Dry matter yield

The dry matter (DM) yield of the aerial part of the plot was obtained by carrying out five random samplings on the plot with squares of 0.25 m² and the forage was harvested at ground level. The biomass of each square was deposited in previously labeled Ziploc-type plastic bags, to later determine the partial moisture in the forage laboratory of the College of Postgraduates, Montecillo campus, and the residual moisture in the animal nutrition laboratory of the Department of Zootechnics of the Chapingo Autonomous University (UACH).

To quantify the partial dry matter (pDM), approximately 100 g of fresh matter was placed in a #8 paper bag and deposited in a forced air oven for 72 h at 55 °C, once the time was completed, the bags were removed from the oven and then weighed on a dibatec balance with 600 g capacity, once the weights were obtained, the pDM was calculated with the following formula: % pDM= (weight of dry matter/weight of fresh matter)*100. Once the samples were partially dry, the total dry matter (tDM) was determined in an oven at 105 °C for 12 h, ^{AOAC (1980)} method 7.003, the DMs obtained were used to determine the tDM with the following equation: % of tDM= (% DM at 55°C)*(% of DM at 105°C)/100.

Intercepted radiation

To calculate the intercepted radiation (IR), a linear ceptometer, model LP-80 (Decagon Devices INC.) manufactured in the United States of America, was used, which was placed horizontally above the plant canopy, with an east-west orientation and the incident light above the canopy was recorded, in μmol of photons m^-2 S^-1, which was assigned a value of 100%. Immediately afterwards, the sensor was placed under the canopy with the same orientation and the incidence of light was recorded. The IR was expressed in (%) using the following equation: % IR= 100- ((reading below the canopy*reading above the canopy^-1)*100).

Botanical composition and leaf:stem ratio

To determine the botanical composition, a subsample of approximately 20% of the forage sample was taken, each subsample was separated into the species of interest (common vetch) and (ryegrass), undesirable species (weeds) and dead material, the weights of each composition were obtained with the help of a digital scale, subsequently the (%) of each fraction was calculated by dividing the weight of each botanical composition by the total weight and multiplied by 100. For the leaf:stem ratio, they were separated into leaves (L) and stems (S), finally the weight was determined, and the leaf component was divided by the stem.

Statistical analysis

An analysis of variance was performed and the PROC GLM procedure of the statistical program SAS 9.0 Statistical Analysis System (2002) was used, with a completely randomized design with five repetitions, in order to evaluate the relationship between the variables studied in the experiment. The comparison of means was performed using the adjusted Tukey test (p= 0.05).

Dry matter yield and intercepted radiation

A progressive increase in dry matter yield was observed as age progressed, from 155 kg DM ha^-1 to 10 052 kg DM ha^-1 from day 38 to day 150 after sowing (Table 1). The percentage of dry matter increased over time from 12.4 to 28.04% from 38 to 150 DAS, 26.67 and 27.2% of DM were obtained in the second and third cuts, respectively. Intercepted radiation (IR) increased as the days of the crop progressed, a significant increase (p< 0.05) in IR was observed until day 80, reaching 90.57%. However, in subsequent DAS, the increases were not statistically significant (p> 0.05).

Figure 1 shows the correlation that existed between DM yield and the (%) of intercepted radiation, as well as a proposal for a prediction equation, in which the (%) of IR is used to determine the yield with an R² of 0.7778. In yield (kg DM ha^-1), there were significant differences (p> 0.05) at 52, 80, 94, 108 and 136 DAS; however, no significant difference (p> 0.05) was found between days 136 and 150 DAS.

With the above it is inferred that the first cut can be made on day 136 without affecting yield. For the second and third cuts, a significant decrease (p< 0.05) was observed in the amount of DM ha^-1 compared to the first cut at 136 or 150 DAS, which could be due to the absence of common vetch in the subsequent cuts (Figure 1).

Figure 1 Behavior of the yield of DM ha^-1 and intercepted radiation on different sampling days. 

The high yield of the first cut with respect to the following cuts is mainly attributed to the association of common vetch-perennial ryegrass since, according to ^{Sánchez et al. (2020}), common vetch can produce an autumn-winter yield of approximately 4 000 kg DM ha^-1 by applying 60 cm irrigation layers with a surface drip irrigation system; likewise, ^{Espinoza et al. (2018}) reported common vetch yields of 2 360 kg DM ha^-1 under rainfed conditions. ^{Mendoza et al. (2018}) reported perennial ryegrass yields of 8 742 and 3 793 kg DM ha^-1 in autumn and winter, respectively and ^{Cueto et al. (2003}) observed a yield of 4 230 kg DM ha^-1 for perennial ryegrass at the first cut.

