Introduction

In the Comarca Lagunera, the production of forage to feed dairy cattle is carried out intensively under irrigation with perennial crops such as alfalfa and three cycles of annual crops of corn, sorghum and autumn-winter cereals. The main winter cereal is oats, while in spring and summer, two crops of corn or sorghum are obtained (^{Santamaría et al., 2006}), all produced with a conventional tillage system. Alfalfa is important in crop rotation in the region because it produces quality forage and contributes to the accumulation of labile C and N, as well as improving the use of N by the following annual species in crop rotation (^{Chen et al., 2019}; ^{Zhou et al., 2019}). However, considering that annual crops are established with conventional tillage, with at least one plow pass in the year and one or two harrow passes before sowing each crop, the transition period from perennial crop to sowing annual crops is critical because losses of organic carbon occur in the soil.

According to ^{Chen et al. (2019}), these losses can reach up to 30% in annual crops with conventional tillage compared to reduced tillage. The addition of agricultural residues to the soil is a practice that is carried out with the aim of improving the physical and chemical properties of the soil and providing nutrients, but for these nutrients to be absorbed by plants, a transformation process called mineralization must occur. During the mineralization process, various factors such as soil organisms, temperature, moisture, texture, soil type, and C and N concentrations are involved in the decomposition of crop residues (^{Monsalve et al., 2017}). Additionally, the biochemical composition of the residues influences in an important way since several studies describe the quality of the residues by the content of N and C and the quality criterion that they commonly use to predict the mineralization is the C/N ratio of the residues (^{Trinsoutrot et al., 2000}).

In the production of annual forages in the region, the amount of harvest residues incorporated into the soil is limited because the largest proportion of the aerial part is harvested in forage crops. In addition, residues of oats, corn and sorghum are considered of low quality for the soil because they have high values of the C/N ratio (31.4 to 56.4). These levels of the C/N ratio indicate that a process of immobilization of C and N in the soil probably occurs, decreasing the availability of N for the use of the following crops in the rotation (^{Lynch et al., 2016}; ^{Singh et al., 2021}). To improve the management of harvest residues, it is important to increase the quantity and quality of organic residues contributed to the soil through greater diversification of forage crops. ^{Zhou et al. (2019}) found that a high diversity of species increases the quality of plant residues with a decrease in the C/N ratio, which indirectly favors the accumulation of carbon in the soil.

In this regard, alternative crops such as canola, beet, chickpeas and forage brassicas showed a potential yield of DM similar or greater than that of traditional species (oats, wheat and triticale), while their nutritional composition was higher, with crude protein contents of 196 to 281 g kg^-1 (^{Reta et al., 2008}). The high content of N in the forage of alternative crops (31.36-44.96 g kg^-1) (^{Reta et al., 2008}) compared to those observed in traditional crops (17.47-22.4 g kg^-1) (^{Reta et al., 2018}; ^{Sánchez-Duarte et al., 2019}) suggests that their residues may be of higher quality, with the potential to improve the availability of N in crop rotation, in addition to favorably affecting the accumulation of C in the soil. The objective of the study was to evaluate the potential contribution of C and N to the soil in alternative and traditional forage residues during the autumn-winter cycle.

Materials and methods

The research was carried out in the autumn-winter cycles 2017-2018 and 2018-2019 in the La Laguna Experimental Field (CELALA, for its acronym in Spanish) of the National Institute of Forestry, Agricultural and Livestock Research (INIFAP, for its acronym in Spanish), located in the municipality of Matamoros, Coahuila, Mexico (103° 13’ 42” west longitude and 25° 31’ 41” north latitude, at an altitude of 1 100 masl). The soil used was one with a loam-clay texture without salinity problems, with a depth greater than 1.8 m and a pH of 8.14. In the 0-30 cm stratum, the soil had an organic matter content of 1.41% and concentrations of NH4-NO3 and P of 11.05 mg kg^-1 and 5.16 mg kg^-1, respectively.

In the two growing cycles, soil preparation consisted of fallowing, harrowing and leveling the land. Before sowing, the experiment was traced and each experimental plot was fertilized with granulated ammonium sulfate and monoammonium phosphate, at a rate of 50 kg N and 80 kg P₂O₅ ha^-1, respectively. Subsequently, between the cuts and after the cuts, a nitrogen fertilization dose of 250 kg ha^-1 was completed. Fertilizers were applied and incorporated manually. Sowing of all crops was done manually in dry soil on September 15, 2017, and October 12 in 2018. Eleven forage species were evaluated in 17 treatments based on alternative and traditional crops with the capacity to regrow and adapt to the autumn-winter production cycle in the region.

