Final report for GNC15-200
Cover crops (e.g., annual cool season forages) often provide opportunity to enhance nutrient cycling and mitigate soil erosion, and loss of nitrogen on tillable lands subsequent to grain harvest. Thus, it is not surprising that amounts of tillable land planted to annual cool season forages subsequent to harvest has linearly increased over the past 10 years. Grazing annual cool season forages (e.g., brassicas, grass, legume, forbs) can allow for greater efficiency of land use by production of plant and animal products on the same land resources. Indeed, grazing annual cool season forages can also allow for increased availability of forage resources to ruminants and reduce needs for stored forages, which could allow for expansion of the United States cow herd.
Many brassicas planted to tillable land allow for appreciable amounts of forage organic matter that contain greater concentrations of metabolizable energy and non-structural carbohydrates in comparison to other forages. However, brassicas often accumulate secondary plant defensive compounds (e.g., nitrates, sulphates, glucosinolates, and S-methyl cysteine sulphoxides) that can have deleterious effects among grazing cattle. A paucity exists among the available data related to optimal management practices for cattle grazing annual cool season forages, and this lack of information prevents optimal use of these forages by cattle. Stocking rate often has large effects on diet digestibility, forage selection, and animal performance. Yet, few data are available on effects of stocking rates among cattle grazing annual cool season forages.
Our goal in this study is to provide information that allows for improved management of cattle grazing annual cool season forages. Improved management strategies among cattle grazing annual cool season forages can allow for greater integration of livestock and crop production systems that can allow for improved use of land resources.
The objective of this research was to determine the effect of stocking rate among cattle grazing brassica (purple top turnip and forage radish) and annual ryegrass planted subsequent oat harvest. We also wanted to identify producer viewpoints and setbacks to the technology.
Specifically, our objectives were to measure: 1) diet selection, 2) diet digestibility, 3) quantify nitrogen balance, and 4) determine animal performance 5) survey 6) interview products.
All protocols involving the use of animals were approved by the South Dakota State University Institutional Animal Care and Use Committee.
Fourteen Angus (initial BW = 267 ± 21.4 kg) and 25 Angus × Simmental heifers (initial BW = 281 ± 21.0 kg) were blocked by initial BW and randomly allocated to 1 of 3 stocking density treatments. An additional 5 Angus (initial BW = 223 ± 51.4 kg) and 5 Angus × Simmental (initial BW = 234 ± 37.7 kg) heifers were surgically fitted with ruminal cannulas to allow measures of diet selection and ruminal parameters. Heifers were vaccinated against clostridial bacterin-toxoids (Calvary 9; Merck Animal Health, Madison, NJ) 19 d prior to and at cannulation. Heifers were also vaccinated against infectious bovine rhinotracheitis, bovine viral diarrhea and bovine respiratory syncytial virus (Bovi-Shield; Zoetis Animal Health, Parsippany, NJ) and papilloma (Wart Shield; Novartis Animal Vaccines, Inc., Larchwood, IA) at 19 d prior to cannulation. Ruminal fistulation surgeries were performed 33 d prior to grazing via a modified one-stage procedure (Kristensen et al., 2010) with cattle standing. Cattle were locally anesthetized (lidocaine-HCl) at the left-flank and provided intravenous analgesia (2.2 mg/kg BW of flunixin meglumine; Banamine; Merck Animal Health) and prophylactic antibiotics by subcutaneous injection at the base of the ear (6.6 mg/kg ceftiofur; Exceed; Zoetis Animal Health) and intramuscularly (20,000 units/kg penicillin; Bactracillin G; Aspen Veterinary Resources, LTD, Greeley, CO) immediately prior to surgery. Subsequently, heifers were provided intravenous analgesia and intramuscular antibiotic daily after cannulation for 3 and 6 d, respectively. If local inflammation was observed at the surgical site or if rectal temperature was greater than 40 °C for more than 3 d following cannulation then analgesia was provided for 5 d. Immediately after surgery, calves were returned to pasture and recovered with their dams for 11 d followed by weaning via fence line separation. Once daily for 14 d following cannulation the surgical site was cleaned with an iodine scrub (7.5% Providine; Purdue Products, Stamford, CT) and monitored for inflammation; rectal temperature was measured as an indicator of infection. Heifers were treated with an anthelmintic (LongRange; Merial Limited, Duluth, GA) 4 d post-surgery. After 14 d, no local inflammation was observed at the surgical site and rectal temperatures were 38 ± 0.26 °C. After recovery, cannulated heifers were randomly assigned to stocking density treatments.