After analyzing these values and obtaining an average of the two species in function, it can be concluded that the yield at the first cut improved when a common vetch-perennial ryegrass association was used. Forage yield (Table 1) decreased for consecutive cuts (178 and 206 DAS) with a significant difference (p< 0.05), even compared to yield at 108 DAS, but ^{Mendoza et al. (2018}) reported that for the growth of the perennial ryegrass monoculture in spring, the season in which the subsequent cuts of ryegrass are made, it was 5 271 kg DM ha^-1 and the data obtained in the present research for the second and third cuts were higher than those reported by these authors, with yields of 6 331 and 6 396 kg DM ha^-1, respectively, which indicates that the common vetch-ryegrass association does not affect the subsequent yield of the perennial crop.

^{Da Silva and Hernández (2010}) reported that, in tropical and temperate grasses, the optimal harvest point is reached with an IR of 95 moles of intercepted photons (100 moles of incident photons)^-1. In the present research work this proportion is reached for the first cut at 108 DAS; however, despite having reached 95% of IR, there is a statistically significant increase (p< 0.05) in yield (Table 1), even for 28 days; nevertheless, to generate suggestions, factors that determine the optimal point at the first cut must be considered, as mentioned by ^{Castro et al. (2017}), who indicate that the increase in the age of ryegrass affects the digestibility of the forage, therefore it is necessary to look for an optimal point between yield and nutritional quality. There is a correlation of 77.87% between yield and the (%) of IR (Figure 2), which coincides with what was reported by Da Silva and Hernández (2010), who observed high correlations between IR and dry matter yield, which can be useful to determine the cut time.

Figure 2 Botanical composition of the perennial ryegrass-common vetch association. 

Botanical composition and leaf:stem ratio (LSR)

Figure 2 shows the behavior in the proportions of common vetch, ryegrass, weeds and dead material. The proportions at 38 DAS were 78 and 20.7% of common vetch and ryegrass, respectively, with greater presence of the annual crop compared to the perennial, but the values were reversed from 80 DAS, with 45 and 53% for common vetch and ryegrass, respectively, a proportion that continued to increase until 150 DAS, when the first cut was made. After the first cut, common vetch disappeared and values less than 1% were observed for the second cut and null values for the third cut, so there was an increase in the proportion of ryegrass of 94 and 93%.

Regarding the presence of weeds during the 150 DAS of the ryegrass-common vetch association, the highest value was 1.3%, which can be attributed to the high competition for light of the desired species, preventing the prosperity of the undesirable species. For the second and third cuts, the proportion of weeds rose to 4%. The presence of dead material was observed at 80 DAS with 0.84%, which increased to 1.2, 3, 4.3, 11.6 and 17.3 at 94, 108, 122, 136 and 150 DAS, respectively, of the dead material approximately 72% came from common vetch.

There was a higher proportion of common vetch vs ryegrass in the first DASs, which could be due to the fact that annual species have a higher growth rate compared to perennials (^{Castro et al., 2012}). The accelerated growth of common vetch competes with weeds, preventing them from proliferating. However, the rapid growth of the annual species presents a rapid senescence, which causes the large percentage of dead material (^{Martínez et al., 2014}).

Table 2 shows the leaf-stem ratio (LSR) for the common vetch and ryegrass plants of the complete plot without considering the contribution of weeds, in general there was a decrease in LSR in the case of common vetch from 2.14 to 1.14, for ryegrass from 2.44 to 0.59 and for the complete plot from 2.24 to 0.88 for 38 to 150 DAS, respectively. ^{Sánchez et al. (2020}) indicate that the crude protein (CP) of common vetch decreases as the cycle ends, the reported ranges of flowering to almost physiological maturity were from 32 to 14.4% of CP, which may be related to the decrease in LSR and the increase in dead material.

There was a higher proportion of dead material from 94 DAS with a value of 3%, coming in a greater percentage (72%) from common vetch. This result coincides with what was reported by ^{Sánchez et al. (2020}), who found that the highest biomass production of common vetch occurred on day 103 with 20.49 t of fresh matter ha^-1 and subsequently the amount of green forage per hectare begins to decrease. Likewise, ^{Wilson et al. (2018}) observed a positive relationship of dead material as the age of the plants increased.

Conclusions

In the association of perennial ryegrass (Lolium multiflorum L.) and common vetch (Vicia sativa L.), the dry matter yield at the first cut increased without affecting the subsequent yields of the perennial crop.

The first cut of the perennial ryegrass and common vetch association should be made at 136 days after sowing, which does not affect the yield compared to 150 days, a lower proportion of dead material (11.6 to 17.3%) and a higher leaf:stem ratio are obtained.