Performance of multibreed beef nurse cows with calves on ecological and fertilized pasture

Seventy beef cows (multibreed crosses with a dominant proportion of Limousin or Blonde d'Aquitaine genes) were assigned to 2 groups of 35 animals each. During the summer, they were grazed with offspring on two separate pastures: one fertilized with nitrogen (120 kg N/ha in the 1 st season and 60 kg N/ha in the 2nd season), group F, and the other unfertilized, group E. The pastures had not been fertilized for the previous 9 years. During the winter, the cows and calves were fed according to the INRA system. Nitrogen fertilization increased pasture yield by 40% (lst season) and by 28% (2nd season). With a similar number of grazing animals, increasing the area of pasture E almost twice compared with pasture F increased the accessibility of herbage but decreased the efficiency of its utilization. The daily intake of beef cows with calves on a pasture fertilized with 120 kg N/ha was 14 kg DM, 1340 g PDI and 12 UFL. After each grazing season, the body condition score of the cows was an average of 3.5 points on pasture F and 3.0 points on pasture E. Winter feeding of the cows made it possible to achieve a body condition score of 2.7 points before grazing. The rations of young bulls and heifers during the winter period helped to achieve predicted weight gains.


INTRODUCTION
Although many studies have already been carried out on the efficiency of rearing calves with beef nurse cows (Dobicki, 1996;Zalewski et al., 1998), the principles and the system of feeding nurse cows under Polish conditions have not been elaborated yet.
The aim of the present work was to determine: -the intake of nutrients by nurse cows with calves, expressed in INRA units, from a fertilized and ecological (unfertilized) pasture, -the effect of summer grazing on a fertilized and ecological pasture on the productivity of cows and rearing efficiency of calves, -the efficiency of the INRA feeding system in the winter feeding of multibreed nurse cows.

MATERIAŁ AND METHODS
The studies were carried out in northeastern Poland on 70 multibreed cows with a large proportion of Limousin and Blonde d'Aquitaine cattle (46 cows and 24 heifers) and their offspring.The animals were assigned to two groups of 35 each that were similar in terms of genotype and age.Animals from group E were grazed on an "ecological" pasture (unfertilized for 9 years) and those from group F on an "intensive" pasture that was supplied with minerał fertilizers during the experiment (including 120 kg N/ha in the lst season and 60 kg N/ha in the 2nd season).Pasture area was 20 ha for pasture F (in both seasons) and 35 and 25 ha for pasture E (in the lst and 2nd season, respectively).Strip grazing was used in both groups for an average of 174 days.A Limousin breeding buli accompanied each group of cows (F and E).
At the end of the grazing season, cows in each group received "winter" rations that were formulated from grass silage according to the INRA system (1988), taking into account the cow's body weight, its body condition after grazing using a 5-point scalę (Barlow et al., 1987), and its physiological status.Rations for the offspring were formulated taking into account body weight at the end of grazing and assuming liveweight gains (g/day) of 700-800 g for heifers and 1100-1300 g for young bulls.
Basal feed analysis was performed according to AOAC (1989).Pasture yield was assessed following the method of Kostuch (1994) and forage intake according to the differential method using the control paddock system by Różycki.Statistical calculations were performed using the Statgraphics ver.5.0 package (1991).

RESULTS
Minerał fertilization increased the yield of pasture F by about 40% in the lst season and by 28% in the 2nd season, improved the efficiency of herbage utilization by 15 and 6 percentage points, respectively (Table 1), and increased the percentage proportion of high ąuality grasses from 60 to 66% while reducing the proportion of legumes from 20 to 14%.Compared to pasture E, herbage allowance on pasture F was about 15% lower in the lst season and about 7% higher in the 2nd season.Cloudy and rainy days were found to be more freąuent in the lst season than in the 2nd.The stocking rate of the pasture, both in cow-calf units and in BH/ha was almost twice as high in the lst season and about 1.5 times as high in the 2nd season on pasture F than on pasture E (Table 1).Dry matter content of the forage from pasture F was an average of 1.2 percentage points higher in the lst season than in the 2nd, and that of the forage from pasture E by about 0.7 percen tage point.On both pastures, cows with calves received less dry matter, crude protein, PDI and UFL in the 1 st season than in the 2nd.
In both grazing seasons, cows from group F were characterized by higher liveweight gains and better body condition than animals from pasture E (Table 2).Com pared to group E, cows from group F were observed to have a slight improvement in calving efficiency and a higher estimated milk yield for 210 days of lactation.
Winter feeding of nurse cows according to the INRA system (1988) made it possible to achieve a body condition score of 2.7 points prior to grazing (Table 2), while the rations ensured daily intakes of energy (UFL) and protein (PDI) as specified under this system for Limousin and Blonde d'Aquitaine cows.
In both grazing seasons, no significant differences (P>0.05) were observed between the groups of calves from pastures F and E for daily liveweight gains and feed conversion (Table 3).Bulls from group N had about 6% better daily gains and heifers in both seasons had about 3% better gains than in group E.

