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 dairy sheep industry in the United States started approximately 25 years ago. In 2003, there were approximately 75 dairy sheep farms in North America, producing over 2 million pounds of milk annually. The majority of sheep dairy flocks in North America are located in temperate regions well suited to pasture production. As the number of sheep dairy farms increases, so does the need for information regarding pasture-based production and supplementation. The research will evaluate dairy sheep production on high quality pastures, mainly kura clover and grass mixtures. The study will determine if there is a significant difference between milk production, milk composition and pasture consumption of ewes at different stages of lactation fed only pasture and ewes fed pasture with grain 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.
Pasture and Ewes
The study was conducted at the University of Wisconsin-Madison, Spooner Agricultural Research Station from May 25 to August 15, 2005 and all procedures were approved by the Animal Care and Use Committee of the College of Agriculture and Life Sciences. Twenty acres of pasture were utilized, and they ranged in composition from a mixture of approximately 60% kura clover (Trifolium ambiguum Bieb.) and 40% orchardgrass (Dactylis glomerata L.) and perennial ryegrass (Lolium perenne L.) to 5% kura clover and 95% orchardgrass, Kentucky bluegrass (Poa pratensis L.) and quackgrass (Agropyron repens L.). The pastures were divided into approximately 1.5 acre paddocks using a combination of permanent and portable electric fencing. Ewes were moved to a new paddock at two to four day intervals, depending on forage availability. The interval between grazing events in each paddock was approximately three weeks. After grazing, each paddock was clipped to a height of 7.5 cm to allow for consistent regrowth.
Fifty-six three-yr-old ewes were arranged in a 2 x 2 factorial treatment design; in early or late lactation and receiving 0 or 0.80 kg DM/d of supplement. The supplement consisted of 75% corn and 25% of a soybean meal based pellet. The final composition of the supplement was 18% crude protein, 11% neutral detergent fiber, and 66% non-fiber carbohydrates. All ewes were weaned from their lambs at 36 to 48 h postpartum and machine milked twice per day in a double-twelve parlor from weaning and fed 0.80 kg DM/d of supplement and 1.9 kg DM/d of alfalfa silage in drylot until grazing began on May 25, when supplementation treatments were applied.
Ten late lactation ewes, averaging 0.79 ± 0.41 kg milk/d and 136 ± 9 days in milk (DIM) prior to the start of the experiment were randomly assigned to the two supplementation treatments. Forty-six early lactation ewes, averaging 21 ± 10 DIM, were randomly assigned to the supplementation treatments. After initial assignment to treatments, a few ewes were reassigned to ensure each supplementation treatment had a similar mean for previous test day milk production in the current lactation (late lactation ewes) or previous year’s milk yield (early lactation ewes). All ewes were on pasture together for approximately 20 h/d and were milked twice daily at 0530 and 1700h. Supplement was divided into two feedings and fed in the milking parlor at each milking. Ewes were provided water in the pasture and a free choice mineral-salt mixture in the parlor holding area; no shade was provided in the pasture.
In addition to the 56 three-yr-old ewes, there were 20 two-yr-old ewes and 19 four-yr-old ewes in late lactation that were divided between the unsupplemented and supplemented treatments. They were managed with the 56 three-yr-old ewes. The two- and four-yr-old ewes were not included in the analysis of the milk production data because this would have resulted in the confounding of the effects of ewe age and stage of lactation, but some of these ewes were used in the collection of dry matter intake (DMI) and milk urea nitrogen (MUN) data.
Sample Collection, Analysis and Calculations
Daily milk production of individual ewes was measured weekly using a graduated Waikato Goat Meter (Inter Ag, New Zealand) by combining the amount of milk obtained at an evening and subsequent morning milking. Bi-weekly milk samples from the morning milking were analyzed for percentage fat and protein (AgSource Milk Labs, Stratford, WI). A compiled milk sample from eight to ten ewes in each of the four stages of lactation and supplementation treatment combinations was analyzed for MUN biweekly (AgSource Milk Labs, Stratford, WI).
Daily milk production, milk fat percentage, and milk protein percentage were used to calculate 6.5 % fat corrected milk (FCM) and 6.5% fat and 5.8% protein corrected milk (FPCM) based on the following equations developed by Pulina et al.(1989):
FCM = M (0.37 + (0.097 x F))
FPCM = M (0.25 + (0.085 x F) + (0.035 x P))
M = milk yield (kg) and F and P = fat and protein concentration (%); respectively.
