CGREC cattle performance: 2017-2018
A comparison of 4 management systems with 4 reps each: 2 PBG treatments vs. no-fire continuous grazing and twice-over rotational grazing
Devan Allen McGranahan
Take-home points
- Cows on PBG pastures consistently put on weight.
- Cow weight gains on PBG were statistically significantly greater than continuous grazing without fire (nearly an additional pound/day extra gain on PBG averaged across both years).
- No difference among PBG approaches (Spring-burn only vs. Spring + Summer burns)
- Already we have evidence of annual variability in cattle performance on continuously-grazed rangeland without fire.
- Too early to tell much about twice-over rotational grazing, but no evidence that these animals out-performed animals on continously-grazed pastures with or without patch-burning.
Raw data and distributions
No reason to believe calf gains differed, but it appears that calf gains in the no-fire, continuously-grazed pastures (Refuge) came at the expense of cow performance. In 2017, cows lost weight in continuously-grazed pastures without fire, presumably because they continued to make milk to support their calves while grazing a lower-quality forage base.
Note that the twice-over rotational system was added in 2018.
Distribution of Average Daily Gains for cows and calves by fire management treatment. Colors denote separate pastures.
Hypothesis testing
There are two different ways to ask questions of these data:
- Which treatments show the greatest gains/are gains different from zero?
- The experimental question: Are gains on fire treatments different than the no-fire treatment?
Comparing performance among treatments within years
- Cows put on weight in both PBG treatments in 2017 and 2018.
- Despite higher average gains within PBG pastures in 2017, high variability around these means caused the analyses to fail to reject the null hypothesis (i.e., 2017 PBG gains were not statistically different than zero).
- 2018 PBG gains were statistically greater than zero.
- Cows on continuously-grazed pastures without fire (Refuge) tended to lose weight in 2017 and put on a bit in 2018.
- In neither year were mean gains statistically different from zero.
- Still, this is close to the theoretical expectation that pastures without patch contrast will show greater inter-annual variability.
- Cows on the twice-over rotational grazing pastures appeared to lose a bit of weight but not statistically different from no change in weight over the season.
Differences from zero were tested with LMER models fit to data within each year followed by post-hoc analysis testing the null hypothesis that mean gains per treatment were not different from zero.
Differences among treatments
In a nutshell, cow gains on both PBG treatments are significantly different than no-fire, but PBG treatments are not different from each other.
Regression coefficients from a linear mixed-effect regression (LMER) model fit to 2017 and 2018 data together serve as measures of treatment effect sizes for cattle weight gains throughout the study period so far:
Average daily gain (lbs/day) for three treatments with associated 95% confidence intervals taken from regression coefficient estimates in linear mixed-effect regression.
Post-hoc pairwise analysis of the LMER used to estimate the above coefficients indicates that gains on both PBG treatments were ssignificantly greater than no fire continuous grazing, and PBG treatments were not different from each other:
| Coefficient | StandardError | z | P | |
|---|---|---|---|---|
| Spring + Summer - No fire | 0.88 | 0.34 | 2.57 | 0.03 |
| Spring only - No fire | 0.90 | 0.34 | 2.61 | 0.02 |
| Spring only - Spring + Summer | 0.01 | 0.34 | 0.04 | 1.00 |