Yield and Nutritive Value of Stockpiled Grasses as Influenced by Cultural Practices and a Freeze

Paul Mislevy, Range Cattle Research and Education Center, University of Florida, Ona 33865-9706; and Frank G. Martin, Statistics Department, University of Florida, Gainesville 32611-0840

Abstract

Cattle grazing tropical and subtropical grasses following a freeze generally require supplementation. However, livestock growers and animal scientists have difficulty determining supplementation needs because there is little information on tropical and subtropical grass nutritive value following a freeze. This study measured yield and nutritive value of nine tropical/subtropical grasses with and without fall fertilization, harvested at date of freeze and 1, 2, and 4 weeks post-freeze over a 3-year period. Grasses were four bahiagrasses (Paspalum notatum Flugge) cv. ‘Argentine,’ ‘Paraguay 22,’ ‘Pensacola,’ and ‘Tifton 9’; two bermudagrasses (Cynodon spp.) cv. ‘Florakirk’ and ‘Tifton 85’; two stargrasses (C. nlemfuensis Vanderyst var. nlemfuensis) cv. ‘Florico’ and ‘Florona’; and one limpograss [Hemarthria altissima (Poir) Stapf. and C.E. Hubb] ‘Floralta.’ Plots were clipped to 3 inches during the last week of October each year and half the plots fertilized with N-P₂O₅-K₂O at 50-30-60 lb/acre + elemental Cu, Zn, Fe, and Mn (sulfate form) at 1.5 lb/acre and S at 4.5 lb/acre, annually. Dry forage mass (FM) increased 70, 180, 170, and 190% for bahiagrass, bermudagrass, stargrass, and limpograss respectively with fertilization. Crude protein (CP) and in vitro organic matter digestion (IVOMD) both increased nearly 2 percentage units due to fertilization. Four weeks following a freeze, average CP and IVOMD across grasses decreased 2 and 15.6 percentage units, respectively. Tifton 85 bermudagrass and Florico stargrass showed the greatest decline of more than 19 percentage units IVOMD. Data demonstrate selected tropical grasses respond positively to fall fertilization. A freeze has little effect on herbage CP, however IVOMD decreases drastically.

Introduction

Bahiagrass, bermudagrass, stargrass, and limpograss are major forage species in peninsular Florida and throughout the tropics. Forage production in many tropical climates is highly seasonal. In the region of southern Florida USA, 70 to 90% of forage production is obtained during the warm/rainy season (11) with little production during the cool, short-day winter period. Similar results occur in the tropics with little forage production during the dry season.

Stockpiling is one method of utilizing forage grown during peak periods for use during periods of deficit. In subtropical regions where cool-season temperatures drop below 32°F, grasses turn brown within 48 h (5,6). When these conditions occur, animal scientists have difficulty advising cattlemen on proper nutritional programs because the nutritional value of forage is unknown following a freeze.

The purpose of this study was to monitor the yield and forage nutritional value of nine tropical and subtropical perennial grasses, with and without a fall fertilizer application, and harvested at date of freeze and 1, 2, and 4 week post-freeze.

Growth and Harvest Procedures

The experiment was established on a sandy siliceous, hyperthermic Ultic Alaquod (Pomona fine sand) during the summer of 1992. The study was conducted during the fall-winter of 1993-1994, 1994-1995, 1995-1996 at the Range Cattle Research and Education Center, Ona, FL. Three bahiagrass cultivars (Argentine, Paraguay 22, and Pensacola) were seeded at 30 lb/acre and Tifton 9 bahiagrass at 10 lb/acre. Bermudagrass (Florakirk and Tifton 85), stargrass (Florico and Florona), and limpograss (Floralta) were all planted using rooted crown material on 2-ft centers.

Immediately after seedling emergence or signs of vegetative growth appeared, N-P₂O₅-K₂O at 50-30-60 lb/acre plus Zn, Cu, Mn, and Fe (sulfate form) at 1.5 lb/acre, and S at 4.5 lb/acre were applied. A second application of N at 50 lb/acre was applied 20 days after the initial to encourage plant establishment. All grass entries were harvested twice during the summer of each year to remove top growth.

