FORAGE YIELD, ELONGATION RATE AND BOTANICAL COMPOSITION OF Lolium multiflorum LAMB. IN RESPONSE TO DIFFERENT GRAZING INTERVALS AND INTENSITIES † DE ELONGACIÓN Y COMPOSICIÓN BOTÁNICA LAMB. A INTERVALOS E INTENSIDADES DE PASTOREO]

GIv and GI; however, at 28-d GIv, GI did not affect this ratio. Implications. The results of the present study contribute to know the productive parameters of an established L. multiflorum meadow in zones of arid climates under two intensities and three grazing intervals. Conclusions. In conclusion, both grazing intensity and grazing interval are effective management tools to modify ryegrass performance during the hot ambient conditions of the Sonoran Desert region, associated with late-spring grazing period.

Reduced forage yield is attributed to several factors, including decreased exposure to light for photosynthetic activity, depletion of organic reserves of the plant, reduced uptake of water and nutrients, and damage to apical meristems (Campbell 1996;Hernández-Garay et al., 1997a). The relative importance of each of these factors is normally influenced by both ambient and grazing management conditions (Chaparro, 1991).
Various authors (Thomas, 1998) have reported that grazing intervals (GIv) and/or grazing intensities (GI) are closely associated with plants carbohydrate reserves. Low grazing intensities reduce forage yield and pastures botanical composition, and increase the abundance of dead material (Bryan et al., 2000). Poor grazing management leads to deterioration of otherwise wellestablished pasture (Milchunas and Lauenroth, 1993;Carvalho 2013).The objective of the present study was to evaluate the interaction of grazing intensity and grazing interval on yield, elongation rate, botanical composition and proportion of morphological components of an irrigated ryegrass pasture in the Mexicali valley, located in the Sonoran desert, in northwestern Mexico.

Site description
This experiment was conducted on an annual ryegrass pasture located 20 km south of Mexicali, in the Ejido Saltillo, Baja California, Mexico (32° 24´LN and 115° 23´LO), with an altitude of 12 msnm. Climatic conditions were monitored at weather station located 2 km from the experimental site.

Pasture establishment, grazing animals and grazing management
Seedbed preparation was carried out by plowing (30 cm deep), double disking (10 cm deep), and floating. The pasture was planted on October 18, 2018, with a seeding rate of 44 kg ha -1 and 70 kg ha -1 of fertilizer (14 % nitrogen, 8 % phosphorus and 21 % potassium oxide; Nutrisol ® , Fertilizantes Tepeyec Gpe., Mexicali, México) was applied at planting and again after each grazing rotation. The pasture was irrigated to field capacity after each grazing rotation. A calendar of irrigation and fertilizer schedules, along with dates of grazing rotation is shown in Table 1. Weeds were not controlled during the productive cycle of the pasture.
A total of 1008 crossbred bull calves (Bos indicus x Bos Taurus, 11 months of age and 120 ± 5 kg LW) were used in the study. Calves were vaccinated against IBR, BVD, PI3, BRSV, Vibriosis, and Leptospirosis (BOVI-SHIELD Gold, Zoetis Kalamazoo, MI, USA), and implanted with Synovex G (Zoetis Kalamazoo, MI, USA). The animals were placed on the grazing plots and allowed grazing for 10-h, from 06:00 a.m. to 4:00 p.m. Plots were grazed either at 4 to 6 cm (Figure,  (Table 1), using an intensive grazing system with stocking rate of 56 calves per ha. Grazing was controlled using electrical fencing. Fencing was moved every hour for 10 hours. Animals had access to a continuous supply of fresh water in each grazing unit. After each day of grazing, calves were then removed from the pasture to a holding area where they had ad libitum access to water and provided with 0.5 kg chopped corn stubble animal -1 .

Treatment structure and experimental design
The total experimental area was 180,000 m 2 (18 plots of 10, 000 m 2 each), divided in nine plots for each of two grazing intensity (GI); 4-6 or 10-12 cm of residual forage height; each containing three plots for each grazing interval (GIv); 21, 28, and 35 days. Treatments were assigned in 3 ˣ 2 factorial arrangement in a randomized complete block design.

Forage yield
For the estimation of pasture forage yield (pre-grazing forage and residual forage post grazing forage), 36 samples were taken before and after grazing (0.25 m 2 per experimental animal unit), as described by Haydock and Shaw (1975). Samples were placed in paper bags, transferred to the laboratory and a subsample, equivalent to 30% of the total material collected, was dried (DM) in a forced air-drying oven (65 °C for 72 h). Ryegrass yield (kg DM ha -1 ) was determined as indicated: FY = ((DM ˣ 10) ˣ 10000)/1000) was determined as described by Hodgson (1990); where: FY= Ryegrass yield, in dry matter bases (kg DM ha -1 ).  interval was subjected to either severe grazing intensity (4-6 cm) or light grazing intensity (10-12 cm) and ‡= the last irrigation and fertilizer was applied to support seed production, after the razing season had finished.