The study was conducted 1.5 km north of Brookings, SD (44°20'22.21"N, 96°48'7.84"W). After oat harvest and removal of oat residue by baling, a binary mixture of grass and Brassicaceae (mustard family; here after referred to as brassica) was planted 6 days after an application of glyphosate. The seed mixture consisted of 66.5% Lolium perenne L., 20% Raphanus sativus L. and 13.5% Brassica rapa L. and was seeded at a rate of 16.6 kg × ha-1 on July 28, 2015. Subsequently, the field was fenced into 12 paddocks for grazing.
Different stocking densities were achieved by assigning 3, 4 or 5 heifers to 1 of 12 paddocks (1.1-ha) and allowing cattle to graze for 48-d to obtain a stocking rate of 1.7-, 2.3- and 2.9-AUM × ha-1, respectively. Heifers were allowed to graze their paddocks beginning October 14, 2015 and remained until November 30, 2015. In 9 of the 12 paddocks 1 ruminally cannulated heifer was included to allow measures of diet selection and nutrient intake. Before the trial initiated, heifers grazed a pasture that consisted of smooth brome (Bromus inermis Leyss. subsp. inermis), creeping foxtail (Alopecurus arundinaceus Poir.), and big bluestem (Andropogon gerardi Vitman).
Forage biomass was sampled 11 d prior to grazing by clipping 30 quadrats (0.25 m2) stratified across the field prior to assignment of the 3 stocking density treatments and 4 replicate paddocks. Clippings were dried for 14 days in a forced air oven (60 °C). After drying, samples were separated into either forage from brassica or grass and composited by paddock. Subsequently, forage composition was determined gravimetrically, and chemical composition (OM, N, NDF, ADF, acid detergent insoluble ash, nitrate-N and nitrite-N) was analyzed.
Sampling Procedures and Laboratory Analyses
Heifer BW was recorded for 2 consecutive days beginning on d 1, 21, and 47. Diet samples were collected by total ruminal evacuation (Reid, 1965) on d 2, 24 and 46. Ruminal contents of cannulated heifers were totally evacuated, weighed and subsampled for determination of ruminal liquid and DM fill (Froetschel and Amos, 1991) and analyses of DM, VFA, ammonia, nitrate-N and nitrite-N concentration. Subsequently, heifers were returned to the appropriate paddock and allowed to graze for 45 min. After 45 min., ruminal contents were removed, weighed, and sampled for analysis of DM, OM, NDF, ADF, N, acid detergent insoluble ash (ADIA) and starch. Ruminal contents removed prior to diet sampling were replaced before heifers were allowed to return to paddocks.
Nitrogen balance was measured from d 18 to 23. Spot fecal samples were collected from d 9 to 14 for determination of background TiO2. At 0800 from d 18 to 23, a bolus of TiO2 was administered for estimation of fecal output (Titgemeyer et al., 2001). Urine (70 g) and feces (50 g) were collected and each was composited daily from d 18 to 23 by spot sampling. Urine was acidified with 5.5 mL H2SO4 (10% wt/wt) before it was composited. All samples were frozen at -20 °C. Time of spot sampling was delayed by 2 h daily so that composites reflected every other hour in a 12 h period. Feces was analyzed for DM, OM, NDF, ADF, ADIA, N and TiO2 concentration. Urine was analyzed for creatinine, and N. Dry matter intake and DM digestion were estimated from measures of fecal output, and concentration of dietary and fecal ADIA.