Total trial production of milk, fat, protein, FCM and FPCM were calculated based on the equations of Berger and Thomas (2004). Total percentage of fat and protein during the trial were calculated from values of total milk, total fat and total protein produced.
Pasture samples were collected from paddocks approximately twice per week. Four quadrats (0.37 m2) were tossed randomly throughout the paddock before a grazing event, and forage was harvested to 2.5 cm stubble height. Forage samples were weighed and oven-dried (37º C forced-air oven) until they reached a constant weight. A composite of the four pre-grazing forage samples from each pasture was ground to pass through a 1-mm screen in a UDY cyclone mill before laboratory analysis. Forage samples were analyzed for dry matter (DM), total nitrogen (N) and neutral detergent fiber (NDF). Crude protein (CP) was calculated as N concentration x 6.25. Nitrogen concentration was determined by rapid combustion (850º C), conversion of all N-combustion products to N2, and measurement by a thermo conductivity cell (LECO Model FP-528; LECO Corp., St. Joseph, MI.). Neutral detergent fiber was determined using the method of Robertson and Van Soest (1981), as modified by Hintz et al (1996) and Mertens et al. (2002).
Titanium dioxide (TiO2) was used as an external marker to estimate pasture intake of ten ewes randomly selected from each supplementation treatment. A pre-treatment fecal sample was taken from ewes from each treatment before administration of the marker. Gelatin capsules of 2.5 g of TiO2 were administered to ewes twice daily for 11 days. After a seven day adjustment period, fecal grab samples were collected once per day, at 1630 h, for the following four days. A compiled fecal sample for each ewe was stored frozen and subsequently oven dried (60º C forced-air for 48 h) and ground to pass through a 1-mm screen in a UDY cyclone mill. Fecal samples were analyzed for the concentration of TiO2 using the colorimetric technique described by Myers et al. (2004).
Total fecal output (FO; g/d) was estimated by the equation: FO = (g TiO2 administered per d) / (g TiO2 /g feces DM). Forage fecal output (FFO) was equal to FO for unsupplemented ewes. For supplemented ewes, FFO was estimated by subtracting the estimated fecal output of grain (0.82 kg grain * 97 % DM * 90 % digestibility) from the total FO.
Based on FFO, pasture DMI was estimated using the equation: pasture DMI = FFO x (100/100-DMDpasture). Fecal and forage samples were analyzed for acid detergent lignin (ADL), using the technique of Robertson and Van Soest (1981), to determine apparent forage digestibility: Dry matter digestibility (DMD) = 100 – ((% ADL forage / % ADL fecal) * 100). For unsupplemented ewes, pasture DMI represents total DMI. For supplemented ewes, total DMI was estimated by adding pasture DMI to grain DM (0.82 kg/d * 97 % DM) offered in the parlor.
Weekly milk production and bi-weekly fat and protein percentage were analyzed using the mixed model procedure of SAS 8.2 (PROC MIXED) for a factorial design (SAS, 1999) with repeated measures. The model for these repeated measures included the fixed effects of stage of lactation, supplementation treatment, test date and their two- and three-way interactions, and the random effects of ewe and the residual. The general linear model procedure of SAS (PROC GLM) was used to analyze DMI and the total trial period production of milk, fat, protein, FCM and FPCM, and percentage of fat and protein. The model included the effects of stage of lactation, supplementation treatment, their interaction, and the residual. All means presented are least squares means, and significance is declared at P < 0.05 unless otherwise noted. Forage CP was related to MUN using the correlation procedure of SAS (PROC CORR).
Berger, Y. M., and D. L. Thomas. 2004. Milk testing, calculation of milk production, and
adjustment factors. Pages 55-62 in Proc. 10th Annual Great Lakes Dairy Sheep Symp. Univ. Wisconsin, Madison.