Plant morphology
In each sampling of each grazing interval and intensity. Samples were collected and separated into leaves and stems to measure the contribution each of these components. Each separated component was dried in a forced air oven, at a temperature of 55 ° C for 72 h and the partial dry matter was determined. The proportions of morphological components were determined by the following formulas by Gustavsson and Martinsson (2004): PStems= Ytems/YRyegrass; PLeaf= YLeaves/YRyegrass; where: PLeaves= proportion of leaves (%) to the total forage yield; and PStems= proportion of stems (%) to the total forage yield (kg DM ha -1 ).

Ryegrass height and elongation rate
Plant height from the ligule to the apex of the green leaves was measured in each experimental unit using a ruler. The elongation rate of plant (elongation rate; cm stem -1 d -1 ), was calculated for growing leaves, by the difference between the sum of the lengths of the final (Lf) and initial (Li), at end of two successive measurements, divided by the number of days elapsed for each grazing frequency (D) between both successive measurements with the following formula, suggested by Cruz-Hernández et al. (2017): elongation rate= (Lf-Li) / D.

Statistical analyses
The trial was analyzed as a randomized complete block design with a 3 ˣ 2 factorial arrangement of treatments. Where grazing interval by grazing intensity interactions were appreciable (P≤ 0.05), individual means comparisons were evaluated using the Tukey test (P≤ 0.05). using the Least-Squares Means (LSMEANS) procedure of SAS version 9.2 (SAS, 2009)..

Weather conditions
Weather conditions during the course of the study are given in Figure 2. During the experimental grazing period (January to May), daily average temperature ranged between 14.4 and 20.1 °C, with minimums between 8 and 14 °C, and maximums between 20.4 and 26.4 °C. Precipitation during the course of the study was characteristically low averaging 20 mm.

Forage yield
Forge yield recorded a significant GI x GIv interaction (Table 2). Figure 3 illustrate how this interaction occurred. The interaction was significant due to the difference of magnitudes of increment between the groups of GIv across the two GI, recording a greater lineal increment as the GIv increased at the 4-6 GI, than at 10-12 GI. The influence of grazing intensities and intervals on yield is shown in Table 2. Forage yield increased linearly (P<0.01) with increasing grazing interval. However, with the 21-d GIv, GI did not affect yield; whereas, with GIv of 28 and 35 days, yield increased 61 and 79 %, respectively, with increased GI. Table 2 shows that the stems fraction of the forage was affected by a significant GI x GIv interaction. The interaction is explained by a fast linear increment of steams in the forage as the GIv increased from 21 to 35 days in the 4-6 cm GI treatment, compared with the increment observed for the 10-12 cm GI. Increasing both GI and GIv increased the stems fraction ( Figure  4). Independently of GI, stems contribution to yield increased linearly with increasing GIv. At GIv of 35 days, a greater contribution of stems to yield was recorded, compared with 21 or 28 days.

Plant morphology
The leaves proportion of the forage was significantly affected by both GI and GIv, however this variable was the only one in the present study that was not affected by the GI x GIv interaction (Table 2). Among GI treatments, the 4-6 cm significantly yielded more biomass as leaves (1,821 kg ha -1 ), than grazing at 10-12 cm (1,433 hg ha -1 ). Among GIv, as the GI deceased, the production of leaves increased, with 796, 1,436, and 2,649 kg ha -1 , for 21, 28, and 35 days intervals, respectively (data not shown). This effect is consistent with corresponding differences in forage yield; leaf fraction as a morphological plant component to forage yield increased as pasture yield increased.
Leaf to stem ratio in the forage recorded a significant GI x GIv interaction (Table 2). Figure 5 illustrates this interaction. The interaction occurred due to the disproportional decrement in leaf to stem ratio in the 4-6 cm GI as GIv increased, as compared with the much more drastic fall in the 10-12 cm GI, particularly when moving from 21 days interval to 28 days interval.

Plant height
Plant height was also affected by a significant GI x GIv interaction (Table 2). This interaction is illustrated in Figure 6. The interaction occurred because the plant height was similar when grazing changed from 28 to 35 days in the 4-6 cm GI, as compared with a significant difference recorded for the same GIv in the 10-12 cm GI. Plant height increased with increasing GIv. However, at 21-and 35-d grazing intervals GI did not affect plant height; whereas, at the 28-d GI plant height was greater with increased GI.

Plant elongation rate
A significant GI x GIv interaction was recorded for plant elongation rate (Table 2). A second order polynomial (quadratic) curve was recorded across GIv in the 4-6 cm group of treatments; while, in the 10-12 cm GI, no significant effect was recorded, as the GIv increased from 21 to 35 days Figure 7).