The field plot layout was a split-split plot with grass cultivars (9) as main plots, with and without fertilizer as subplots, and 4 harvest treatments (date of freeze and 1, 2, and 4 weeks post-freeze) as sub-subplots. Whole plots were randomized in three complete blocks. The main plot measured 16 ft by 56 ft, subplots 16 ft by 28 ft, and sub-subplots 7 ft by 16 ft. Grasses were cut to a 3-inch stubble the last week of October each year. After cutting, the plots to be fertilized received 50-30-60 lb/acre N-P₂O₅-K₂O + 1.5 lb/acre elemental Cu, Zn, Fe, and Mn (sulfate form) and 4.5 lb/acre S as combined sulfur. Grasses were then allowed to grow until first freeze which occurred 13 December 1993, 45 days after clipping; 25 January 1995, 88 days after clipping; and 25 December 1995, 57 days after clipping (5,7), when harvest treatments were initiated. The first harvest was during the day following the freezing temperature the preceding night

A 19-inch by 10-ft strip was harvested to a 3-inch stubble to determine dry FM. Subsamples were dried at 140°F, ground, and analyzed for total N (3,4). Crude protein concentration was calculated as 6.25 × N. Additionally, IVOMD was determined for forage samples by the two-stage procedure of Tilley and Terry (14) modified by Moore and Mott (12).

Data were analyzed using PROC GLM (SAS Institute Inc., Cary, NC) with the model statement for a split-plot experiment in a randomized complete block design. Differences among grasses were examined using Duncan’s Multiple Range Test. Polynomial regression equations for harvests were fitted to the interaction means. Separate equations were fitted for each grass × fertilizer combination.

To investigate the grass × fertilizer interactions, Duncan’s Multiple Range Test was applied to grass entry means for the four harvests within each fertilizer level. Fertilizer means were compared for each grass entry using the t-test (LSD). A 0.05 significance level was used for all tests. For significant grass × harvest interactions, Duncan’s Multiple Range Test was applied to the grass entry means for each harvest. A different method was used to compare harvest means for each grass entry. The means for 1, 2, and 4 weeks after a freeze treatments were compared with the date of freeze means using Dunnett’s test. This test compares a single treatment "date of freeze" with each of the other treatments (1, 2, and 4 weeks post-freeze harvests) in the experiment.

Research Results

Dry FM, CP concentration, and IVOMD are presented for forage grown from late October to the first freeze and pooled over 3 years because there was no difference (P > 0.05) between years. There were differences (P < 0.05) between grass cultivars due to fertilizer for FM, fertilizer × harvest treatment for CP and IVOMD, and grass cultivar × harvest treatment for CP and IVOMD.

Dry biomass. The application of fertilizer in late October increased FM (P < 0.05) of all grasses over the non-fertilized treatment (Table 1). Average FM of all grasses not fertilized and fertilized was 0.50 and 1.23 ton/acre, respectively, or a 146% FM increase due to fertilization. The bahiagrasses responded least to late fall fertilization increasing only 0.26 ton/acre. Similar results were also observed by Evers et al (2) in Texas. This indicates that it would not be economical to fertilize bahiagrass in late October under Florida conditions.

Table 1. Influence of fall fertilizer application on dry biomass yield of perennial grasses harvested immediately after a freeze (data pooled over 3 years) at Ona, FL.

Grass entry		Fertilizer applied (ton/acre)		Yield increase (ton/acre)^x	F ratio
Grass entry		Yes	No	Yield increase (ton/acre)^x	F ratio
Bahiagrass	Argentine	0.57 de^y	0.29 d	0.28	*^z
	Paraguay 22	0.67 de	0.37 cd	0.30	*
	Pensacola	0.49 e	0.28 d	0.21	*
	Tifton 9	0.77 d	0.52 a-c	0.25	*
	Avg.	0.63	0.37	0.26	—
Bermudagrass	Florakirk	1.78 b	0.69 a	1.09	*
	Tifton 85	1.51 c	0.46 b-d	1.05	*
	Avg.	1.65	0.58	1.07	—
Stargrass	Florico	1.71 b	0.64 ab	1.07	*
	Florona	1.61 bc	0.59 ab	1.02	*
	Avg.	1.66	0.61	1.05	—
Limpograss	Floralta	1.98 a	0.69 a	1.29	*
Overall avg.		1.23	0.50	0.73	—

^x Yield increase due to late October fertilizer application.