Partial DM of diet samples, ruminal digesta and feces was determined by drying in a forced air oven (55 °C) for 24 h, and samples were allowed to air equilibrate prior to weighing. After measures of partial DM samples were ground to pass a 1 mm screen (Thomas Wiley Laboratory Mill Model 4; Thomas Scientific USA, Swedesboro, NJ). Measures of DM were determined by drying for 16 h at 105 °C. Organic matter was determined by combustion (500 °C). Neutral detergent fiber was measured with addition of a-amylase and sodium sulfite; ADF was measured non-sequential to NDF (Van Soest et al., 1991). Acid detergent insoluble ash was determined by combustion subsequent to determination of ADF. Diet, fecal and urinary N was determined by the Dumas procedure (method no. 968.06; AOAC, 2012; Rapid N III; Elementar, Mt. Laurel, NJ). Urinary creatinine was measured colorimetrically (DetectX; Arbor Assays, Ann Arbor, MI) by the modified Jaffe reaction described by Slot (1965) and Heinegard and Tederstrom (1973). Ruminal ammonia was analyzed with a colorimetric reaction catalyzed by phenol-hypochlorite (Broderick and Kang, 1980). Titanium dioxide in feces was measured as described by Short et al. (1996). Starch in diet and forage samples was determined using the glucogenic assay described by Herrera-Saldana and Huber (1989); glucose was quantified by glucose oxidase (Gochman and Schmitz, 1972), and unpolymerized glucose was determined when no a-amylase was added. Ruminal VFAs were determined by gas chromatography as described by Vanzant and Cochran (1994).
Dry matter was calculated as partial DM (55 °C) multiplied by DM measured after drying at 105 °C. Fecal output was calculated as the quotient of amount of TiO2 bolused daily by TiO2 concentration in feces corrected for amount of TiO2 in feces prior to administering TiO2 (Titgemeyer et al., 2001). Daily creatinine output was assumed to be 28 mg/kg of BW (Chizzotti et al., 2008). Urine output was estimated to be the quotient of creatinine output and urinary creatinine concentration. Urinary N output was calculated as the product of urine output and urine N concentration. Fecal excretion of N, OM, NDF, ADF and ADIA were calculated by multiplying daily fecal output by fecal concentration of N, OM, NDF, ADF, and ADIA, respectively. Dry matter intake was calculated as described by Merchen (1988):
Intake of OM, NDF, and ADF was calculated as the product of DMI and OM, NDF, ADF in the diet sample collected at d 24, and N intake was calculated as the product of DMI and N intake from forage. Proportional intake of brassica and grass were calculated from NDF content in diet samples and NDF content in each forage. Subsequently, intake of each forage was determined as the product of proportional intake of brassica and grass and DMI. Forage N intake was the summed product of predicted brassica and grass intake and brassica and grass N content, respectively. Nutrient digestion was calculated as the difference of intake and fecal output. Nitrogen balance was calculated as the difference of N intake and N output from urine and feces. Ruminal DM fill was calculated as the product of the weight of wet ruminal contents and DM content. Ruminal liquid fill was calculated by difference between the weight of wet ruminal contents and DM fill.
On d 24 one of the cannulated heifers was injured and replaced with another cannulated heifer which was used for the final ruminal evacuation on d 46. Additionally, data from a cannulated heifer grazing in an intermediate stocking density paddock and a non-cannulated heifer grazing in paddock with the least stocking density were removed from calculations of pen means for digestibility analyses because of an apparent marker failure. Data were analyzed with the MIXED procedures of SAS 9.3 (SAS Inst. Inc., Cary, NC). Measures of nutrient digestion and performance were analyzed as a completely randomized block design and paddock was the experimental unit. Stocking density was a fixed effect and block was a random variable. Differences between stocking density were determined by linear and quadratic contrasts. Measures of diet selection and ruminal parameters were analyzed as repeated measures. Paddock was the experimental unit, and the model contained stocking density with block as the random term. The repeated term was day, with paddock serving as the subject. Compound symmetry was used for the covariance structure. Effects of stocking density and day were determined with linear and quadratic contrasts. When effects of stocking density × day was significant (P ≤ 0.05) differences of stocking density within day were separated by linear and quadratic contrasts.
Temperature and precipitation data were obtained from a weather station located 3 km southeast of the grazing site. Average temperature during the grazing period was 5.1 ± 0.86 °C, and wind speeds averaged 11.8-± 1.1 km × h-1. Additionally, rain events occurred on 6 days (11 ± 3.4 mm/d) and total precipitation was 66 mm. It is likely that little OM growth of brassica and grass occurred after heifers grazed paddocks due to cold temperatures and diminishing photoperiod.
