Do lambs grazing diverse pastures have low urinary N excretion as well as fast growth rates?
Updated: May 3
There have been a few studies published that compare lamb growth rates for lambs grazing ryegrass and white clover pastures and diverse pasture mixes. These have been completed in both the North and South Island of New Zealand, focusing on the summer dry areas in the Manawatu and Canterbury.
The summer is a time where feed demand for growing animals is high but can be limited depending on the season. Diverse pastures such as clover, plantain and grass mixes can provide a higher quality feed and greater growth when things start to dry up when compared to traditional ryegrass and white clover pastures.
The impact that farming has on the surrounding water bodies is something communities are becoming more aware about. Previous studies have shown that diverse pastures can increase lamb growth rate, however, this study considers the urinary excretion as well. So if the use of diverse pastures could also reduce the environmental impact as well as increasing lamb growth, then that would be an additional benefit to grazing diverse pastures.
This study looks at the effect of grazing a diverse pasture of plantain, Italian ryegrass, and red clover versus a ryegrass-white clover pasture on the liveweight gain and urinary nitrogen (N) excretion of ram lambs.
Lambs grazing diverse pastures had greater liveweight gain compared with lambs grazing perennial ryegrass and white clover.
There was a similar total urinary output for lambs grazing both diverse and ryegrass pastures.
Lambs grazing diverse pasture excreted less urinary N per 100g live weight compared with lambs grazing perennial ryegrass and white clover.
In this experiment there were 82 Coopworth ram lambs (8 months old) measured over a 3 month period. They were assigned evenly to a treatment group as follows:
Treatment 1: a diverse pasture containing plantain, Italian ryegrass, and red clover (Diverse).
Treatment 2: a conventional ryegrass-white clover pasture (Ryegrass).
The Diverse pasture sward comprised of approximately 17 % plantain, 42 % Italian ryegrass
and 15 % red clover (26% weeds), while the Ryegrass sward comprised 63 % perennial ryegrass, 12 % white clover (25% weeds).
There were no difference between the initial live weight for each group. There was also no difference in ME of the pastures, the diverse pasture was 11.6 and the ryegrass white clover pasture was 11.3. Both high quality pasture.
The final live weight of each group was significantly different. The graph below shows the live weight (kg) for the lambs that grazed the Ryegrass pasture (blue) and Diverse pasture (purple). The lambs that were grazing the Diverse pasture were 63 kg, whereas the lambs that were grazing the Ryegrass pasture were 55.8 kg on average. The letters indicate significant differences.
The Diverse pasture lambs achieved a daily liveweight gain of 240 g per day compared with the lambs grazing the Ryegrass of 160 g per day. This shows that lambs grazing Diverse pasture over a 3 month period had greater growth rates resulting in greater live weight.
Did they grow faster because they ate more?
Apparent dry matter intake (DMI) was estimated as the difference between pre- and post-grazing herbage mass plus daily herbage growth. The study reported no significant difference in dry matter intake between the two treatment groups, although the Diverse group ate slightly more (2.1 vs 1.8 kgDM/lamb/day). The metabolisable energy intake was also slightly higher, although it was not statistically different.
The nitrogen intake was significantly higher in the diverse pasture group at 68.4 g/lamb/day compared with 56.3 g/lamb/day. This could be the main reason why the Diverse pasture group had higher growth rates. More nitrogen in the diet allows for greater muscle growth.
Feed conversion efficiency is another indicator of high growth as it is a measure of how much feed was converted into growth.
Feed conversion efficiency were greater in Diverse than Ryegrass treatment, 111 vs 86 g liveweight per kg dry matter intake.
This means that the lambs grazing Diverse pasture could eat less compared to the lambs grazing the Ryegrass pasture to achieve similar live weight gains. You could theoretically grow more lambs on the Diverse pasture to target live weight than grazing Ryegrass pasture.
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We have established the growth rates were higher on the Diverse pasture, but did they excrete more urinary N as a result of a higher N intake?
Even though the Diverse pasture group had a greater nitrogen intake, they had similar total N excretion to the ryegrass group.
When adjusted for live weight, the Diverse pasture treatment had a significantly lower urinary N excreted. The graph below shows first the total urinary N excretion in g/day and the bars to the right show the adjusted urinary N output in g/100g live weight.
The adjusted live weight urinary N is calculated as the total urinary N divided by the live weight of the lamb. So although each treatment group had similar urinary N output, the Diverse treatment lambs had greater live weight so the urinary N was spread over more weight. You would expect heavier animals to drink and eat more, therefore excrete more. However this was not the case, in fact excreting less per unit of live weight.
The authors concluded that there is a potential to achieve target live weight of lambs in a shorter time and with less N loss to the environment by using a diverse mix containing plantain, Italian ryegrass and red clover in comparison to ryegrass-white clover mix in autumn.
The lambs grazing Diverse pasture can achieve live weight targets earlier as they have greater growth rates. As a result of reaching target live weights sooner, they could be sent off the farm earlier and free up feed for other stock classes on farm.
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Al-Marashdeh, O., Cook, G. A., Anderson, F. C., Meyer, J. P. H., Logan, C. M., Edwards, G. R., & Maxwell, T. M. R. (2020). Liveweight gain and urinary nitrogen excretion of lambs grazing diverse (plantain, Italian ryegrass and red clover) or ryegrass-white clover pasture in autumn. New Zealand Journal of Animal Science and Production, 80, 70-75.