^y Means within a column followed by the same letter(s) are not different at the 5% level (Duncan’s Test).

^z An asterisk indicates a significant difference (P < 0.05) in dry biomass yield between fertilized and non-fertilized treatments.

The bermudagrasses, stargrasses, and limpograss all responded well to fall fertilization increasing 184, 170, and 186% in FM over the non-fertilized treatments. Forage mass increases due to fertilization averaged 1.07 for bermudagrass, 1.05 for stargrass, and 1.29 ton/acre for limpograss, making the application of late fall fertilization economically feasible on these grasses. Other studies (1,13) have also demonstrated that late season fertilization of grasses increased dry FM. However, Kretschmer et al (9) indicated that late fall fertilization of mature (unclipped) grasses did not influence FM.

Forage nutritive value. Fall fertilization of most grasses provided higher (P < 0.05) CP concentration when compared with non-fertilized grasses for all harvest treatments. However, the application of fertilizer had no effect (P > 0.05) on CP concentration of Florakirk bermudagrass (Table 2). The average CP increase due to fertilizer over all grasses was 1.6, 1.8, 2.1, and 1.5 percentage units at date of freeze and 1, 2, and 4 weeks post-freeze, respectively. The CP concentration tended to decrease slowly for most grasses from date of freeze to 4 weeks post-freeze. Generally, the decrease in CP between date of freeze and 4 weeks post-freeze was similar for both the fertilized and non-fertilized treatments averaging 1.7 and 1.6 percentage units, respectively (Table 2). Earlier studies by Miselvy (10) using Cynodon and Paspalum grasses also indicated decreases among entries existed for CP after a freeze. Earlier data indicate a 4-percentage-unit decrease in CP concentration at the 4 weeks post-freeze harvest.

Table 2. Influence of fertilizer × harvest treatment on crude protein (CP) concentration of grasses harvested at date of freeze and 1, 2, and 4 weeks post-freeze pooled over 3 years at Ona, FL.

Grass entry	Fertilizer	Date of freeze	Post-freeze (week)			Avg.
		Date of freeze	1	2	4	Avg.
		CP (%)
Bahiagrass
Argentine	No	10.3 a^w	9.0 b	9.7 ab	9.0 b	9.5^y
	Yes	13.1 a	11.7 b	11.9 b	11.2 b	12.0
	Yes vs. no	*^x	*	*	*	—
Paraguay 22	No	10.8 a	9.9 ab	9.7 b	10.1 ab	10.1
	Yes	11.8 a	11.8 a	11.6 a	11.5 a	11.7
	Yes vs. no	*	*	*	*	—
Pensacola	No	10.6 a	10.4 ab	8.8 c	9.4 bc	9.8
	Yes	13.0 a	12.1 ab	11.7 b	11.7 b	12.1
	Yes vs. no	*	*	*	*	—
Tifton 9	No	10.1 a	10.2 a	9.5 a	9.7 a	9.9
	Yes	12.6	12.2 ab	11.4 b	10.0 c	11.6
	Yes vs. no	*	*	*	*	—
Bermudagrass
Florakirk	No	12.6 a	10.9 b	9.7 c	10.1 bc	10.8
	Yes	12.4 a	11.8 a	10.3 b	10.4 b	11.2
	Yes vs. no	NS	NS	NS	NS
Tifton 85	No	11.4 a	10.9 a	9.0 b	9.0 b	10.1
	Yes	13.4 a	12.7 a	11.8 b	10.5 c	12.1
	Yes vs. no	*	*	*	*	—
Stargrass
Florico	No	11.9 a	10.8 b	9.0 c	8.4 c	10.0
	Yes	12.3 a	12.0 ab	11.2 b	11.1 b	11.7
	Yes vs. no	NS	*	*	*	—
Florona	No	11.6 a	10.8 a	9.8 b	9.8 b	10.5
	Yes	13.7 a	12.9 ab	12.1 bc	11.6 c	12.6
	Yes vs. no	*	*	*	*	—
Limpograss
Floralta	No	8.4 a	8.4 a	7.6 a	7.8 a	8.1
	Yes	10.1 ab	10.3 a	9.4 ab	9.2 b	9.8
	Yes vs. no	*	*	*	*	—
Avg. CP increase^z		1.6	1.8	2.1	1.5	1.8
Avg. CP		11.7	11.0	10.2	10.0	10.8

^w Means in a row followed by the same letter(s) are not different at the 0.05 level of probability (Duncan’s test).