Paddocks consisted of a binary mixture of brassica and grass that contained 26.5 ± 1.9% and 60.8 ± 1.2% NDF, respectively. We estimated relative intake of brassica to grass from NDF content of masticate that was ruminally collected during measures of diet sampling and observed an interaction (P < 0.01) among stocking density and amount of time heifers grazed on estimates of diet selection. As expected, we observed no difference (P ≥ 0.68) in estimates of relative intake of brassica to grass after 2 d of grazing, but increased stocking density increased (quadratic, P = 0.02) brassica intake after 24 d of grazing. Interestingly, increased stocking density decreased (linear, P < 0.01) relative intake of brassica to grass after 46 d of grazing (Figure 1).
A precise understanding of how plant material is selected by grazing cattle remains equivocal; however, grazing pressure (Van Soest, 1994), previous grazing experience (Launchbaugh and Provenza, 1991), bulk-fill and chemostatic feedback mechanisms (Allen et al., 2009), and plant flavonoids (Provenza, 1995, 1996) can affect forage selection. Importantly, however, ruminants often avoid selection of plants with toxins that can reduce performance or digestion (Provenza, 1996; Catanese et al. 2012). In our experiment, cattle grazed greater amounts of grass at d 2 compared to d 24. Grass contained no nitrate, but brassica contained 0.16 ± 0.03% nitrate-N (Table 1). Forages containing nitrates are potentially toxic to cattle that are not adapted to nitrates (Emerick, 1988). Amount of nitrate intake, liquid and particulate passage rate, and rates of ruminal nitrate reduction to ammonia can effect nitrate poisoning in cattle. Ostensibly, ruminal capacity for nitrate reduction to ammonia can quickly adapt to large amounts of nitrate intake (Farra and Satter, 1971; Allison and Reddy, 1984; Emerick, 1988; Van Soest, 1994; Leng, 2008). Small daily amounts of nitrate intake can augment ruminal capacity for nitrate reduction to ammonia (Lee et al., 2015a; Lee et al., 2015b). Perhaps after only 2 d of grazing, heifers foraged mostly grass because of previous grazing experience, and larger amounts of nitrate in brassica. Generally, reductions in diversity of plant species and increases in grazing pressure reduce selection of plant species among ruminants (Black and Kenney, 1984; Van Soest, 1994). Indeed, brassica intake was greater among heifers grazing pastures at greater stocking density after 24 d. It is possible that increased grazing pressure allowed for incremental increases in intake of brassica and concomitantly increased nitrate intake. If stocking densities in our study allowed for small daily increases in brassica and nitrate intake it is likely that ruminal nitrate reductive capacity of heifers was increased. Ruminal nitrate concentration tended to decrease (linear, P = 0.06; Table 2) even though estimates of brassica intake increased. Greater ruminal reductive capacity of nitrate could allow greater intake of brassica. Typically, cattle select plants with greater nutrient density, but increased grazing pressure decreases diet quality because selection is reduced (Cook et al., 1953; Pieper et al., 1959; Bryant et al., 1970; Ellis et al., 1984). Data are limited on digestibility of brassicas among cattle; however, several authors (Nicol and Barry, 1980; Sun et al., 2012) have reported that, in sheep, forage from brassica had greater total-tract digestion in comparison to grass. Thus, increased grazing pressure may have facilitated increased intake of the more nutrient dense brassica after less amount of time spent grazing. Heifers in paddocks with the least amount of stocking density had the greatest intake of brassica relative to grass after 46 d of grazing. It is possible that heifers housed in paddocks with greater intake of brassica nearer to the beginning our study had less amounts of forage OM available from brassica at the end of the study because of greater brassica intake earlier in the experiment and greater amounts of trampling.