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Mertens, D. R., M. Allen, J. Carmany, J. Clegg, A. Davidowicz, M. Drouches, K. Frank, D. Gambin, M. Garkie, B. Gildemeister, D. Jeffress, C. S. Jeon, D. Jones, D. Kaplan, G. N. Kim, S. Kobata, D. Main, X. Moua, B. Paul, J. Robertson, D. Taysom, N. Thiex, J. Williams, M. Wolf. 2002. Gravimetric determination of amylase-treated neutral detergent fiber in feeds with refluxing in beakers or crucibles: Collaborative study. J. AOAC Int. 85:1217-1240.
Myers, W. D., P. A. Ludden, V. Nayigihugu, and B. W. Hess. 2004. Technical Note: A procedure for the preparation and quantitative analysis of samples for titanium dioxide. J. Anim. Sci. 82:179-83.
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energetico di latte di pecore di razza sarda. Pages 1867-1870 in Proc. XLIII Congress S.I.S.VET., Italy.
Robertson, J. B., and P. J. Van Soest. 1981. The detergent system of analysis and its
application to human foods. Pages 123-158 in The Analysis of Dietary Fiber Food. W. P. T. James and O. Theander, ed. Marcel Dekker, New York, NY.
SAS. 1999. SAS/STAT. User’s Guide, Version 8.2 SAS Institute Inc., Cary, NC.
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.
Late lactation ewes had a significantly higher milk fat percentage than early lactation ewes before the start of the trial, but the differences between stage of lactation treatments were not significant for any of the test days during the trial or averaged over all test days. This contradicts previous work, in which milk fat percentage tends to increase throughout lactation as milk production decreases (Pulina, 2002).
Supplemented ewes had a lower milk fat percentage than unsupplemented ewes on all test days except day 82, and the difference was significant at days 12 and 26. The substitution effect of a low-fiber supplement may have affected total NDF intake, leading to milk fat depression in supplemented ewes (Bencini and Pulina, 1997). Averaged over all test days, unsupplemented ewes had a higher (P < 0.05) milk fat percentage than supplemented ewes (6.00 vs. 5.75 %, respectively), which may be related to their significantly lower milk production.
Milk protein percentage was higher (P < 0.05) on all test days except day 82 for late lactation ewes compared to early lactation ewes and significantly higher on days 12 and 26. Averaged over all test days, late lactation ewes had a greater (P < 0.01) milk protein percentage than early lactation ewes (5.02 vs. 4.86 %, respectively). This supports previous reports in which milk protein percentage increases throughout lactation as milk production decreases (Cappio-Bolino et al., 1997). Milk protein percentage is less influenced by diet than milk fat percentage (Nudda et al., 2002) and shows less variation during the trial than milk fat percentage. Unsupplemented ewes had a higher milk protein percentage than supplemented ewes on each test day, and the values averaged over tests days were significantly different (4.84 vs. 5.04 %, respectively).
Early lactation ewes produced more (P < 0.01) total milk (+ 41.3 kg), total FCM (+ 39.0 kg), and total FPCM (+ 37.3 kg) than late lactation ewes during the trial period. Ewes in both stages of lactation produced milk with a similar fat percentage, and early lactation ewes produced more (P < 0.01) milk fat that late lactation ewes (7.77 vs. 5.31 kg, respectively) due to the greater milk production. Late lactation ewes had a similar milk protein percentage compared to early lactation ewes, but they produced less (P < 0.01) total milk protein (4.61 vs. 6.27 kg, respectively) due to lower milk production.
Even though supplemented ewes had greater mean values for total milk production than unsupplemented ewes (milk: + 19.7 kg; FCM: + 14.0 kg; FPCM: + 13.3 kg), the differences between supplementation treatments were not significantly different from zero. Since supplemented ewes produced more (P < 0.01) milk than unsupplemented ewes on individual test days, the lack of statistical significance between supplementation treatments for total milk production is probably due to the fewer number of observations for the latter trait. As in the case of total milk production, total fat and total protein production were greater for supplemented than unsupplemented ewes, but the differences were not statistically significant.
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 %. The paddocks with the most kura clover and least orchardgrass had the highest CP and lowest NDF values, and the paddocks with the greatest percentage of grass and the least kura clover had the lowest CP and highest NDF values.
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).