The traditional crops and cultivars used were the following: of oats (Avena sativa L.), varieties Cuauhtemoc and Karma; of barley (Hordeum vulgare L.), varieties Cántabra and Narro 95; of triticale (x Triticosecale Wittmack), varieties Río Nazas and AN 105; of wheat (Triticum aestivum L.), varieties Salamanca and AN 265; of berseem clover (Trifolium alexandrinum L.), cultivar Multicut; of annual ryegrass (Lolium multiflorum Lam.), Tetraploid Oregon. The alternative species and cultivars evaluated were the following: of chickpea (Cicer arietinum L.), porquero chickpea variety; of canola (Brassica napus L.), spring cultivar Ortegón and winter cultivar Riley; of beet (Beta vulgaris L.), cultivar Starmon; of brassicas, cultivar ‘Winfred’ (Brassica oleracea L. x Brassica rapa L.) and cultivar Hunter (Brassica rapa L. x Brassica napus L.); of forage radish, cultivar Graza (Raphanus sativus L. x Brassica oleracea L., Raphanus maritumus L.). The crops ‘Winfred’ brassica, ‘Hunter’ brassica and ‘Graza’ radish were only evaluated in the 2018-2019 cycle.

The experimental area was irrigated by a system of PVC plastic pipes with gates. The volume of water applied in each plot was measured, adjusting the water flow in the gates of the pipes installed for irrigation and considering the irrigation time in each experimental plot. Irrigation was applied on the same day of sowing with an irrigation sheet of 150 mm; eight days later, a light irrigation with a 60 mm sheet was applied. During the production cycle, six supplemental irrigations were applied with a total sheet of 75 cm in oats, triticale, wheat, clover and Hunter brassica, while in barley, Winfred brassica and Graza radish, five supplemental irrigations with a sheet of 63 cm were applied.

The nitrogen fertilization dose (250 kg ha^-1) was also completed, with 55 kg ha^-1 at 33 das, 90 kg ha^-1 after the first cut in each species between 77 and 112 das, and 55 kg ha^-1 before the second cut between 112 and 135 das. The experimental design used was one of randomized complete blocks with four repetitions, in each of which the 17 treatments were randomly distributed. Each of the 68 experimental plots consisted of 20 furrows 0.18 m apart and 6 m long. At harvest, fresh forage and DM yields were determined. The useful plot to determine forage yield was 14.4 m², harvesting 16 central furrows of 5 m in length.

DM content was obtained in a 0.72m² sample taken at random from the sample used for DM yield measurements. For this, two of the central furrows of each plot of 2 m in length were sampled. The sampled plants were dried at 60 °C in a Shel Lab forced air oven Model FX28-2 until reaching a constant weight. DM yield was determined by multiplying the fresh forage yield by the DM content of each plot. In the 2017-2018 cycle, three harvests were carried out in the cereals in the booting stage, five harvests were carried out in ryegrass and berseem clover in the vegetative stage; canola and beet were harvested three times also in the vegetative stage, while chickpeas were harvested only once in the flowering stage.

In the 2018-2019 cycle, the cereals were harvested twice in the booting stage; chickpeas were harvested only once in the stage of flowering and formation of pods; while in the rest of the species, the harvests were carried out in the vegetative stage, with two harvests in brassicas, beet, radish and canola, while ryegrass and berseem clover were harvested on four and three occasions, respectively. After the harvest of all crops, on March 20, 2018, corresponding to the first cycle and on March 26, 2019, of the second cycle, the harvest residue was collected at two sampling points on an area of 0.25 m² (0.5 x 0.5 m) taken at random in each experimental plot, where the root (20 cm depth) and organ residues of the aerial part were included. In each sample, the soil was sieved, separating the plant residues, which were then washed to remove the adhered soil.

The samples were dried in a Shel Lab forced air oven Model FX28-2 at 60 °C until reaching a constant weight, and subsequently, the weight of the dry matter was obtained. The dry forage samples were ground in a Wiley mill (Thomas Scientific Swedesboro, NJ USA) with a 1 mm mesh. A quartering of the ground sample was carried out in order to obtain a representative subsample of each plot in all the harvests carried out and subsequently, the sieving process was carried out using a 100 μm sieve. To determine the contents of C and N by the dry combustion method, between 10 and 15 mg of forage were weighed with an OAHUS PA224C analytical balance, previously dried at room temperature and sieved at 100 μm. The samples were calcined in the Elemental Analyzer (Thermo Fisher Scientific Model Flash 2000) at 950 °C using oxygen as an oxidizing agent (^{AOAC, 2005}).

Meteorological data were obtained from a climate station located at the experimental site. Climate conditions in the two years of the study were similar in average temperatures, with higher rainfall in September 2018, which delayed the establishment of the second cycle until October 12, 2018 (Table 1). Due to this situation, together with the fact that some species were only evaluated during the second cycle, statistical analyses of the variables obtained were performed by year. Analyses of variance (p≤ 0.05) were performed for the total forage DM yield data and for the contents of N and C, C/N ratio and DM yield, N and C in the residues (aerial part and roots). The protected Fisher’s least significant difference test (p≤ 0.05) was used to compare the means. The analysis of the information was carried out with the statistical program SAS (^{SAS Institute, 2011}).

Conclusions

A greater diversity of forages in the autumn-winter production cycle with alternative forage species, such as brassica, radish, canola and beet, can contribute to improving the contribution of C and N to the soil in their harvest residues with better quality compared to traditional forages and at the same time to maintain or increase the yield of forage in the production cycle.