Increased ruminal reduction of nitrate has been associated with increased acetic acid concentration and decreased concentration of propionic acid (Farra and Satter, 1971; Klop et al., 2015; Olijhoek et al., 2016) and methane (Johnson and Johnson, 1995). Ruminal nitrate concentration (mmol/L) and amount (mmol) tended to decrease (linear, P = 0.06; Table 2.) as heifers grazed brassica and grass for greater amounts of time. Total ruminal liquid (P < 0.01) and DM (P < 0.01) fill, and VFA concentration (P < 0.01) and amount (P < 0.01) decreased with greater amounts of time. Interestingly, acetic acid concentration was least and propionic acid concentration was greatest after 24 d, but concentration of acetic and propionic acid was similar to amounts observed at the beginning of the study after 46 d. Similarly, Sun et al. (2012) reported reduced concentrations of acetic acid relative to propionic acid among sheep fed brassica than when sheep were fed perennial ryegrass. Phillips and Horn (2008) reported that ruminal liquid and particulate fill decrease when cattle transition to diets with greater nutrient density. It is likely that ruminal concentration and amounts of fermentative end products and ammonia measured at the beginning of our study (d 2) were reflective of the common mixed grass pasture that heifers grazed before grazing brassica and grass. However, differences in concentration and amount of ruminal end products was also likely reflective of greater selection of grass relative to brassica at the beginning our study. Increased intake of brassica could have depressed activity of ruminal fibrolytic bacteria (Marais et al., 1988). Additionally, brassica contained less NDF than grass (Table 1), and generally brassica contains larger amounts of more rapidly fermentable carbohydrate (e.g., soluble sugars, pectins) relative to amounts of structural carbohydrate (i.e., cellulose and hemicellulose; Barry, 2013). Ruminal starch content was also greatest at d 24 (quadratic, P < 0.01). Greater fermentation of more rapidly fermentable carbohydrate is often associated with increased ruminal concentration of propionic acid and decreased ruminal concentration of acetic acid (Owens and Goetsch, 1988; van Gastelen et al., 2015; NRC, 2016).
Nutrient intake, total-tract digestion and DMI
Overall, diet OM (82.6 ± 1.5%), NDF (42.2 ± 1.7%), ADF (33.5 ± 0.51%) and ADIA (4.9 ± 0.51%) content was not affected by stocking density (P ≥ 0.25; Table 3). However, diet OM was greatest and NDF and ADIA content were least after 24 d of grazing (quadratic, P ≤ 0.01). Diet ADF content (33.5 ± 0.6%) was not affected by amounts of time that heifers grazed. Even though we did not observe any differences among stocking density at different amounts of time, diet selection for OM was numerically greatest among heifers grazing paddocks with greater amounts of stocking density (84.5% OM) and numerically least (82.9% OM) among heifers grazing paddocks with lesser stocking density after 24 d. Diet NDF and ADIA were numerically least among heifers in paddocks with greater amounts of stocking density (34.5% NDF; 2.8% ADIA) compared to heifers in paddocks with lesser stocking density (37.5% NDF; 4.2% ADIA) after 24 d. These data correspond with differences in estimates of relative intake of brassica and grass and to effects of stocking rate and time among ruminal measures. Further, measures of DM digestibility obtained from 18 to 23 d of grazing were greatest (quadratic, P < 0.01) and digestion of OM, NDF and ADF tended (quadratic, P ≤ 0.09) to be greatest among heifers in paddocks with increased stocking density (Table 4).
Several authors have reported that brassica DM digestibility in sheep (Nicol and Barry, 1980; Sun et al., 2012) and OM digestibility in cattle (Clark et al., 1997) is greater in comparison to DM and OM digestion of grass. Nicol and Barry (1980) and Sun et al. (2012) reported DM digestibility of forage turnips among sheep was 89.3% and 80.8%, respectively. Typically, ryegrass DM digestibility ranges between 65% and 80% (Ellis et al., 1984; Sun et al., 2012), and increases in grazing pressure often decrease diet nutrient density and in vitro OM digestibility (Cook et al., 1953; Blaser et al., 1959, 1960). Relatively few authors have reported measures of total-tract diet digestibility among grazing cattle in response to increased grazing pressure. Ellis et al. (1984) reported that increased grazing pressure among cattle grazing ryegrass reduced total-tract DM digestion. Similarly, Olson et al. (2002) reported that increased stocking density reduced total-tract OM digestibility among steers grazing short-grass prairie. We are unaware of any reports of effects of increased grazing pressure on total-tract digestion among cattle grazing brassicas and annual ryegrass. We observed increased digestibility in response to greater stocking density that was also concurrent with a greater proportional intake of brassica. Cattle often select familiar plants and avoid plants containing toxic secondary compounds (Launchbaugh and Provenza, 1991; Provenza, 1995; Provenza, 1996; Catanese et al., 2012). Increased stocking density in our study may have facilitated more rapid adaptation to and greater intake of brassica despite a lack of previous grazing experience among cattle and measurable amounts of nitrate in brassica.
