Another investigation¹⁴ in Oaxaca reported that 14 varieties of lucerne grown in greenhouse and with fertigation, does not register variations in DM yield; however, the highest yield and height were recorded on April and May, so the temperature is closely related to the forage performance¹⁵.

In several research studies has been shown the importance of regrowth and the effect of the harvest frequency in the persistence, and the environmental effect on yield, which allows for more efficient handling of animal-vegetal production per hectare¹⁶. In a study conducted in Oaxaca¹⁰ recorded that in spring, lucerne variety Valencia, reached its maximum performance at the 6th wk (4.7 t DM ha^-1), while the variety Oaxaca (creole lucerne) recorded it at 7th wk (4.1 t DM ha^-1), concluding that the optimum time of cutting varied with the season of the year.

The quality, yield, persistence and longevity of the pasture depends on seasonal frequency of defoliation management, so defining the cuts calendar based on the re-emergence of the crop in each season is of paramount importance¹³. However, in the central valleys of Oaxaca, lucerne is harvested every 40 d regardless of the season of the year, causing changes in its morphological and biochemistry composition. Therefore, the objective of the present study was to evaluate the dynamics of seasonal growth in a pastureland of lucerne Var. Oaxaca or Creole, to determine the optimum time to harvest by season, to get the highest yield and forage quality.

The study was conducted in a meadow of lucerne Var. Oaxaca creole (Medicago sativa L.) of 3 yr of established during a year of assessment (February 2011 - January 2013), located in the community of the Villa de Zaachila, Oaxaca, at 16° 56' 38.58” N and 96° 44' 12.61” W, at an altitude of 1,509 m. The climate is temperate subhumid C(w), with rains in summer, with an average annual temperature of 17.5 ^oC, and average rainfall of 730 mm. The type of soil is shallow, dark grey, with horizon A Malic acid, horizon B argillic, clayey texture, strong adhesiveness and plasticity, slightly pH alkaline, of medium fertility, classified as a Foazen livic¹⁷.

For standardization, lucerne was cut at a height of 5 cm at the beginning of the experiment. The blocks were delimited based on the ground slope and established 28 experimental units of 5 x 2 m, using a randomized complete block design with seven treatments (week of cutting) and four replications. The pasture was not fertilized and in the dry season (October to May) irrigation was provided every 2 wk to field capacity.

Forage yield was measured with a steel frame of 0.25 m² per replicate¹⁵; in the randomly selected sample place, all the fodder contained within this place was cut to a height of 5 cm; the harvested plant material was deposited in paper bags previously marked with the number of treatment and the corresponding repetition, it was washed and weighed fresh, then dried in a microwave oven until constant weight to obtain the dry matter value¹⁸.

The forage height was recorded before each cut with a graduated ruler 1 m length and a precision of 0.5 cm. There were ten measurements within each treatment, plants chosen at random, with the settled rule completely vertical from the base of the plant to the youngest higher leaf^19,20.

Forage harvested in each experimental unit was mixed to homogenize the sample and subsequently took a sub-sample of approximately 20 % of the forage and plants were classified by species, dead material, other grasses and weeds; it was separated into its morphological components (stems, leaves, inflorescence and dead material); each component weighted in fresh and recorded their weight in dry basis¹.

The growth rate (GR) was calculated with data of dry matter yield through the following formula: GR= HF/t. Where HF= harvested forage (kg DM ha^-1), and t= days between a cut and the next.

The leaf:stem ratio was obtained by dividing the morphological component leaf yield between the component stem yield.

Monthly data average temperature (maximum, average and minimum) weather and precipitation during the study period were obtained from the meteorological station of the airport Benito Juárez from the city of Oaxaca, located 13 km from the study area.

The grouped values were graphed using the statistical software SigmaPlot vol.12²¹ and analyzed with the PROC MIXED procedure of SAS²². When the assumption of normality of variables was not fulfilled, they were transformed according to the suggestions of SAS. The Akaike criterion was used to select the variance and covariance matrix²³, with what the effects of the sources of variation were determined: days of regrowth (7, 14, 21, 28, 35, 42, 49), time of the year (spring, summer, autumn and winter), that were considered as fixed effects, and the effect of blocks considered as random²⁴. Treatments means were estimated using LSMEANS and comparison among them was carried out by the probability of the difference (PDIFF), based on the “t” test of “Student” to a level of significance of 5 %.