While the DMI estimates of this study are similar to the findings of Young et al. (1980), DMI of lactating dairy ewes is stated to reach 4 to 6 % of BW (Cannas, 2002). Estimates of both pasture DMI (1.3 % of BW) and total DMI (1.8 % of BW) in this study were below those stated by Cannas (2002), but his recommendations are based on ewes with much lower BW (Cannas, personal communication). The larger ewes in this trial may have a greater intestinal capacity and may consume less DM as a % of BW due to increased feed retention time and fiber utilization. Analysis of the supplemented and unsupplemented rations using the Cornell Net Carbohydrate and Protein System for Sheep (Cannas et al., 2003), predicted DMI estimates higher than observed DMI for both groups (supplemented 3.28 vs. 2.25 kg/d, unsupplemented 3.01 vs. 1.63 kg/d), suggesting that the titanium dioxide procedure may have underestimated pasture DMI.
The fecal recovery of the external marker may be affected by forage type (Judkins et al., 1990). Support for the use of TiO2 in sheep was demonstrated by the excretion pattern of TiO2 and chromic oxide (Cr2O3) in sheep consuming bromegrass hay (Myers et al., 2006), which may have different recovery rates in ewes consuming the highly digestible kura clover- orchardgrass pastures used in this study. Myers et al. (2006) found that the concentration of TiO2 in the feces was consistently higher than Cr2O3, which may account for low estimates of fecal output and DMI. In addition, estimates of digestibility based on ADL may be affected by the low lignin content in the forage sample, which averaged 3.2%. Van Soest (1994) recommended using ADL to determine digestibility in forage samples above 5% lignin due to incomplete recovery.
Milk urea nitrogen
Milk urea nitrogen (MUN) levels are closely related to blood urea nitrogen levels in sheep and can be used as an indicator of protein utilization (Cannas, 2002). There was no significant effect of stage of lactation or supplement level on MUN levels (P = 0.96 and 0.41 respectively). 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. Supplemented ewes had energy available from the concentrate to utilize dietary protein, but the supplement also was 18 % CP, confounding the effects of energy from the supplement on protein utilization from the pasture.
Milk urea nitrogen analysis, commonly used in the United States, measures the concentration of nitrogen, which makes up approximately 46.66 % of the urea molecule. Milk urea nitrogen values can approximate the concentration of the total urea molecule, or milk urea (MU), which is the general analysis in European countries. Cannas (2002) developed a linear relationship between MU and CP (% of DM), based on studies with varying sources of protein, in which MU (mg/dl) = 4.5 CP – 38.9 In this trial, the regression equation generated from MUN data collected on the unsupplemented ewes is MU (mg/dl) = 3.43 CP – 21.24. Both equations give similar results, so an average pasture CP value of 23 % corresponds to 64.6 and 57.0 mg/dl MU, respectively, or 30.1 and 26.6 mg/d MUN, respectively.
These high MUN values in all treatments suggest that pasture protein content was sufficient to meet the nutritional requirements of the ewes. Additional NFC in the supplemented treatment did not improve the utilization of pasture protein, due to the protein content of the supplement. High MUN values have been associated with excess protein intake and low reproductive efficiency in Italian dairy ewes and Cannas (2002) suggests that ammonia detoxification may increase the energy requirements of ewes. In addition, providing supplemental protein to grazing ewes is expensive and, if unnecessary, will reduce farm profitability.
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Educational & Outreach Activities
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 and a Masters Thesis completed in August 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. Finally, a paper will be submitted to a peer reviewed journal for publication.
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.
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.
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.
The integration of information regarding supplementation of grazing dairy ewes into the management practices of dairy sheep operations is a long term goal of this research project. At producer meetings, the results of this trial generated conversations about pasture quality, pasture management, pasture species and the nutritional needs of high producing dairy ewes. While the integration of these research results depends on individual farmer adoption of management practices, the dissemination of information has stimulated interest in monitoring and improving pasture quality, and ration formulation.
Areas needing additional study
Further research is needed to address the dynamic relationship between temperate pasture quality and supplementation of lactating dairy ewes. As farmers begin to improve pasture quality, further work may be needed to investigate protein and forage digestibility and the interaction of various supplements in the rumen. One approach may be the use of milk composition to determine the intake and utilization of pasture protein by further analysis of milk urea nitrogen. If farmers can asses pasture composition and consumption based on milk components, they may be able to modify the supplement to meet the energetic and protein needs of the ewes.