DISCUSSION
An increased yield of pasture as a result of using various levels of nitrogen fertilization was also observed in other studies (Mikołajczak, 1996;Mc Grath et al., 1998).An increase in the yield of pasture of 20 kg DM/kg N in the lst grazing season and of 26 kg DM/kg N in the 2nd season suggests that even smali nitrogen doses may ensure good efficiency of fertilization and production on pastures that had not been fertilized for many years.Increasing the percentage proportion of grasses on pasture F compared with pasture E, accompanied by a decrease in the proportion of legumes, which do not respond to nitrogen fertilization, is a commonly known phenomenon (Ostrowski, 1994;Grynia et al., 1997).According to Marshal et al. (1998) changes in the botanical composition of grass-legume pasture do not reduce its nutritive value.This is confirmed by the production parameters obtained on pastures F and E in both grazing seasons.An approximately 615 kg DM/ha higher herbage allowance in the lst grazing season on pasture E compared with pasture F (despite its lower yield) was because pasture E was almost double the size of pasture F, with a similar number of grazed animals (Morrison et al., 1999).
The production efFects of cows and calves in both grazing seasons were favourably affected primarily by nitrogen fertilization, although the influence of weather conditions related to rainfall (Goszczyński and Witkiewicz, 1996;Mikołajczak, 1996;Zalewski et al., 1998) and the fact that the calves originated from multibreed herds (Barlov, 1987;Velde, 1996) cannot be disregarded.
The body weights of calves could also be affected by the intake of more milk from crossbred nurse cows, whose milk yield is higher than that of purebred beef cows (Velde, 1996).Daily nutrient intake from the pasture fertilized with 120 kg N/ha and the productivity results of cows and calves indicate that daily intake from the pasture of about 14 kg DM, 1340 g PDI and 12 UFL with a ratio of (PDIE -ODIN)/UFL = -10 g may ensure adeąuate body condition of multibreed nurse cows (about 3.5 points) and liveweight gains of young animals of about 1 kg/day in bulls and 850 g/day in heifers at the end of grazing.The body condition of cows before turning out to pasture and at the end of the grazing season was similar to that reported in other countries (Barlow et al., 1987;Petit and Agabriel, 1988).During the winter, daily intakes of energy (UFL) and protein (PDI) by nurse cows were similar to those specified by the INRA system (1988) for Limousin and Blonde d'Aquitaine cattle when taking into account the body condition and physiological period.

CONCLUSIONS
The summer feeding of multibreed beef cows with calves on a pasture ferti lized with 120 kg of N/ha makes it possible, with a stocking rate of 1.7 cow-calf units (at an average weight of 730 kg), to meet the nutritional reąuirements of the cows and to achieve adeąuate fertility and body condition of the cows at the end of grazing (about 3.5 points) and high weight gains of the calves of about 1 kg/day for bulls and 850 g/day for heifers, with a daily nutrient intake by the cow-calf unit of 14 kg DM, 1340 g PDI and 12 UFL.
Winter feeding of multibreed beef cows makes it possible to obtain a body condition of 2.7 points prior to grazing, while daily intakes of energy (UFL) and protein (PDI) conform to the reąuirements specified by the INRA system (1988) for Limousin and Blonde d'Aquitaine cows.

TABLE 1
Yield and efficiency of pasture, feed and nutrient intake of cow-calf unit TABLE3Liveweight and daily gains of calves in the grazing period 1 in the winter period and feed utilization 3as in Table1