^x An asterisk indicates a significant difference between fertilizer levels at each harvest period for each grass. NS not significant (t test LSD).

^y Average CP of four harvest treatments.

^z Average CP increase due to late October fertilizer treatment.

About 2 weeks after the initial freeze a second freeze occurred driving temperatures down to a low of 29, 28, and 27°F during the harvest years of 1993-1994; 1994-1995; and 1995-1996, respectively (5,6,7,8). However, the second freeze had little effect on CP and means were 10.2 for 2 weeks post-freeze and 10.0% for 4 weeks post-freeze when averaged over both fertilizer treatments.

There were differences (P < 0.05) between grass entries in each harvest treatments for CP concentration when pooled over 3-years. Crude protein concentration in bermudagrasses (12.5) and stargrasses (12.4) was always higher than bahiagrasses (11.5) and limpograss (9.2%) at the date of freeze (Table 3).

Table 3. Comparison of grass cultivars harvested at date of freeze and 1, 2, and 4 weeks post-freeze for crude protein (CP) concentration, and pooled over 3 years at Ona, FL.

Grass entry	Date of freeze	Post-freeze (weeks)			Dunnett’s test			CP de-crease (%)^z
	Date of freeze	1	2	4	Dunnett’s test
	CP (%)				1	2	4
Bahiagrass
Argentine	11.7 bc^w	10.3c A^x	10.8 a A	10.1 ab A	*^y	*	*	1.6
Paraguay 22	11.3 c	10.9 bc A	10.7 a A	10.8 a A	NS	NS	NS	0.5
Pensacola	11.8 a-c	11.3 ab A	10.2 a B	10.5 ab B	NS	*	*	1.3
Tifton 9	11.3 c	11.2 a-c A	10.4 a B	10.3 ab B	NS	*	*	1.0
Avg.	11.5	10.9	10.5	10.4	—	—	—	—
Bermudagrass
Florakirk	12.5 ab	11.4 ab A	10.0 a B	10.2 ab B	*	*	*	2.3
Tifton 85	12.4 ab	11.8 ab A	10.4 a B	9.8 b B	NS	*	*	2.6
Avg.	12.5	11.6	10.2	10.0	—	—	—	—
Stargrass
Florico	12.1 a-c	11.4 ab A	10.1 a B	9.7 b B	NS	*	*	2.4
Florona	12.7 a	11.9 a A	10.9 a B	10.7 ab B	NS	*	*	2.0
Avg.	12.4	11.6	10.5	10.2	—	—	—	—
Limpograss
Floralta	9.2 d	9.4 d A	8.5 b B	8.5 c B	NS	NS	NS	0.7
Overall avg.	11.7	11.1	10.2	10.1	—	—	—	1.6

^w Means within a column followed by the same letter(s) are not different at the 5% level (Duncan’s test).

^x Means in a row (post-freeze, 1, 2, and 4 weeks) followed by the same upper case letter are not different at the 5% level (Duncan’s test).

^y An asterisk indicates that post-freeze harvests 1, 2, and/or 4 means are different (P < 0.05) from the date of freeze harvest (Dunnett’s test). NS = not different.

^z Values are calculated as the difference in CP concentration (percentage units) between date of freeze and 4 weeks post-freeze.

Using Dunnett’s test to monitor change in CP concentration between time of freeze and 1 week post-freeze revealed all grasses except Argentine bahiagrass and Florakirk bermudagrass were not different. This would indicate little or no change in CP concentration within 1 week following a freeze (Table 3). However, most grasses sampled 2 and 4 weeks after the freeze exhibited a decrease (P < 0.05) in CP concentration, except Paraguay 22 bahiagrass and Floralta limpograss. Paraguay 22 decreased only 0.6 and 0.5 percentage units and Floralta limpograss decreased 0.7 percentage units CP for the 2 and 4 weeks post-freeze harvest, respectively.

