We weighed a group of steers and heifers off of ryegrass the other day, and many ranged from 770-850 lbs. That made me think about an old rule of thumb I was once told regarding growing cattle on wheat pasture. The cowboy logic I was told “once they hit 800 lbs, they’re done”, and as a nutritionist, I see the logic in that. Assuming that maintenance requirements are driven by metabolic body weight (emphasis on the word body weight), as that animal gets bigger, more and more of its diet is going to simply maintain weight. In a feedlot, dry matter intake is fairly easy to measure, however, it is quite the opposite in a pasture scenario, and there is not a good method to measure DMI. Nonetheless, this concept has been further examined by researchers at Arkansas who compiled data of cattle on wheat pasture and determined that to maintain their rate of gain, a range of 3.7 to 7 lbs of forage is required per pound of body weight in the animal. Based upon their review, these researchers felt that 5 lbs of forage (dry matter basis) per pound of body weight is required for weight gain. While we cannot reliably measure DMI in cattle grazing pastures, we can effectively measure forage allowance. Using the time honed tools of the trade, grazing stick, rising plate meter (the homemade one works well too), or simply cutting weighing and calculating (for those who really want a good estimate). Similar to wheat, ryegrass pastures are highly digestible and high in CP, so the logic would seem to fit in ryegrass cattle scenarios as well. Additionally, like wheat, the overall nutrient content of ryegrass throughout the grazing season (based upon samples collected at White Sand Unit) is fairly high, and based upon calculations from NRC (2000), it should support 1.75 lb ADG in an 850 lb yearling even at its lowest level. However, if we use the figures compiled by Arkansas, that 850 lb steer will need at least 4,250 lbs of forage to meet those gains. Examining the data from MSU Forage Variety data, Poplarville is the only location that has all varieties with an average yield that high on a per acre basis. In fact, based upon that data in South MS we could in theory keep cattle gaining up to 1,100 lbs (stocked at one animal per acre). However, we must keep in mind that those are averages for yield, calculated over a growing season, which means that yield data will differ within a season. Additionally, those numbers generated by our forage colleagues are based upon small plot harvest data, continuously grazed pastures might have different averages. Moreover, fertilization regimen and other management scenarios might offer different yields. The question of economics then plays into the scenario. Using NRC equations, and our forage allowance assumptions, we can determine how many cattle we can effectively stock per acre, and still maintain good weight gain. Rather than worry about individual animal gain, the area that should be focused on is gain per acre. Examining the scenario presented in Example 1, will give you an idea why that old saying about the 800 lb steer being done comes about.
Example 1. Two management scenarios for grazing beef cattle on ryegrass for 28 d
Two Stocking Scenarios
850 lb animal 425 lb animal
Pounds of forage available 9800 in 2 acres 9800 in two acres
Stocking rate @ 5 lbs/lb of BW 2 animals total 4 animals total
Calculated ADG (NRC, 2000) 1.75 lb/d 1.40 lb/d
Total gain 98 lbs 156.8 lbs
The above scenario shows that while more cattle per acre might yield less individual gain, the overall gain was greater on that pasture. Oftentimes, the individual ADG is the factor that is most talked about, however, when our expensive inputs (NPK) are applied on a pasture level, pasture gain is more important than individual ADG. Unfortunately, to most optimally manage stocker cattle grazing pastures, requires a lot of flexibility and resources to be able to move heavier cattle off, and bring in light weight cattle.
In conclusion, knowing how much forage is available can help determine forage allowance and stocking rate. Additionally, there comes a time when the heavier cattle are not as efficiently using resources as well as a lighter animal, and the decision needs to be made whether that heavy animal is “done”.