Pasture botanical composition
Ryegrass. There was a significant GI × GIv interaction on plant botanical composition (Table 2). Figure 8 illustrates this interaction. With lesser GI, BC increased linearly with increasing GIv. With greater GI, BC increased at 21-and 28-d grazing intervals.

Weeds and dead material
A significant GI × GIv interaction was recorded for WDM (Table 2). WDM production in the 4-6 cm GI changed little as the GIv changed. In contrast, in the 10-12 cm GI, a clear reduction was recorded when moving to 35 days DIv, as compared with either 21 or 28 days (Figure 9).

Forage yield
Forage yield in the present study are consistent with several studies involving annual ryegrass (Velázquez et al., 2009). Bribiesca et al. (2002)  of interaction between GI and GIv may have been due to differences in forage yield with increasing grazing intervals, as reported by Kallenbach et al., (2004) and Mullenix (2016). Cruz-Hernández et al. (2020) observed a yield increase of 1,500 kg DM ha -1 , as grazing interval increased from 21 to 35 d. Whereas, in the present study, as the grazing interval increased from 21 to 35 days the impact of grazing intensity on forage yield was much greater (7618 and 4761 kg DM ha -1 , for GI of 4-6 vs 10-12, respectively).

Plant morphology
The increased proportion of stems production with increasing GIv may have been due to climatic conditions favoring growth of the reproductive stems as plant matured (Kallenbach et al., 2002). Consistent with Cruz-Hernández et al. (2020), leaf to stem ratio decreased with increasing GIv. This effect was due to the increased contribution of stem and dead material to total biomass yield. There was a positive correlation (P˂0.01) between leaves and stems of ryegrass with increasing the GIv (data not shown). Likewise, in an evaluation of Maralfalfa grass pasture (Calzada-Marín et al., 2014) observed a positive correlation (r= 0.85) between leaf to stem ratio and GIv. Bribiesca et al. (2002) observed leaf to stem ratios of 1.4 and 1.1 in a perennial ryegrass pasture harvested at a plant height of 3 and 15 cm, respectively. Plant age affects yield of plant morphological components (Soares et al., 2019;Beecher et al., 2015;Hopkins et al., 2006).. As plants age with increasing grazing interval, leaf production drops and the proportion of both stem and senescent material increases or negatively affecting leaf to stem ratio (Soares et al., 2019;Beecher et al., 2015;Hopkins et al., 2006). Results in the present study are consistent with previous studies in that plant height increased with increasing GIv (Barthram and Grant, 1984;Hurley et al., 2013). Furthermore, the findings on plant height in the present study are in agreement with Lemaire et al. (2009) and Santos et al. (2013), who observed increased plant height with increased grazing intensity. In regard to the response observed for elongation rate, environmental temperatures and relative humidity seems to be closely associated with elongation rate of grasses (especially the Lolium species). In an evaluation of L. perenne associated with Trifolium repens pasture, (Castro et al., 2013) observed that elongation rate during the spring was 0.2 cm stems -1 day -1 greater than during the winter. With different GIv, plant elongation rate following grazing is dependent on root carbohydrate reserves and the number of initiated vegetative buds (Duru and Ducrocq, 2000).

Pasture botanical composition
The results of the present study regarding BC coincide with another study with alike climatic conditions, conducted at a close location (about 10 km away); Tilus et al. (2020) observed that the ryegrass fraction to the total yield decreased with increasing GIv; while the proportion of WDM increased. These finding agree with previous studies (Duchini et al., 2016;Duchini et al., 2014;Barth et al., 2013), where increasing grazing intensity was observed to decrease WDM. Da Silva et al. (2010) noted that once the plant intercepts 95% of the solar radiation, there is a progressive increase in WDM. Elevated ambient temperature (greater than 50 ºC) could have been another factor contributing to increased WDM, due to the death of leaves in the upper strata (Duru, 2000). This is particularly relevant to pastures were grazing management maintained plants at a greater height (Beecher et al., 2015).

CONCLUSIONS
In conclusion, both grazing intensity and grazing interval are effective tools to modify plant growth performance, morphological components, as well as weeds and dead material fractions, height and elongation rate of ryegrass. Research is needed to further evaluate grazing strategies during the warmer ambient conditions of the Sonoran Desert region associated with the late-spring phase of the annual ryegrass grazing period.

Acknowledgments
The authors presented special acknowledges the commercial feedlot (Engorda Rancho Nuevo S.P.R. de R.L. de C.V.; Sr. Antonio Valdivia) for the financial support provided to this research. The authors are also grateful to Consejo Nacional de Ciancia y Tecnología (CONACyT) for the scholarship provided to the first and third authors.
Funding. The authors did receive and appreciate the funding received from Engorda Rancho Nuevo S.P.R. de R.L. de C.V.; Sr. Antonio Valdivia for the conduction of the present research.

Conflicts of interest.
The authors declare no conflicts of interest regarding the publication of this paper. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data; in writing of the manuscript, or in the decision to publish the results.