Greater amounts of readily fermentable carbohydrate allow for increased rates of ruminal degradation, passage rate and subsequently DMI. Additionally, increases in diet digestibility allow greater DMI when DMI is limited by ruminal fill (Ellis et al., 1984; Redmon et al., 1995). Brassica has less structural carbohydrate and greater apparent digestibility (Sun et al., 2012) than grass. Increased stocking density tended to increase (quadratic, P = 0.07; Table 4) DMI in our study. It is likely that greater proportional intake of brassica allowed for increased DMI compared to heifers that had a greater proportion of intake that was grass. When ruminal fill is not limiting chemostatic mechanisms control DMI (Allen et al., 2009) and cattle consume food to a constant energy end-point (Lofgren and Garrett, 1968; NRC, 2016). Brassica has nearly 41% more ME (3.1 Mcal ME/kg DM) compared to ryegrass (2.2 Mcal ME/kg DM; Sun et al., 2012; Lindsay et al., 2007). Plegge et al. (1984) concluded that DMI was maximized and likely regulated by chemostatic mechanisms when diet ME was near to 3.1 Mcal ME × kg-1 DM. If increased grazing pressure increased brassica intake it seems likely that heifers were able to more nearly meet predicted amounts of ME intake. Our estimates of proportional brassica and grass intake together with measures of DMI and apparent ME of brassica and grass indicate that ME intake among heifers in paddocks with the least stocking density was limited to 83% of predicted ME intake (NRC, 2016); however, increased grazing pressure resulted in estimates of ME intake nearly 24% greater than predicted ME intake (NRC, 2016).
Nitrogen Retention and Performance
There is a paucity of data related to effects of increased stocking density on measures of N retention, and we are not aware of any reports related to effects of increased stocking density on N retention among cattle grazing brassica and grass-based pastures. Increased stocking density did not affect N intake, or N excreted in urine or feces (P ≥ 0.13; Table 5). However, numerical differences in amounts of N intake and N excreted contributed to increased estimates of N retention in response to increased stocking density (quadratic, P = 0.05). Generally, measures of N retention are sensitive to greater lean tissue accretion; however, we did not observe an effect of increased stocking density on changes in BW (Table 6). Typically, increased stocking density decreases performance (Petersen et al., 1965; Bryant et al., 1970; Smart et al., 2010). We observed small, but statistically significant decreases (linear, P = 0.05) in performance in response to increased stocking density after 22 d, and BW gains were not different from 22 to 48 d of grazing. It is interesting, however, that even though we observed no difference in BW gains during the latter half of our experiment amounts of BW gain were nearly 6.7-times greater than amounts of BW gain from d 1 to 22. Typically, energy first-limits BW gains in cattle of similar size to the heifers in our study (Lofgren and Garrett, 1968; NRC, 2016), and several authors have reported that energy content of brassica exceeds energy content of ryegrass (Barry et al., 1994; Lindsay et al., 2007; Sun et al., 2012). Therefore, increased intake of brassica could allow for greater rates of BW gain. However, reports of BW gains among ruminants grazing brassica relative to grass-based pastures have differed. Campbell et al. (2011) reported that BW gains among sheep grazing brassica were nearly 14% less in comparison to sheep that grazed a grass and clover based pasture. Alternatively, Lindsay et al. (2007) observed a more than 38% increase in BW gains when sheep grazed brassica compared to grass. Phillips and Horn (2008) reported that calves required 7 to 14 d to adapt to wheat pastures before performance was increased. Data are limited on performance of cattle grazing brassica based pastures; however, Barry et al. (1981) reported that, even when mineral deficiencies do not limit gain, BW gains were 27% less among cattle grazing brassica monocultures compared to cattle grazing grass during the first 42 d of grazing. Nonetheless, these authors (Barry et al., 1981) observed a 17% greater ADG compared to grass after cattle were allowed to graze brassica for greater amounts of time (i.e., 42 to 168 days of grazing). We observed increases in BW that were nearly twice the rates of gain reported by Barry et al. (1981) among cattle adapted to brassica monocultures. Many factors (e.g., BW, breed, sex, age, and grazing pressure) could have led to differences between our measures of BW gains and those reported by Barry et al. (1981); however, increases in stocking density in our study may have allowed for a more rapid adaptation to brassica and subsequently larger BW gains compared to those observed by Barry et al. (1981).
