Forage yield

Average seasonal yield was highest in spring (9,447 kg DM ha^-1), although this did not differ with that of summer in POV1. This variable was lowest in autumn (3,120 kg), which did not differ from winter in the same period (Table 1). Yield behavior was directly related to optimal temperatures (22 °C) for growth in L. corniculatus³. Temperatures were favorable in spring (Figure 1), which benefited growth and production. This agrees with yield behavior reported in a study of five populations of L. corniculatus using grazing intervals of 20 and 40 d¹⁷. Seasonal changes in forage species growth performance can therefore be attributed to seasonal environmental conditions¹⁸. For instance, in L. corniculatus seasonal distributions in forage production have been reported of 32 % in spring, 30 % in summer, 23 % in winter and 15 % in autumn; 62 % of production occurred in spring-summer¹⁹. Slightly lower average yields (7,700 kg DM ha^-1) have been reported for L. corniculatus at a 45-d cutting interval in Texcoco, which were influenced by climate, management and genotype growth habit³.

Accumulated forage production differed between treatments (P≤0.05) (Table 1). In POV1, yield was lowest in the FC treatment, with 27 % less production than in the 95 % IL treatment (28,417 vs 20,730 kg DM ha^-1). In POV2 the FC treatment had a cumulative yield 29 % less than the average of the three IL treatments (26,952 vs 19,100 kg DM ha^-1). This may be related to the shorter harvest interval (< 33 d) in the FC treatments versus the longer average harvest interval (70 d) in the IL treatments (Figure 2). This coincides with a study in which L. corniculatus (cultivar 202700) grown in Texcoco exhibited a lesser adaptation to a season-defined fixed cut than cuts based on IL percentages; the fixed cut yielded 29 % less forage than the IL treatments²⁰. In some forage species, frequent cuts decrease yield and foliar area, leading to greater presence of undesirable species²¹, and consequent greater competition with the desired species and depletion of their carbohydrate reserves²².

90 = 90 % IL; 95 = 95 % IL; 100 = 100 % IL; CF = season-defined fixed cut (autumn = 35-d interval, winter = 42-d interval, spring-summer = 28-d interval).

Figure 2 Average cut intervals for L. corniculatus, genotype 255301 

Botanical and morphological composition

The morphological component contributing most to yield was the leaf (average= 14,273 kg DM ha^-1; 56 %), followed by the stem (30.5 %), dead material (8.5 %) and weeds (4.5 %). Of the IL treatments, the 95 % IL produced the highest average leaf yield (16,526 kg DM ha^-1), which was higher (P≤0.05) than the other IL treatments during POV2. The FC treatments in both study periods produced the lowest average yields of leaves (12,276 kg DM ha^-1) and stems (4,710 kg DM ha^-1). Dead material and weed percentages did not differ between treatments (P ≥ 0.05) in either period (Table 2).

The higher leaf and stem production observed in the 95% IL treatment may be related to greater crop growth age²³. In addition, the higher leaf production in the 95% IL versus the FC treatment may have resulted from a compensation for greater stem biomass caused by a longer growth period, which implies more time producing photosynthates²⁴.

The leaf component also contributed most to forage yield in different seasons (Figure 3). Average yield in both periods was highest in the spring (5,141 kg DM ha^-1) and lowest in the winter (1,580 kg DM ha^-1), with the 95% IL treatment having the highest yield (5,852 kg DM ha^-1) and the FC the lowest (1,163 kg DM ha^-1). Stem yield was also highest in the spring (3,347 kg DM ha^-1) and lowest in the winter (772 kg DM ha^-1). These variations in component yields are probably responses to changes in environmental conditions between different seasons (Figure 1). For example, changes in stem production are known to be a function of seasonal variations in pasture light quantity and quality, precipitation and temperature¹⁸. Dead material (873 kg DM ha^-1) and weed (268 kg DM ha-¹) production was highest in summer, although these yields did not differ (P≥0.05) from the corresponding spring yields during POV1. These yields were lowest in autumn (237 and 133 kg DM ha^-1, respectively). This may result from self-shading of the basal area by the plant since growth is greater in seasons with favorable development conditions²⁵.

Figure 3 Botanical and morphological composition of L. corniculatus, genotype 255301, as a function of intercepted light (90, 95 and 100 %) and season-defined fixed cut (FC) (autumn = 35-d interval, winter = 42-d interval, spring-summer = 28-d interval) 

Plant height

Plant height differed between treatments and seasons (P≤(0.05). Height was greatest in the 90, 95 and 100 % IL treatments, which averaged 21.5 cm. This is slightly taller (P≥0.05) than in the 90 % IL treatment in POV2 and noticeably taller than the 17 cm average for the FC treatments in both periods (Table 3). These results are related to pasture age because in the IL treatments the plants had 70 d for regrowth compared to the FC treatments with an average of 33 d in both periods (Figure 2). In the FC treatments, plants were younger and cut more frequently, resulting in lower forage yield¹⁷. Greater height is reported to correspond to higher forage yield¹⁵, and for L. corniculatus yield and height have also been linked to erect and prostrate growth habits³.

In both study periods the highest average height (24 cm) was recorded in spring, although this did not differ (P≥0.05) from average height in the summer and autumn in POV1. Average height was lowest in winter (17 cm), which, in POV2, did not differ (P≥0.05) from autumn. Plant height was greatest during the seasons with optimal humidity and temperature conditions for growth and yield in L. corniculatus (Figure 1). These results coincide with those in a study of twelve L. corniculatus genotypes in the State of Mexico, Mexico³. Adequate photoperiod, temperature and humidity can allow forage plants to accelerate growth and exhibit changes in height between seasons²⁶. This directly affects forage yield¹², which, in L. corniculatus genotype 255301, is associated with its prostrate growth habit³. Pasture height is therefore predictive of forage production⁸. Based on this assumption and using the present results, calculations were done of the equivalence (per centimeter of plant height) of the highest yields by season and treatment. In the spring each centimeter of height corresponded to a yield of 167 kg DM ha^-1, as an average of both study periods. In POV1, height in the 95 % IL treatment corresponded to a 144 kg DM ha^-1 yield, while in POV2 the height in the 90, 95 and 100 % IL treatments corresponded to an average yield of 192 kg DM ha^-1.

Leaf:stem ratio

Average leaf:stem ratio values in both periods were highest in the fixed cut treatment (P≤0.05), followed by the 90, 95 and 100 % IL treatments (Table 4). In POV1, this ratio in the FC treatment was 36 % higher than the average of the 90, 95 and 100 % IL treatments (2.8 vs 1.8), while in POV2 the ratio in the FC was 44 % higher than in the 100% IL treatment (3.2 vs 1.4). The larger leaf:stem ratio in the FC treatments was caused by more frequent harvests (average= 33-d interval)(Figure 2). Lotus corniculatus is in the accelerated growth phase at this interval, which is not optimal for harvest, since it exhibits the highest percentage of young leaves and fewer stems²⁷. In addition, post-cut incident light quality and quantity in a pasture are altered by cutting interval, leading to variations in leaf and stem production and consequent changes in the leaf:stem ratio¹⁰.

The leaf:stem ratio was higher in winter (P≤0.05): 2.6 in winter POV1; 2.5 average in autumn and winter POV2. In some forage species leaf:stem ratio values are lower in seasons when plant growth is lower (e.g. autumn and winter) due to higher stem density coupled with lower weight²⁷. The lower ratio values (P≤0.05) observed here during the spring and summer in both periods resulted from higher individual stem weight²⁶, which is a possible response to greater translocation of assimilates from the leaves to the stems during these seasons⁷.