The effect of fertilizer treatment on IVOMD at date of freeze and post-freeze harvests was similar to CP. The IVOMD was 1.0, 1.4, 2.3, and 2.3 percentage units greater for fertilized plots at date of freeze and 1, 2, and 4 week post-freeze, respectively (Table 4). Most grasses except Florakirk increased in IVOMD due to fertilizer. Argentine and Tifton 9 bahiagrass increased an average of 2.6 and 2.5 percentage units IVOMD due to fertilization across harvest treatments, respectively. Similar to CP, the average IVOMD change due to fertilizer, tended to occur 2 and 4 weeks post-freeze increasing by 1.3 percentage units when compared with time of freeze. This increase may be due to high quality forage of young developing tillers in the base of the sward. Under subtropical conditions, temperatures tend to warm rapidly following a freeze (5,6,7,8).

Table 4. Influence of fertilizer × harvest treatment on in vitro organic matter digestion (IVOMD) of grasses harvested at date of freeze and 1, 2, and 4 weeks post-freeze pooled over 3 years at Ona, FL.

Grass entry	Fertilizer	Date of freeze	Post-freeze (weeks)			Avg.
		Date of freeze	1	2	4	Avg.
		IVOMD (%)
Bahiagrass
Argentine	No	55.3 a^w	47.7 b	47.8 b	39.1 c	47.4^y
	Yes	59.4 a	50.6 b	50.2 b	40.0 c	50.0
	Yes vs. no	*^x	*	NS	NS	—
Paraguay 22	No	57.4 a	48.6 b	48.0 b	39.0 c	48.3
	Yes	58.6 a	50.1 b	48.7 b	40.7 c	49.5
	Yes vs. no	NS	NS	NS	NS	—
Pensacola	No	58.7 a	55.3 b	53.2 b	46.5 c	53.4
	Yes	60.8 a	56.5 b	56.1 b	49.7 c	55.8
	Yes vs. no	NS	NS	*	*	—
Tifton 9	No	59.2 a	56.0 b	54.1 b	48.7 c	54.5
	Yes	61.6 a	58.7 b	57.3 b	50.3 c	57.0
	Yes vs. no	NS	*	*	NS	—
Bermudagrass
Florakirk	No	59.4 a	54.6 b	51.0 c	40.4 d	51.4
	Yes	57.5 a	51.7 b	50.0 b	42.6 c	50.5
	Yes vs. no	NS	*	NS	NS	—
Tifton 85	No	62.2 a	55.1 b	51.6 c	40.7 d	52.4
	Yes	61.2 a	56.9 b	55.1 b	43.9 c	54.2
	Yes vs. no	NS	NS	*	*
Stargrass
Florico	No	59.5 a	53.0 b	48.3 c	36.7 d	49.4
	Yes	58.5 a	54.8 b	50.9 c	42.3 d	51.6
	Yes vs. no	NS	NS	*	*	—
Florona	No	53.9 a	47.7 b	44.8 c	36.8 d	45.8
	Yes	54.3 a	49.2 b	47.9 b	37.9 c	47.3
	Yes vs. no	NS	NS	*	NS	—
Limpograss
Floralta	No	61.3 a	58.2 b	57.6 b	53.5 c	57.7
	Yes	64.2 a	60.3 b	61.0 b	54.4 c	60.0
	Yes vs. no	*	NS	*	NS	—
Avg. IVOMD increase^z		1.0	1.4	2.3	2.3	1.8
Avg. IVOMD		59.0	53.6	51.9	43.5	52.0

^w Means in row followed by the same letter(s) are not different at the 0.05 level of probability (Duncan’s test).

^x An asterisk indicates a significant difference between fertilizer levels at each harvest period for each grass. NS not significant (t test LSD).

^y Average IVOMD of four harvest treatments.

^z Average IVOMD increase due to late October fertilizer treatment.