Growing cattle often experience compensatory gains in BW when realimented from diets that limit growth to diets that allows optimal rates of BW gain (Fox and Black, 1984; Choat et al., 2003; NRC, 2016). Santra and Pathak (1999) reported greater N retention in cattle during periods of compensatory growth. Perhaps increased grazing pressure allowed for more rapid increases in diet nutrient density by greater proportional intake of brassica, and subsequently, cattle grazing paddocks with increased stocking density realized increases in N retention nearer to the beginning of our experiment compared to cattle housed in paddocks with lesser stocking density. Yet, apparent linear increases in proportional brassica intake among heifers grazing paddocks with the least amount of stocking density could have facilitated similar compensatory gains after our measures of N balance and obviated differences in BW gain.
Survey of producers and interviews:
Educational & Outreach Activities
A journal article has very recently been published in Journal of Animal Science titled Effect of stocking density on performance, diet selection, total-tract digestion, and nitrogen balance among heifers grazing cool-season annual forages. Also, a survey as well as interviews with 3 cooperating producers has been conducted over the duration of the project. Extension professionals will use the information gathered to educate about and advocate for the addition of cover crops to current production systems. The survey and interviews asked questions to identify production goals and current practices of participants and further identify views and roadblocks toward the use of cover crops in their operations.
Information has been disseminated to a number of scientists and producers through the poster and presentation produced from research project data. It has also helped substantially progress my own education as a graduate student, and was the basis of my master’s degree completed in April of 2017. Plans are in place to produce extension material to aid producers in determining if implementation of cover crops to extend the grazing season and increase land use efficiency would be feasible for their own production systems. A survey and interviews have been conducted to gauge current producer perspectives in order to most effectively write and distribute extension materials.
These data have been presented as a poster1 to regional stakeholders, and an abstract presented to scientists at the American Dairy Science Association- American Society of Animal Science Joint Annual Meeting in July 2016. Additionally, data were presented at the Canadian Forage and Grassland Association in January 2017. In addition to these presentations, the work is the basis of my master’s thesis in April 2017 and was published in Journal of Animal Science in August 2017.
I did not know that cover crops had much more to do with soil health than controlling erosion, nor did I think of them as much of a forage resource before this work. I also learned that there are many road blocks and variables in deciding if, or when, to plant cover crops and what species to use. From the project I learned that stocking density can be a tool to influence grazing behavior when novel or undesirable feedstuffs are present. Nitrogen retention increased and average daily gain more than doubled as calves increased intake of the unfamiliar brassica plants. Therefore, I learned that our mixture of cover crop species specifically was nutritious to cattle and beneficial to cattle producers. Grazing cover crops in a field that would be barren after the main crop is harvested is a great way to increase utilization of land resources for food production.
A crop and cattle producer from east central South Dakota reported less compact soil after planting and grazing cover crops as well as his cows easily maintaining condition from the end of summer to early fall on a mixture containing winter wheat, oats, and peas. Another producer in north central South Dakota has been planting cover crops and using them as a forage source since the early 1990's before he was aware of the multitude of soil benefits. He reported a successful crop in all but a couple instances of severe drought in the last 15 years; even in years of little rain he says, "They [cover crops] can't grow if you don't plant them." He observes greater water retention on ground with them versus without and passionately says there are no reasons he can see for people not to utilize such a simple and beneficial technology.