Effect of Supplementation and Lactation Stage on Performance of Grazing Dairy Ewes
A comparison of the effect of stage of lactation and supplementation found that supplementation had a positive effect on milk production of ewes in both early and late lactation. Based on the current pricing of sheep milk, trial supplement levels would be expected to increase profitability of grazing dairy sheep farms. Based on milk urea nitrogen levels, supplementation did not improve the utilization of pasture protein. The rotationally grazed, kura clover and orchardgrass pastures maintained high levels of protein and moderate levels of fiber. Pasture dry matter intake was not affected by supplementation or stage of lactation, but total dry matter intake was increased by supplementation.
The goal of the project is to provide farmers with information regarding supplementation of grazing dairy ewes. The short-term outcomes were to determine the effect of supplementation and stage of lactation treatments on milk production, utilization of pasture protein, and dry matter intake of grazing dairy ewes. In addition, we monitored the quality of rotationally grazed kura clover and orchardgrass pastures. Intermediate outcomes included the transfer of information regarding supplementation of grazing dairy ewes through producer meetings and publications. This information emphasizes the use of managed pastures in providing the majority of nutrients to dairy ewes to maximize milk production, pasture utilization, and farm profitability.
In the analyses of test day milk yield, milk fat percentage and milk protein percentage, none of the two-way or three-way interactions were statistically significant. Therefore, the least squares means within test days for these three traits by the main effects of stage of lactation and supplementation treatment are reported. On each test day during the trial, early lactation ewes produced more milk than late lactation ewes and supplemented ewes produced more milk than unsupplemented ewes; however, none of the differences within a test day were statistically significant. When these differences were averaged across test days, large and significant differences were observed between stage of lactation and between supplementation treatments. Early lactation ewes produced more (P < 0.0001) daily milk (+ 0.53 kg), FCM (+ 0.36 kg), and FPCM (+ 0.34 kg) than late lactation ewes. Since milk production generally peaks in the fourth week of lactation and then slowly declines (Carta et al., 1995), the early lactation ewes are expected to have greater daily milk yield than late lactation ewes.
Supplemented ewes produced more (P < 0.01) daily milk (+ 0.23 kg), FCM (+ 0.16 kg), and FPCM (+ 0.14 kg) than unsupplemented ewes. This supports previous work in which supplementation increased milk production in lactating ewes (D’Urso et al., 1993). However, it is somewhat surprising that the response to supplementation was similar in ewes at both stages of lactation. Cannas (2002) suggests that supplementation in late lactation may contribute more to an increase in body weight than milk production.
The quality of available pasture remained high throughout the trial as a result of pasture management, which included intensive rotational grazing and clipping paddocks after grazing. Fiber and protein levels varied throughout the trial as a result of changes in pasture composition, including changes in the relative proportions of grass and legume. Pasture CP averaged 24.2 % but ranged from 16.6 to 30.6 %, and NDF averaged 36.0 % and ranged from 22.6 to 51.9 %.
Trial MUN values tended to be higher than recommended levels for sheep (14 to 22 mg/dl; Cannas, 2002), indicating an excess of protein intake. This can be explained by the high quality pastures, which ranged in CP from 16 to 30 %. The utilization of dietary protein depends both on protein and energy intake. Across all treatments, the correlation between pasture crude protein and MUN was 0.65. Within the unsupplemented treatment, the correlation (0.78, R2 = 0.61) was numerically higher, but not significantly different, than the correlation (0.52, R2 = 0.27) within the supplemented treatment. Unsupplemented ewes were more dependent on pasture for both protein and energy than supplemented ewes so a higher correlation between pasture CP and MUN would be expected in unsupplemented ewes.
DMI was not significantly different between supplementation or stage of lactation treatments. Pasture DMI averaged 1.63 kg/d for unsupplemented ewes and 1.53 kg/d for supplemented ewes. Early lactation ewes consumed, on average, more pasture DM than late lactation ewes (1.66 vs. 1.50 kg/d, respectively). Total DMI of supplemented ewes was estimated by adding pasture DMI to the DM of grain offered in the parlor (0.80 kg DM/d). This calculation led to a significant difference (P < 0.01) in total DMI between supplemented and unsupplemented ewes (2.33 vs. 1.63 kg/d, respectively). These results support a previous study of lactating ewes grazing pasture with a high forage allowance in which supplemented ewes consumed more organic matter than unsupplemented ewes (2.2 vs. 1.9 kg/d, respectively; Young et al., 1980). Total DMI was not significantly different between early and late lactation ewes (1.90 vs. 2.05 kg/d, respectively).
In the United States, sheep milk prices are generally based on milk quantity without regard to milk composition. Supplemented ewes produced 19.7 kg more milk over the 82 d trial period than unsupplemented ewes, and this milk is valued at $1.32/kg. As long as supplement costs are less than $ 0.39/kg (or $ 0.18/lb.), the supplementation will be profitable. Since $ 0.39/kg is about twice the normal commercial value of the supplement provided in this study, supplementation is expected to be profitable in most situations.
Cannas, A. 2002. Feeding of lactating ewes. Pages 123-166 in Dairy Sheep Feeding and
Nutrition. G. Pulina, ed. Avenue Media, Bologna, Italy.
D’Urso, G., M. Avondo, and L. Biondi. 1993. Effects of supplementary feeding on grazing behavior of Comisana ewes in a Mediterranean semi-extensive production system. Anim. Feed Sci. Technol. 42:259-272.
Young, N. E., J. E. Newton, and R. J. Orr. 1980. The effect of a cereal supplement during early lactation on the performance and intake of ewes grazing perennial ryegrass at three stocking rates. Grass Forage Sci. 35:197-202.
Impacts and Contributions/Outcomes
The outcomes of this project can directly benefit producers by providing information regarding the effect of supplementing dairy ewes on pasture. From the results of this study, supplementation had a positive effect on milk production in both early and late lactation. The increase in milk production observed in both stages of lactation is of benefit to producers who are currently paid based on milk volume. In addition, the results demonstrate the potential amount of milk the dairy ewes can produce in a pasture-based production system. Many dairy sheep producers are currently utilizing pasture as the main forage source during the summer and fall. This research supports and promotes this management practice. In addition, the project encourages the use of pasture sampling and ration balancing to meet the dietary needs of lactating ewes.
Trial results were presented to both producer and research groups at various meetings. Preliminary data were presented at the 2005 Spooner Sheep Day and included in the proceedings. Final results were presented in an oral presentation and abstract at the annual meeting of the American Society of Animal Science in July 2006. The results were also presented in a poster at the national SARE conference in August 2006. In addition, the results were distributed directly to dairy sheep producers through an oral presentation and the proceedings of the 12th Great Lakes Dairy Sheep Symposium held in November 2006. This annual meeting of dairy sheep producers included over 50 farmers and 95 attendees from around the world.
After presenting the results of the study to farmer groups, there were many questions regarding legume varieties, pasture establishment and pasture management. Based on the pasture quality data, this research project has successfully generated interest in the renovation of pastures among a few dairy sheep producers.