by Tamara Scully
In a recent eOrganic webinar, Utah State graduate student Michael Greenland presented research which was designed to explore how dairy breed might interact with forage type. Previous studies at the university had shown that Jersey heifers have increased dry matter intake (DMI) when grazing pastures where grass was mixed with bird’s-foot trefoil (BFT), a legume, resulting in increased weight gain, and that forages with good leaf texture and increased carbohydrates and lower fiber also led to increased animal intake.
Seeding pasture with legume mixtures also decreases overall feed cost, as legumes don’t require as much fertilizer as do grasses. Some legumes can cause bloat, but due to high levels of tannins, BFT does not, Greenland said.
Yet the BFT in pasture forages had been shown to contribute to 50% of the increase seen in DMI. Greenland wanted to explore other factors that impact intake.
Greenland’s study utilized four breeds of cow and eight types of pasture forages. Each breed (Jersey, Holstein, Jersey x Holstein and a Holstein-Montebeliarde-Swiss (or Viking) Red cross) was rotationally grazed on pastures planted to grass monocultures of meadow brome, orchardgrass, tall fescue and perennial ryegrass as well as each of those grasses mixed with BFT. Three pastures were divided into 10 paddocks. The grasses gradually changed from one predominant type to another as the cows progressed through the paddocks.
Each of the breeds grazed through their own individual section of each paddock, and each breed was grazed on all mixtures of pasture forages. The cows were in each paddock for 3.5 days, then rotated to the next paddock. After grazing through 10 paddocks, each group of cows began the paddock rotation again, on a new pasture, for a total of 105 days of grazing.
Each group of cows were evaluated as a whole, not as individual cows, to establish how the type of forage and the dairy breed impacted DMI, grazing adaptation and feed efficiency. The paddocks – either all grass or grass with BFT – were not analyzed for performance on individual grass type, but rather on how a grass monoculture or a BFT mix impacted cow intake.
In order to quantify the amount of intake, a rising plate meter (RPM) was used to measure forage height. The shaft sits on the soil surface while the plate rises to the top of the forage, measuring height. Thousands of these measurements were taken to calibrate the RPM, and several dozen RPM measurements were used at each data-collecting point in the study.
Forage samples were also collected at the site of the RPM measurements. The forage was cut and bagged, then dried and weighed, to help determine the actual amount of dry matter available in each paddock at various stages of the grazing rotation.
The measurements were taken pre-grazing, 24 hours after cows were turned into the paddock, 48 hours after turn-in and again at 84 hours as the cows were rotated out of the paddock. Researchers attempted to account for the amount of forage left trampled in the paddocks, as well as the nutritional value of the forage, which was assessed by grinding the forage and determining the amount of fiber, protein and tannins. The nutritional data have not yet been released.
Because different breeds of cow are relatively different sizes, the researchers accounted for this variable by utilizing cows estimated to be at 40% of their mature body weight, so all cows were at the same stages of growth as the study began. Although larger-sized breeds consumed more food overall, “the animals should be gaining about the same rate of their percent mature body weight,” Greenland said.
A 150-pound gain for a Holstein was treated as the equivalent of a 100-pound gain in a Jersey, as each amount of gain represents the same percentage of the mature body weight of the breed, respectively 1,500 and 1,000 pounds.
“Another thing we use is that we convert the weight to metabolic live weight,” Greenland said. “We hope that helps to compare the breeds better.”
One animal unit (AU) was defined as 40% of the metabolic mature body weight of the breed. The metabolic body weight is the weight of active tissue only. The AU for Jerseys was considered to be 180 kg, while the AU for Holsteins was 270 kg for the purposes of the study. By dividing the intake data by AU, the size of the animal was neutralized.
The data showed that on pure grass pastures, Holsteins consumed the most forage, followed by the crosses, with the Jerseys consuming the least. The same was true when the cows grazed the grass/BFT pasture mixes. Forage type did not impact grazing consumption across breeds.
But the amount of forage consumed by each breed did depend on the pasture makeup. When BFT was in the mix, all cow breeds had a higher DMI than they did on grass alone.
“For every single breed, they ate more on the mixture than they did on the monoculture,” Greenland said.
The study’s second objective was to see if there are differences in grazing ability between breeds. The grazing rate was calculated by measuring kgs consumed/hour/AU.
This was measured by determining how the DMI changed on a paddock over time, comparing the intake after 24 hours to that which occurred between 24 and 48 hours, as well as to the DMI between 48 and 84 hours. Because the remaining forage quality is decreasing the longer the cows graze a paddock, and if some breeds are better grazers, the DMI over time should differ by breed.
For all breeds, DMI lessened the longer the cows were in any given paddock. A substantial drop in grazing rate occurred at a steady pace between 24 and 48 hours post turn-in, with a slower rate of decline seen from 48 to 84 hours. This rate was extremely similar across all four breeds, with no differences in the rate of the decline. The breed did not influence the grazing rate of consumption of lower quality forages.
“The order stayed the same. That was a real surprise. We really thought there would be one breed that was better at eating lower quality forage than others,” Greenland said. “We’re not finding any forage type and breed interaction. If there was an interaction, we’d expect the breed order to be different on the mixture side compared to the monoculture side.”
The last measure was feed efficiency, or the ability to convert what is consumed into weight gain. All breeds were found to be more efficient on monoculture than on mixed BFT pastures. Lower energy or lower intake equates to better feed efficiency, but it’s unknown why.
Breed differences show that Jerseys are most efficient, with the other breeds statistically the same, whether on monoculture or mixture. When percent mature body weight is factored in, Holsteins become close to Jerseys in feed efficiency, whether monoculture or grasses mixed with BFT is grazed, and they are more efficient than either crossbreed.
“Holsteins have the highest intake … All breeds have higher intake on BFT/grass mixtures … No breed had an advantage on higher or lower quality forage,” Greenland concluded, and “The Jerseys seem to be most efficient.”