Forage digestibility on date of freeze and 1, 2, and 4 weeks post-freeze averaged 59.0, 53.6, 51.9, and 43.5% over all cultivars with and without fertilizer (Table 4). On average, IVOMD decreased 15.5 percentage units between date of freeze and 4 weeks post-freeze or about 4 units/week. Therefore, under grower conditions when these subtropical grasses are exposed to a freeze, forage must be utilized within one or two weeks post-freeze before forage IVOMD gets too low. Floralta limpograss had the highest IVOMD 1 week after a freeze averaging 59.3%, followed closely by Tifton 9 bahiagrass (57.4%) (Table 5). Argentine and Paraguay 22 bahiagrass and Florona stargrass all had the lowest IVOMD averaging 49.1, 49.4, and 48.4%, respectively. These three grasses decreased 8.2, 8.6, and 5.7 percentage units, respectively within one week after a freeze. Similar results were obtained with other tropical grasses (10,13). Delaying harvest until 2 weeks after a freeze revealed the bahiagrasses and Floralta limpograss did not change in IVOMD when compared with 1 week, however the bermudagrasses and stargrasses decreased significantly (Table 5). Harvesting grasses 4 weeks after a freeze again showed a major decrease in IVOMD with an overall average of 43.5% or 8.4 percentage units lower than grasses harvested 2 weeks after a freeze. At 4 weeks post-freeze, limpograss had the highest IVOMD of 53.9% and Florona stargrass the lowest IVOMD (37.3%). The bermudagrasses and stargrasses appear to be very sensitive to freezing temperatures and IVOMD decreased 18 percentage units during the four weeks following a freeze. This information can be utilized by commercial growers when making cattle supplementation decisions, especially when grazing subtropical grasses after a freeze. Since IVOMD dropped rapidly following a freeze a readily available energy supplementation is required. In Florida the most economical energy source supplementation is molasses.

Table 5. Comparison of grass cultivars harvested at date of freeze and 1, 2, and 4 weeks post-freeze for in vitro organic matter digestion (IVOMD), and pooled over 3 years at Ona, FL.

Grass entry	Date of freeze	Post-freeze (weeks)			Dunnett’s test			IVOMD de- crease (%)^z
	Date of freeze	1	2	4	Dunnett’s test
	IVOMD (%)				1	2	4
Bahiagrass
Argentine	57.3 d^w	49.1e A^x	49.0 c A	39.5 de B	*^y	*	*	17.8
Paraguay 22	58.0 cd	49.4 e A	48.3 cd A	39.8 d B	*	*	*	18.2
Pensacola	59.8 b-d	55.9 bc A	54.7 b A	48.1 b B	*	*	*	11.7
Tifton 9	60.4 a-c	57.4 ab A	55.7 b A	49.5 b B	*	*	*	10.9
Avg.	58.8	52.9	51.9	44.2
Bermudagrass
Florakirk	58.5 cd	53.2 d A	50.5 c B	41.5 cd C	*	*	*	17.0
Tifton 85	61.7 ab	56.0 bc A	53.3 b B	42.3 c C	*	*	*	19.4
Avg.	60.1	54.6	51.9	41.9
Stargrass
Florico	59.0 cd	53.9 cd A	49.6 c B	39.5 de C	*	*	*	19.5
Florona	54.1 e	48.4 e A	46.3 d B	37.3 e C	*	*	*	16.8
Avg.	56.5	51.1	47.9	38.4
Limpograss
Floralta	62.7 a	59.3 a A	59.3 a A	53.9 a B	*	*	*	8.8
Overall avg.	59.1	54.4	51.9	43.5				15.5

^w Means within a column followed by the same letter(s) are not different at the 5% level (Duncan’s test).

^x Means within a row followed by the same upper case letter are not different at the 5% level (Duncan’s test).

^y An asterisk indicates that post-freeze harvests 1, 2, and/or 4 means are different (P < 0.05) from the date of freeze harvest (Dunnett’s test). NS = not different.

^z Values are calculated as the difference in IVOMD content (percentage units) between time of freeze and 4 weeks post-freeze.

Conclusion

The application of fertilizer to bahiagrasses in late October was uneconomical from the FM standpoint, however CP and forage digestibility was increased. However, October fertilization of bermudagrasses, stargrasses, and limpograss was beneficial. The effect of fertilizer on CP concentration and IVOMD of grasses was not great enough that commercial growers would be advised to apply fertilizers in late fall to increase forage CP and IVOMD alone. However a fall application of fertilizer ($40/acre) would be profitable with increased FM and nutritive value. Following a freeze subtropical grasses change little in CP concentration but decrease drastically in IVOMD over a 4-week period. When high-quality forage of subtropical grasses is frozen, forage should be removed within 2 weeks, to minimize loss of IVOMD. When grazing these grasses that were exposed to a freeze is extended beyond 2 weeks, some type of energy supplementation will be required.

Acknowledgment

This research was supported by the Florida Agricultural Experiment Station.

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