Authors: Evan Blank, Undergraduate Student; Thomas E. Aquino, Graduate Student; Jessica L. Miller, Research Technician and Graduate Student, Mary E. Drewnoski, Prfessor, Animal Science, Lincoln.
Summary with Implications
A sorghum-sudangrass hybrid was swathed in the hard dough stage and grazed by growing steers from November 2023 to January 2024. Steers appeared to select the seedheads when grazing. The seed made up approximately 12.6% of swathed forage mass, but its energy contribution appears limited due to poor digestibility. When incubated whole, only 23% of dry matter and 13% of starch was digestible. Grinding improved digestibility but still left 40% of the dry matter and over 85% of starch unavailable. Weathering over two months did not improve digestibility, indicating the seed coat remains intact through winter. Compared to sorghum grain, the hybrid seed had significantly lower starch digestibility. Because cattle are unlikely to sufficiently chew the small seeds to break the tough seed coat, the actual energy benefit is minimal. Although seedheads are readily consumed, producers should not assume they contribute significant nutritional value in grazed systems.
Introduction
When cattle were grazing the sorghum x sudangrass forage that was swathed in the hard dough stage in the experiment reported in 2025 Nebraska Beef Report, p. 16, cattle appeared to preferentially select the seedheads of the sorghum x sudangrass. However, sorghum x sudangrass seed appears to have a hard waxy outer seed coat similar to one of its parents, sorghum. This could result in the poor digestion of the seed by cattle and an over estimation of energy value when analyzed by a forage testing laboratory. The objectives of this study was to determine dry matter digestibility and starch digestibility of the sorghum x sudangrass seed, and compare the digestibility of sorghum x sudangrass seed to sorghum grain, as much more is known about its feeding value. Additionally, there was an opportunity to look at the potential effect of weathering on the durable, hard seed coat that encapsulates sorghum x sudangrass seed to determine if there was an increase digestibility due to seed coat degradation from exposure to Nebraska winter weather conditions. If weathering had an effect on the seed coat there may be an increase in the digestibility and thus energy value of the seed later in the winter.
Procedure
The initial sampling for this experiment was conducted at the University of Nebraska-Lincoln Eastern Nebraska Research, Extension, and Education Center (ENREEC) near Mead, Nebraska. All laboratory analysis was conducted onsite at the ruminant nutrition laboratory inside the Department of Animal Science, University of Nebraska-Lincoln, Lincoln, NE.
Crop Information
The crop field that was utilized covered a 38-acres that was seeded with a sorghum x sudangrass hybrid, Canex® Brown mid-rib (BMR) 210 Hybrid Sweet sorghum on June 16, 2023. Prior to seeding, the field was fertilized with 40 lb of N/ac from urea (46-0-0). The sorghum x sudangrass forage was allowed to grow for the full season with the first frost occurring on October 7, 2023. On November 3, 2023, the crop was mowed with a 15 ft wide mower and swaths were left in field (2025 Nebraska Beef Report, p. 16).
Grain and Forage Sampling Procedure
Sorghum x sudangrass quality samples were collected every two weeks starting on November 9th, 2023. A 1 ft section of a windrow in each paddock was cut and removed from an area that had not been allocated for grazing. The samples were dried in 140°F forced air oven until reaching a constant weight. Then the forage and seeds were separated and weighed. The forage nutritive value of the sorghum x sudangrass forage (i.e. without the seedhead is reported in 2025 Nebraska Beef Report, p.16). In the current experiment, the seeds from two dates (November 9th and January 4th) and four paddocks were used. This resulted in a total of 8 samples across the two dates. A sorghum grain sample was also obtained and split into 4 replicates to act as a control.
Laboratory Analysis
Sorghum x sudangrass seed and control sorghum grain were both processed following the same methodology. There were two processing methods used, with “whole” seed, being left “as-is” and “6 mm” seed, ground through a Next Generation Model 4 Wiley® Mill with chamber size of 7.75 in, 920 rpm cutting speed, 6 cutting blades and a 6 mm screen. This resulted in a total of 16 sorghum x sudangrass seed samples (4 paddock x 2 dates x 2 processing methods) and 8 sorghum grain samples (4 samples x 2 processing methods).
Five grams of each sample was weighed into ANKOM® R1020 10 x 20 cm forage in-situ bags with micron porosity of 50 ± 10. This was replicated over 5 bags per sample with 3 being used for ruminal incubation and 2 to determine washout for a total of 120 bags. In-situ bags were placed into two cannulated steers that were both fed a diet of ground hay, rolled corn, dried distillers grain, and a supplement containing minerals. Samples were divided in half with all the samples from 2 paddocks of sorghum x sudangrass and 2 sorghum samples being placed in a laundry bag and incubated in one steer and the other 2 paddocks and 2 sorghum samples being incubated in the other steer. Bags were incubated in the rumen for 24 hours. Bags were then placed in a washing machine and washed in cold water to remove outside feed contaminates and rumen fluid. At the same time, the washout bags were added to the washing machine to adjust for any sample loss through the pores of the bag (i.e. washout). The bags were then dried in a 140°F forced air oven until they reached a constant weight. Samples were then allowed to air-equilibrate, and a final weight was recorded. The in-situ bags in triplicate by paddock were then composited into singular whirl-pak bags that represented each paddock. Representative samples were then ground in the Wiley mill to 0.5 mm to prepare them for the starch assay. Starch assay was performed using the NEOGEN® MEGAZYME total starch assay procedure for no resistant starch w/ incubation at pH 5.0.