For the linear regression between the height of the pasture and forage yield, the height was performed just prior to the sampling of the forage. The equations of linear regression, and 95 % confidence intervals and prediction of performance for time of the year, were obtained by the wizard regression module of the SigmaPlot V.12 software using a polynomial linear model²¹.

The greatest dry matter yield (P<0.01) was recorded in winter at 14 d compared with yields in spring, summer and fall, with lower yields in the fall (Figure 1A). Table 1 shows that all seasons showed a gradual increase in forage accumulation, reaching greater yields at 49 d in spring, summer and autumn with 2,794, 2,680 and 2,116 kg DM ha^-1, respectively, while in winter it was at 42 d (3,416 kg). Taking into account the maximum performance obtained in each of the seasons of the year, the winter exceeded on 22, 27 and 61 %, to spring, summer and autumn, respectively. In particular in spring there were no differences (P>0.05) in the yield among d 35, 42 and 49 of regrowth.

Table 1 Forage yield, growth rate, forage height and leaf: stem ratio in native lucerne grasslands subjected to growth cycles of 7 wk 

^abcd Means with the same letters in each row are not different. SE = standard error.

* (P<0.05); ** (P<0.01).

Figure 1 Dry matter yield (A), Leaf:stem ratio (B), Growth rate (C), Forage height (D). In seven weeks growth cycles 

These results differ with those reported elsewhere^1,25, which concluded that the best seasonal lucerne yield in Montecillo, State of Mexico, was observed in summer; and agrees partially to reports with the same lucerne variety¹⁰, which mention that the higher dry matter accumulation was at 49 d in spring, but do not agree to what was reported in summer, autumn and winter, where higher biomass accumulation occurred on d 28, 35 and 42, respectively.

Differences in results may be due to temperature in the winter, as during those months the average temperature fluctuated between 19 and 21 °C while in other studies the presence of frost affected the net lucerne accumulation^1,12,16. According to what was reported in the literature²⁶, the optimum temperature for lucerne growth is of 15 to 25 °C during the day and 10 to 20 °C at night. Yield decrement at wk 7 all times of the year may be because the plant enters its reproductive phase, and with maturity, senescence is increased in the total population of plants. But this yield loss can be compensated by other components of the pastureland as the relationship size:density²⁷.

With respect to the plant height (Table 1) during spring and winter there were no differences after 35 d of regrowth (56 to 60 cm and 58 to 61 cm, respectively); in the summer on d 42 and 49 (49.5 to 54.7 cm); and only in autumn was recorded a higher height (P<0.01) until wk 7 (55 cm). Figure 1D, shows that with the exception of winter and spring, after 28 d of regrowth summer and autumn recorded lower heights (P<0.05). Forage height is a variable that is highly correlated with dry matter yield²⁰, if the correlation coefficients are high (> 0.80), this variable can be considered to determine the yield and define the optimum harvesting time. In this experiment in the four seasons of the year the r² were higher than 0.8 (Figure 2), which indicates that the parameters obtained are acceptable and similar to other researchs^19,20. Other authors¹⁴ reported that with lucerne in greenhouse and under fertigation conditions, the highest yields coincided with higher values of forage height. Other stuudies²⁸ mention that the heights of the forage varies only in the first year of establishment of the crop, but from the second year onwards, the heights are not significant (P>0.05), so regardless of the variety, the lucerne presents similar heights; however the temperature influences yield and height by time of year.

Slope of the line (black line), confidence interval (blue line), prediction of the data (red line).

Figure 2 Linear regressions of the yield vs height, confidence interval at 95, and prediction of the data, in the four seasons of the year 

The growth rate is an approximation of plant biomass accumulation in time after defoliation. The age at which lucerne reached the highest GR varied depending on the time of year (Table 1), and the higher GR values recorded were in the winter, at 21, 28 and 42 d of regrowth (81 kg DM ha^-1 d^-1)(P<0.05); in the spring there was not a difference from 21 to 57 d of growth; however, the greatest record was at 5 wk (68 kg); in summer the highest value was at d 28 (58 kg DM ha^-1 d^-1); and in the fall to 28 and 35 d (47 kg). With the exception of the fall (P<0.05) there were not reported differences among days of cutting in the GR, just in winter at wk 1 presented the lower GR, but subsequently in wk 2 to 6, it was superior to the other seasons of the year (Figure 1C). These values are similar to those observed in another study¹⁰, with the same lucerne variety from a year of established; the higher GR was in spring between d 28 and 35, while in winter only matched by the 6th wk of regrowth. These results differ to results from other studies^25,29, where report lower GR values in winter, unlike this work; however, in both studies^25,29 the presence of frost led to a lower growth rate, unlike the present study where there were no frost in winter (Figure 3).

