Lactation curves of commercial ewes rearing lambs.

Three-hour milk production measurements determined by machine milking at 3-d intervals throughout a 63-d lactation period were used to describe lactation curves for crossbred ewes lambing at 1 and 2 yr of age and rearing single and twin lambs. Age of ewe, type of rearing, and day of lactation affected (P < 0.05) milk production. Over the 63-d lactation, average daily milk production was 2.56 and 2.63 kg, respectively, for 1- and 2-yr-old ewes rearing single lambs and 2.73 and 3.47 kg, respectively, for 1- and 2-yr-old ewes rearing twins. Milk production of 2-yr-old ewes rearing twin lambs peaked at 21 d of lactation, and that of 1- and 2-yr-old ewes rearing singles peaked between 27 and 30 d of lactation. The largest differences in the lactation curves among age and rearing ewe classes were found in early lactation. These differences were reduced by midlactation, and by late lactation, milk production for all ewes was similar. Diurnal variation in milk production by ewes was evaluated in an 8 x 8 Latin square design. Diurnal variation in milk yield measurements of eight mature ewes, each bearing and rearing twin lambs, was similar between d 21 and 24 of lactation. Time of milk production measurements within a day did not affect yield determinations. Extrapolation from 3-h production estimates to daily milk production is valid in determining a ewe's milk contribution in support of lamb growth.

Grazing methods and stocking rates for direct-seeded alfalfa pastures: I. Plant productivity and animal performance.

A 4-yr study was conducted to determine the effects of two grazing methods (GM) at two stocking rates (SR) on alfalfa pasture plant productivity and animal performance and to ascertain the effect of grazing systems on subsequent performance of steers fed a high-concentrate diet. Eight pasture plots (.76 ha) were seeded in 1988 with alfalfa (Medicago sativa L. var. WL225) and divided into two blocks of four pastures each. Grazing methods consisted of a traditional four-paddock or an intensive 13-paddock system. Pastures were managed to allow a 36-d rest period with an average grazing season of 110 d. The low and high SR were 5.9 vs 11.7, 5.3 vs 10.5, 5.3 vs 7.9, and 5.3 vs 7.9 steers/ha for years 1989 to 1992, respectively. Following the grazing season, steers were placed in a feedlot and fed a high-concentrate diet (81% high-moisture corn, 14% corn silage, 5% protein-mineral supplement) for an average of 211 d. There was no effect of GM on herbage mass, pasture phase ADG, or live weight gain/hectare. Increasing the number of paddocks was beneficial when herbage mass was limited and stocking rate was above 7.9 steers/ha. Increasing SR above 7.9 steers/ha decreased herbage mass and pasture-phase ADG. As forage allowance increased, pasture-phase ADG increased quadratically (R2 = .82, P < .001), reached a plateau, and then decreased. Previous grazing system did not influence the performance of steers in the feedlot or their carcass characteristics. Optimum SR is dependent on herbage mass produced.

Grazing methods and stocking rates for direct-seeded alfalfa pastures: II. Pasture quality and diet selection.

A 2-yr study was conducted to determine the effects of two grazing methods (GM) and two stocking rates (SR) on alfalfa (Medicago sativa L. var. WL225) pasture quality and diet selection by Holstein steers. Eight pasture plots (.76 ha) were seeded in 1988 and divided into two blocks of four pastures each. Pastures were managed to allow a 36-d rest period with an average grazing season of 105 d. Before steers entered the next paddock, canopy heights (CH) of alfalfa plants were determined and pasture-forage samples were collected. Forage samples were analyzed for DM, OM, CP, and in vitro OM digestibility (IVOMD). At 12-d intervals beginning with the second grazing cycle, extrusa samples were collected from steers with esophageal fistulas. Extrusa samples were frozen, freeze-dried, and analyzed for OM, CP, IVOMD, in situ ruminal DM degradation, and ruminal undegradable protein. There were no effects of GM on alfalfa CH or pasture DM, OM, CP, and IVOMD. Increasing the SR increased pasture CP content in both years and increased DM, OM, and IVOMD in the 2nd yr. There was no effect of GM or SR on the quality of forage selected by esophageally fistulated steers. Esophageally fistulated steers selected forage that had greater OM, CP, and IVOMD than the average nutrient content of the forage. Although forage quality was greater when stocking rates were increased, the quantity of forage available per animal may have limited gains.