Data Analysis
Data was analyzed with SAS 9.4 (Cary, NC) using the PROC mixed procedure. The effect of date of sampling on the starch content of the sorghum x sudangrass seed was analyzed with the fixed effect of date and paddock was included a random variable. To evaluate the effects of sampling date and processing method of the sorghum x sudangrass seed on in-situ dry-matter digestibility and starch digestibility the model included the fixed effects of processing method, date and their interaction. The steer used for incubation was included as random and the paddock as the subject. To compare the dry matter and starch digestibility of sorghum x sudangrass seed to sorghum grain, the sorghum-sudangrass data was averaged across date by paddock before being statistically analyzed. The model included the fixed effects of seed type, processing method and their interaction with steer used for incubation as a random effect.
Results
There was no change (P ≥ 0.31) in the amount of forage or seed in the swathed material from November to January (Table 1). Overall, the seed made up 12.6% of the swathed sorghum x sudangrass mass (DM basis). When the seed was left whole, we had hypothesized that weathering would cause some cracking of the seed coat and allow for increased ruminal microbial activity inside the seed to digest more of the endosperm which contains the starch. However, there appeared to be no weathering effect of the winter environment on the hard seed coat of the sorghum-sudangrass, as the dry matter (DM) digestibility, starch content and starch digestibility showed no change (P ≥ 0.59) between the November and January sampling dates (Table 2).
| November | January | SEM | P-value | |
| Forage, lb/ac | 4,230 | 5,314 | 419 | 0.31 |
| Seed, lb/ac | 635 | 715 | 89.6 | 0.64 |
| Seed, % of mass | 13.1 | 12.1 | 1.38 | 0.68 |
November | January | P-value | ||||||
| 6 mm | Whole | 6 mm | Whole | SEM | Date | Process | Date×Process | |
| Starch, % DM | 60.7 | 59.5 | 3.51 | 0.83 | NA | NA | ||
| Dry matter digestibility, % DM | 57.0 | 14.0 | 58.4 | 14.8 | 2.69 | 0.62 | < 0.01 | 0.90 |
| Starch digestibility, % DM | 35.5 | 6.25 | 36.3 | 8.77 | 3.01 | 0.59 | < 0.01 | 0.78 |
There was a significant (P <0.01) effect of processing method for both DM and starch digestibility. Whole sorghum x sudangrass seed was lowly digestible with only 14.4% of DM and 7.5% of starch being available. Grinding the sorghum x sudangrass seed through a 6 mm screen significantly increased digestibility of both DM and starch. However, DM and starch digestibility was still relatively low when ground at 58% and 36%, respectively. Given the small seed size, it would not be expected for cattle to masticate (chew) the seed to the extent of the 6 mm grind. Thus, it would be expected for the digestibility achieved by cattle to be somewhere between the whole and 6 mm when consuming the seed from the swathed material.
Similar to the sorghum x sudangrass seed, grinding the sorghum grain increased (P < 0.01) the DM and starch digestibility (Table 3). The increase in both starch and DM digestibility can be attributed to the increased exposure of the starch rich endosperm due to the cracking of the hard outer seed coat. When the sorghum x sudangrass seed was compared to sorghum grain (Table 3), there was no difference (P = 0.15) in DM digestibility but sorghum did have greater (P < 0.01) starch digestibility than sorghum-sudangrass seed.
Sorghum | Sorghum x sudangrass | P-value | ||||||
| 6 mm | Whole | 6mm | Whole | SEM | Type | Process | Type×Process | |
| Starch, % of DM | 77.4 | 60.1 | ||||||
| Dry matter digestibility, % DM | 71.4 | 23.0 | 65.5 | 23.0 | 3.72 | 0.15 | < 0.01 | 0.27 |
| Starch digestibility, % DM | 47.9 | 19.8 | 41.1 | 12.7 | 1.83 | < 0.01 | < 0.01 | 0.87 |
Conclusions
Although sorghum × sudangrass seed comprised approximately 12.6% of the swathed forage mass (about 675 lb/ac), its contribution to cattle performance is likely minimal due to poor digestibility. The tough outer seed coat remained intact even after two months of winter weathering, and in-situ analysis showed that only a small proportion of the seed's dry matter and starch was digestible when incubated whole. Grinding the seed improved digestibility, but grinding to the extent in this trial does not likely mimic what happens during normal chewing. This has important implications for forage testing, as TDN values from the lab when the seed head is included will overestimate the true energy available to the animal.
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