Figure 3 Climate data during the experimental period. Source: Meteorological station of the Benito Juárez airport, of the Oaxaca city 

The leaf:stem ratio is the variable most correlated with the nutritious forage quality, and in this experiment that was so, in that except for the winter (P<0.05), there were no differences at other times (Figure 1B). Differences between regrowth days were only in spring and summer (P<0.01), being the d 28 and 35 which recorded the greatest value (1.7 and 1.5, respectively). In autumn the best value was at d 28 and in winter to d 35 (Table 1). Rivas et al¹³ mention that the lowest ratio occurs in lucerne varieties that produce the greatest dry matter yield, and corroborates the importance of the variables stem (height and weight) in relation to the leaves, for the accumulation of total dry matter and the quality or nutrition in the plant; i.e. that the higher height the lower leaf proportion, and this is related to the growth phase of the forage. Other works⁶ mentioned that the amount of protein in the leaves and stem, is affected by age of the crop, but it has no influence on the content of dry matter and crude fiber; therefore, the cut must be defined by a combination of variables of the species assessed. Other authors⁹, mention that the emergence of primary leaves is increased when lucerne is in their regrowth stage, during reproduction or destined to the production of seed, and this has repercussions on the interception of light, biomass root accumulation, and therefore in the total biomass yield and nutritive quality of the forage.

The obtained results differ to those reported with the same lucerne variety and in the same area¹⁰, explaining that the lowest leaf:stem ratio presented the following descending order: autumn > summer > winter > spring, and that regardless of the season of the year, the relationship tended to decrease as the week of regrowth increased. In the same way it has been reported²⁵ that greater leaf:stem ratio was presented in autumn and winter. A progressive decrease in the ratio was observed in another study³⁰ in which as regrowth age of lucerne increased, registered greater forage accumulation at the 5th wk in summer, autumn and spring while in winter it was at the 6th wk. These differences could be explained because in the present study the temperature conditions in winter led to a greater dry matter accumulation and the development of more tall stems (Figure 1D), which increased its yield contribution; this, coupled with a faster replacement tissue as it could be observed by greater growth rates in this period (Figure 1C), led to the presence of senescent material, and the loss of leaves by this process caused the leaf:stem ratio (Figure 1B) be lower than in the other seasons.

Regarding the botanical composition (Figure 4), components recorded a variation in their contribution according to the seasons of the year; the biggest contribution of lucerne was recorded in winter and autumn, and the increased presence of other grasses and weeds was in spring and summer. In the winter and summer seasons the biggest contribution of lucerne was at 28 d of growth, in spring at 21 and 35 d, and in the fall until the d 49 (Figure 3). These results agree with those reported elsewhere²⁵, which in spring and summer were increased presence of weeds; it has also been reported a greater percentage of weeds in autumn and lesser in summer seasons, while in spring there was an increase in the percentage of weeds and a reduction in lucerne²⁹. This is consistent with the increase in rainfall in both times, which along with the temperature, led to the emergence of other grass species that came into competition with lucerne.

Figure 3 Botanical composition of native lucerne, in cycles of growth of 7 wk, at different times of the year 

In conclusion, for the cultivation of lucerne Var. Creole Oaxaca, in the village of Zaachila, the best time for cutting is every 30 to 35 d in winter and spring, and every 35 to 40 d in summer and autumn, to obtain the highest forage yields and better leaf:stem ratio. Cutting should be based on the dynamics of growth, the growth rate, leaf:stem ratio, botanical composition and height variables, in order to obtain the optimum defoliation time for the best content of leaf with respect to stem, best height and lower proportion of dead material, which allows to obtain a better forage quality. The growth dynamics determine the phenological crop performance at different times of the year, which varies depending on the environmental conditions.