Social status regulates growth rate: consequences for life-history strategies.

The life-history strategies of organisms are sculpted over evolutionary time by the relative prospects of present and future reproductive success. As a consequence, animals of many species show flexible behavioral responses to environmental and social change. Here we show that disruption of the habitat of a colony of African cichlid fish, Haplochromis burtoni (Günther) caused males to switch social status more frequently than animals kept in a stable environment. H. burtoni males can be either reproductively active, guarding a territory, or reproductively inactive (nonterritorial). Although on average 25-50% of the males are territorial in both the stable and unstable environments, during the 20-week study, nearly two-thirds of the animals became territorial for at least 1 week. Moreover, many fish changed social status several times. Surprisingly, the induced changes in social status caused changes in somatic growth. Nonterritorial males and animals ascending in social rank showed an increased growth rate whereas territorial males and animals descending in social rank slowed their growth rate or even shrank. Similar behavioral and physiological changes are caused by social change in animals kept in stable environmental conditions, although at a lower rate. This suggests that differential growth, in interaction with environmental conditions, is a central mechanism underlying the changes in social status. Such reversible phenotypic plasticity in a crucial life-history trait may have evolved to enable animals to shift resources from reproduction to growth or vice versa, depending on present and future reproductive prospects.

Comparison of weigh-suckle-weigh and machine milking for measuring ewe milk production.

Thirteen crossbred ewes were used to compare weigh-suckle-weigh (WSW) and machine milking (MM) methods for determining milk production of ewes that were rearing single or twin lambs. At parturition, ewes were 13 mo of age and produced six single lambs and seven pairs of twin lambs. Milk production estimates were initiated on d 6 of lactation and a 3-d rotation of the two techniques was implemented. On d 6, milk production was measured using WSW; on d 7, MM was used. No measurement was made on d 8. The 3-d rotation was repeated 20 times throughout a 63-d lactation, resulting in 20 point estimates of milk production for each method of measurement for each ewe. The WSW procedure consisted of a 3-h period in which lambs were withheld from suckling their dams. This was followed by a suckling period, a second 3-h withholding period, and a second suckling period. Differences in pre- and postsuckling lamb weights of the second suckling period were defined as milk consumption and, indirectly, 3-h milk production. The MM procedure included an administration of 10 IU of oxytocin (i.v.), followed by evacuation of the udder with a machine using commercially available sheep milking equipment, and the milk was discarded. Lambs were withheld from suckling the ewes for a 3-h period, followed by a repetition of the oxytocin and machine milking procedures. Milk from the second milking was weighed. Milk production estimates determined using the WSW and MM techniques were similar (P = .42). Average 3-h milk production was 340 and 351 g for WSW and MM, respectively. Machine milking provides a reliable tool in evaluating the milk-producing ability of ewes that are rearing single or twin lambs.

Occurrence and range of dicumarol concentrations in sweet clover.

The dicumarol concentration in 272 cured sweet clover samples from 4 counties in North Dakota was determined. Dicumarol concentrations ranged from 0 to 164.7 mg/kg of sweet clover, with 64.6% of the 272 samples containing less than 10 mg/kg. Round bales were significantly (P less than 0.05) higher in dicumarol concentration (mean of 22.9 +/- 3.10 mg/kg) than were stacks or silage (means of 1.8 +/- -6.3 mg/kg and 0.6 +/- 2.1 mg/kg, respectively). The outer section of core samples taken from round bales contained significantly (P less than 0.0021) higher concentrations of dicumarol than did the inner section of the core, with means of 30.8 mg/kg and 16.6 mg/kg, respectively. A significant (P less than 0.0001) correlation existed between crude protein and dicumarol values (r = 0.44).