Late summer diet on cow milk production and urinary N output
Updated: May 3
Dairy cows in New Zealand get the majority of their diet from grazing high-quality perennial ryegrass and white clover pastures.
Compared with total mixed ration systems used on dairy farms with cows in confinement, ryegrass-based pastures are high in crude protein (CP). This high CP that has been reported as high as 30% of dry matter, can result in high nitrogen (N) excreted as urine that is subsequently lost to water.
When pasture supply becomes limiting during the summer dry period in the southern North Island of New Zealand, dairy cows often receive silage and forage crop dietary supplements. This supplementation allows the opportunity to reduce the CP% of the diet by feeding maize silage and leafy turnips.
This study investigated the effect of dietary change in late summer-autumn on cow’s urinary N excretion.
Urinary N was greater for the cows fed high crude protein compared with the cows fed a medium or low crude protein diet.
Urine urea was more responsive to dietary changes than total urine nitrogen.
Offering a mixture of pasture, leafy rape and maize and grass silages was a viable option to reduce excretion of N to the environment without compromising productivity of lactating cows during summer and autumn.
The dairy cows' diet was changed from an all grazed pasture diet, to grazed pasture supplemented with leafy rape plus maize and grass silages with the aim of reducing the dietary crude protein (CP) concentration and total cow intake.
These three different feeding treatments resulted in the following groups with 18 dairy cows in each, with similar milk production, age, live weight, body condition score and days in milk were:
High crude protein (HCP) - Pasture and Meal
Medium crude protein (MCP) - Pasture, Meal, Turnips and Silage
Low crude protein (LCP) - Meal, Turnips and Silage
The following Table outlines when the different feed was offered and how much was offered. Both the MCP and LCP groups were offered turnips and then silage in between milkings. The LCP group was then returned to the freestall barn so that they could be offered silages over night.
The following graph shows the estimated dry matter intake (DMI), CP and metabolisable energy (ME) of each group. The HCP group had the lowest DMI, but the highest CP intake compared to the MCP and LCP groups. The ME intake was similar between the treatment groups.
The lower CP intakes of the MCP and LCP groups also resulted in lower nitrogen (N) intakes.
The N intakes for HCP, MCP and LCP were 647, 506 and 402 gN/day, respectively. These lower N intakes of the MCP and LCP groups resulted in lower urinary N concentrations as shown in Graph 2 below.
Even though the LCP group had a lower CP intake than the MCP group (13.5 vs 18.6, Graph 1), the Total urine N output was similar. Meaning that increasing the CP% of the diet from 13.5 to 18.6 resulted in the additional N in protein being utilised by the cow and not excreted as urine as it was in the HCP group.
The difference in total urine N output between the MCP and HCP groups shows that an increase in CP% from 18.6 to 25.0 (Graph 1) resulted in some of the additional N being excreted in the urine.
This shows that cows being offered predominately pasture in late summer can increase the urine N concentrations, therefore, increase estimated N loading in urine patches. Maize and pasture silage and/or turnips could be used to help reduce the concentration of N excreted in urine patches.
Urine urea was more responsive to dietary changes and management than total urinary N. Graph 3 below shows that there is a difference between all three treatment groups in urine urea concentration excreted. Low protein intake reduces the amount of protein degraded in the rumen, leading to reductions in the amount of urea released.
Milk production was not significantly different between the HCP and MCP treatments (Graph 4), despite a reduction of about 20% in dietary N intake. This means that the HCP diet had excess CP relative to production requirements.
The lower N intakes of the LCP group resulted in lower urinary N and urine urea concentrations, however, milk production was impacted by the very low CP intake of these cows as shown in Graph 4 and Graph 5 below.
Graph 4 shows the milk volume of the LCP group is significantly negatively impacted on a CP intake of 13.8%. The same relationship is reflected in milk solids (Graph 5).
This study has shown that the use of different feeds can be utilised to alter crude protein intake and therefore, urine N excreted. Feeding a mix of silage and turnips in addition to grazing pasture resulted in a reduction of total urine N excreted with no change in milk production. Therefore, feeding a mix of silage and turnips in addition to grazing pasture was a viable option to reduce excretion of N to the environment without compromising productivity of lactating cows during summer and autumn.
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Kyamanywa N, Tait IM, Mitchell CM, Hedley MJ, Pacheco D, Bishop P. Effect of a late summer diet change from pasture to brassica crop and silages on dairy cow milk production and urinary nitrogen excretion. New Zealand Journal of Agricultural Research. 2020 Feb 1:1-20.